How To Fix A Bad Alternator Diode

It still puts out great voltage but it drains the battery when turned off. Any info on special tools I might need or other special parts I might want to replace would be nice. Recheck as per Step 02. The diodes of the alternator can go bad, failing to transmit the required voltage. Much better than $400, and I didn't have to wait any longer. There are many shops that offer alternator repair and services anywhere you are. Posted to the Waveform Library on 4/10/2015. So, all you would do in this circumstance is drive your vehicle to the nearest auto body shop or auto parts store and purchase a new battery for your vehicle. If you can see into the housing, you can pick up a burned stator by the darker winding color as compared to the other stator windings. it's pretty much a bridge rectifier that converts AC to DC, and enough reverse bias on the diodes can cause them to short. The rectifier is made up of a series of diodes which together convert AC power from the charging coil to DC power used to charge the battery and run other electrical appliances on the mower such as the headlights. Back to Top. 00 or so for a new Diode Trio for a 27-si alternator, put it in and go find something else to work on. Order Alternator Repair Kit for your vehicle and pick it up in store—make your purchase, find a store near you, and get directions. This is really obvious when headlights are on at night but also noticeable in dash lights and the center dome light. Car batteries can (and do) fail on their own. Any info on special tools I might need or other special parts I might want to replace would be nice. Dimming Lights The alternator by itself can be the cause of dimming lights, fading radio, and/or stalling. Diodes are like one way check valves that allow current to flow in only one direction. Those little cheap pencil type 30 watt jobs just don't do the job. A forward-biased diode acts as a closed switch, permitting current to flow. We have an AUDI Q7 that has had starting issues. Below, we're going to explore five of the most common signs of a dying alternator that just about every driver notices. Another important aspect to consider is the alternator's diodes, which can in turn affect the vehicle's voltage performance. The risk of slow discharge is the alternator input. 5 Volts AC (that's point five i. I replaced my bad alternator with the new one and started the car. Bad alternator symptoms: Dim lights. The diodes of the alternator can go bad, failing to transmit the required voltage. The Solar BP9 showed 180mv AC ripple at the alternator. 2 bad diodes in the rectifier. Connect black lead of DMM to pin E in harness plug. For others that might be searching for this potential problem, the diode that I purchased at Radio Shack is an epoxy rectifier diode, 3amp - 400PIV, part number 276-1144. (First diagram) To check alternator diode leakage, connect the multimeter in series with the alternator output terminal when the car is not running. It still puts out great voltage but it drains the battery when turned off. if it was a really bad short, as in full on metal to metal contact, you may have damaged the rectifier diodes inside the voltage regulator. Get it as soon as Fri, Oct 25. Visual cues. Even though the alternator is supplying ample DC power, a little AC current escaping from it will cause sensors to have erratic readings. If you need to replace the alternator you should ask that the alternator be inspected and that you be given a reason for it failing. Diodes are responsible for converting alternating current (AC—what the alternator produces), into direct current (DC— which is what the vehicles battery and other electrical components run on). Step 2: Connect positive test lead (+) to alternator AC post (TP-1) and negative test lead (-) to alternator B+ post. The way I understand it is that the diodes "convert" the A/C generated by the alternator (the Alt ( - ernating current) in Alternator) to DC so when you get above. Typical AC ripple on a healthy alternator/battery system is about 10-20 mV as read on a DVM. I had one fail on my truck and it was letting nearly 1v of AC into the system which made the computer have a few issues amongst other things. Next, test the alternator and pay close attention to the condition of its diodes. If the diodes are exposed and very easy to get to I could get it fixed here also, I'm just afraid to take it out of car without a fix in the works or parts in hand to swap it with. Then again, it is possible you just got a rare bad alternator. When recifier diodes go bad, you usually see the output voltage dropping in units of 4 volts or so as one of the 3 field windings go out of service. So what can you do to determine if your alternator is going bad before it is completely dead? Car Battery or Alternator? Learn How to Tell The Difference. Bad diodes will cause a fluctuating voltage which the multimeter will pick up as AC voltage. When the battery initially powers the car, the alternator belt, or V-belt, spins the pulley on the alternator, causing the rotor inside the alternator to spin very quickly. I've rebuilt some of mine before. Open the hood and observe the alternator belt's condition, along with its tension. Previous owner converted to 12v using EZ wire harness. In case it has lost tension, get your car to the nearest car repair center to get the belt tension adjusted precisely. 5 or so VAC out of an alternator at the battery, the odds are at least one diode went bad. yo! Is it Possible that a bad diode in the alternator rectifier can put out a sq wave into the ABS sensor (also called the differential speed sensor or VSS) ckt & then be picked up by the PSOM/and or PCM resulting in numerouse PSOM, E4OD shifting or PCM problems?. wires are connected per diagram and double checked with tech support. Bad diodes can also allow current to leak from the battery back through the alternator to ground, causing the battery to run down overnight. This is such a straightforward sign that it's an actual sign. 169 - Testing for Short in Stator, Diode or Charge Lead TESTING DIODE Disconnect connector plug. If the pulley is the culprit, you'd see the battery symbol light turning on and off at random times as the pulley cuts in and out. Diodes, epecially cheap ones, can leak AC through them. Visual cues. Re: Alternator diode faulty - Repair or replace alternator? Post by Pinelli » Sun Aug 05, 2012 11:15 am Sorry to drag up an oldish post, I've got a TD27T with a grotty, oily vacuum pump, obviously leaking, and I would like to sort it out while its out of the car. Since load testing an alternator will conclusively confirm the presence of bad diodes or failed windings, the value of measuring or viewing ripple exists mainly as a quick test. You can check the amount of A/C voltage leaving the alternator by putting your multimeter leads on the battery (Positive lead on +, and Negative lead on -), with you multimeter set on A/C voltage you should read no greater than. A multimeter's Diode Test mode produces a small voltage between. the brushes,in a replaceable cartridge in the denso alternator,wear down and failure occurs. To check for a possible bad alternator diode, switch your voltmeter to a low setting on the AC (alternating current) voltage scale; With the engine running, touch the meter probes to the battery terminals; Your voltmeter should read 0 AC volts. How to check your car Alternator Diode with a multi meter ( Testing the Voltage Regulator, Diode rectifier and Stator How to Repair Your Own Alternator (With Simple Tools. The alternator could still have bad diodes. The diodes fail by either going open circuit or short circuit. When it fails, that current no longer flows. This puts 12 volts on the #1 terminal, but keeps the alternator from feeding back 12 volts to the ignition circuit with the ignition switch turned off. A Delco reman. The extra "diode-trio" was used on the original Motorola Alternator Patent and on some of the one-wire alternators. The alternator gets hot at the rectifier and whines. There's no in between and there's no such thing as "about to fail". This could be due to some faulty alternator wiring. The battery will COMPLETELY effect running voltage and mimic a failed alternator (Bad cable connections too) Find more about the effect of battery condition to running voltage here. Electrical parts working below full strength, not working at all, or interference in your electrical systems could mean a bad alternator or rectifier. Many of us assume that our battery is dead when our car doesn't start. Most of the newer designs have done away with the diode-trio and the idiot lamp. The good news is that replacing a factory alternator with a high output alternator, in and of itself, isn't going to damage the rest of the electronics in your car, even if the new alternator is much, much bigger than the old one. Testing the Alternator. There is also another one up front that I have yet to get to but from what I understand those go bad too. If you can see into the housing, you can pick up a burned stator by the darker winding color as compared to the other stator windings. Bad outer and inner alternator bearings (large and small silver cylinders in the cutaway image above) can produce a whining noise. How to Replace an alternator. I'm trying to figure out what could cause this. Autozone says bad diode in the alternator So, I have had hard starting for a few days, so I went to my local Autozone and the battery checked out okay. You may ultimately have an intermittent starter as well, however bad diodes in the alternator are definitively verifiable on test equipment, and I'd be inclined to believe their diagnosis. How much should it cost to replace a diode in a 2001 Ford Taurus alternator - Answered by a verified Ford Mechanic We use cookies to give you the best possible experience on our website. the most common problem in all alternators,including this one is a simple fix. Bad Alternator Sign #5: Electric Failures Since the alternator is the primary source of electrical power in your car, electrical failures are signs that it is going bad. 3? I saw a few items that might work. It's a question of time before the alternator goes out. Or you can get it tested at an autoparts store. Touch the big post on the alternator (the positive terminal post on the alternator) with the red lead of the multimeter, and touch the other lead on the alternator case. Repair kit is $17. Before condemning the alternator, it's a good idea to check the drive belt. For DC Amp alternators, replacing the stator, diode or regulator may be required. The most common problem with alternator wine is one or more diodes have opened up or worse a diode has shorted. The diode's simply rectify the AC from the alternator into DC. A worn out or slackened alternator belt may be the reason for the alternator's low voltage output. , one-half a volt) with the engine running. If you can find a starter and alt shop that repairs them then a diode is the way to go. So who is offering to replace the diodes? I asked the people I know and they all shied away from replacing diodes in an alternator for whatever reason or another. Paying attention to the 10 trouble signs on this list can make it a little easier to diagnose car trouble caused by the alternator. I also checked the diode after desoldering it with the diode tester of my multimeter but the strage this was that it seems fine there. Your order may be eligible for Ship to Home, and shipping is free on all online orders of $35. If your alternator has a bad diode , your battery can drain. Bad diodes are a common cause of alternator failure. Just go ahead and replace it. Took it off, took it apart, and tested all the diodes only to find a bad one in the tri-diode pack. When several accessories utilize a vehicle's body as a ground, they form a common return path. If the reg is the source of the noise replace it. Once the engine runs, the alternator takes over the electrical delivery, supplying voltage to the spark plug and sending extra current to the battery for recharging. You can check the amount of A/C voltage leaving the alternator by putting your multimeter leads on the battery (Positive lead on +, and Negative lead on -), with you multimeter set on A/C voltage you should read no greater than. It can even cause a "Check Engine" light. I just changed the alternator and diode in my stairmaster 4000pt. This could be due to some faulty alternator wiring. All electrical parts in the automobile are compatible with direct current. Disconnect B+ battery cable at alterna-tor. the brushes,in a replaceable cartridge in the denso alternator,wear down and failure occurs. It's built into the alternator. Set meter on ohm scale. Here's some info I've been exchanging with SoS and Cantrell. 5) Volts with the headlights on or off and/or with everything on (so if the the test above gave you steady, and I emphasize steady,13 and up to 14. If your alternator is in fact bad after so few miles, I would suspect there is a problem somewhere else that caused it to fail so quickly. Bad diodes can also allow current to leak from the battery back through the alternator to ground, causing the battery to run down overnight. In case it has lost tension, get your car to the nearest car repair center to get the belt tension adjusted precisely. So, all you would do in this circumstance is drive your vehicle to the nearest auto body shop or auto parts store and purchase a new battery for your vehicle. our repair kit comes with the new brush set and front and rear bearings,and instructions,now 34. It was previously pointed out, however, that the use of an alternator rewound only at an optimum wind turbine locations makes sense! Note: In poor locations, the permanent magnet - generator clearly the better choice!. Diodes are essential to the charging process, and if they begin to fail, the alternator will quickly go bad. It still puts out great voltage but it drains the battery when turned off. If your vehicle won't start, it's usually caused by a dying or dead battery, loose or corroded connection cables, a bad alternator or an issue with the starter. I need to fix it because it's an expensive high output alternator for a sound system. There is only one diode and it is mounted to the rear of the alternator in a brown block with 3 screws. Usually the brushes go bad, and sometimes the core gets worn out from too much heat over time or too much current drawn. If the diode is defective, the conversion of AC to DC will be affected, thereby causing the energy generated to be insufficient for the high loads needed by your parts. When the battery initially powers the car, the alternator belt, or V-belt, spins the pulley on the alternator, causing the rotor inside the alternator to spin very quickly. Many of us assume that our battery is dead when our car doesn't start. You may ultimately have an intermittent starter as well, however bad diodes in the alternator are definitively verifiable on test equipment, and I'd be inclined to believe their diagnosis. Additional reasons for testing include a buzzing radio noise through the speakers signaling the alternator diodes have gone bad and are leaking AC voltage into the system. Move the positive test lead (+) to (TP-2) and (TP-3). I still had the charging light coming on, so I took it to a shop. 5 out of 5 stars 48. Rebuilder says the alternator has a bad diode which will give fluctuations and sometimes give DC and sometimes ac power. Bob further explained that one of the Diodes inside the Alternator could be shorted, causing the drain on the Battery, but the Alternator would still put out 12 Volts, and therefor pass the Voltage Output Test, however it could not put out full Current, and would fail the Current Load Test, IF the Shop actually ran that Test. They will explain the extent of the repair and answer honestly and. The diodes of the alternator can go bad, failing to transmit the required voltage. I go to a Dancing Academy as my daycare and I'm doing this dance in the recidal I'm having in the summer C:. With our how-to troubleshoot alternator charging systems article, you can Make sure your vintage Mustang's battery is charged up and ready to go - Mustang Monthly Magazine. What does it cost to have diodes replaced and does the entire alternator needs (2004 Honda Accord) to be replaced or just the above diodes, and what are the dangers in driving with that problem. Although the construction of some brands will vary slightly, the basic anatomy of an alternator consists of the same components. It has just been diagnosed that it is apparently a bad diode in the alternator. I replaced my bad alternator with the new one and started the car. Use of the terms "open diode" and "closed diode" refer to the flow of current through the diode. Diodes should allow current to pass in only one direction, but a bad diode will keep the charging circuit open even when the engine in not running, allowing the battery to go dead. The alternator output will vary with engine speed and electrical load. Diode overheating also occurs when the alternator is used to bring an undercharged battery up to a fully charged condition. Re: Bad Alternator Causing Engine to Die?? It could be a bad alternator. Any thoughts? ----- Hi Chris, having some alternator problems here. Alternator Repair at Cottman. The extra "diode-trio" was used on the original Motorola Alternator Patent and on some of the one-wire alternators. Disconnect charge lead connector at alternator. Much better than $400, and I didn't have to wait any longer. it's pretty much a bridge rectifier that converts AC to DC, and enough reverse bias on the diodes can cause them to short. Or if you want to, see if you can find a rebuild kit and give it a try if you want. Once the dash lights or headlights dim, it is a clear indication of alternator malfunction. Any info on special tools I might need or other special parts I might want to replace would be nice. I need to fix it because it's an expensive high output alternator for a sound system. A bad diode will cause 33% of the alternator's current output to be lost,. But either way, it's very unusual to have to replace the alternator on a Honda under 100,000 miles. Took it off, took it apart, and tested all the diodes only to find a bad one in the tri-diode pack. A worn or broken fan belt, a loose or damaged wire, even a blown fuse can cause a problem that could appear to be a bad alternator. Other things might be causing these failures, but it's worth taking it to a trusted auto repair service to have it checked out. Repair or replace as needed. simazoran/ThinkStock The next three symptoms are less obvious than a signal from a warning light; however, these signs of alternator trouble can tip off a driver that there's something going on that is out of the ordinary. And this is how much money you could save at the same time. Because your car often will not start when the alternator is dying, an alternator that's on its way out can easily be confused with a dying battery or starter. If you can see into the housing, you can pick up a burned stator by the darker winding color as compared to the other stator windings. You may need a new battery. The diode's simply rectify the AC from the alternator into DC. It was previously pointed out, however, that the use of an alternator rewound only at an optimum wind turbine locations makes sense! Note: In poor locations, the permanent magnet - generator clearly the better choice!. Test for complete alternator. Make sure there is no rust and corrosion between. If you can see into the housing, you can pick up a burned stator by the darker winding color as compared to the other stator windings. Removed alternator, took it to a rebuilder and had it tested. How to Tell if Volkswagen Needs New Alternator By Product Expert | Posted in Tips and Tricks, VW Service on Wednesday, January 25th, 2017 at 5:38 pm. I had one fail on my truck and it was letting nearly 1v of AC into the system which made the computer have a few issues amongst other things. Alternator repair with pics Here is how I did it: Since my Alero only had 27,500 miles on it when the alternator went, and I could assume that the bearings were OK (spun good), and because I've always wanted to do it, I decied to try to repair it. Starting Problems? How to Tell If It's the Battery or Alternator. 2002 Honda Accord Alternator Bad Diode. Once the engine runs, the alternator takes over the electrical delivery, supplying voltage to the spark plug and sending extra current to the battery for recharging. Diodes can fail in three ways; they produce an AC ripple that injects AC voltage or "noise" into the car's electrical system, the diode shorts-to-ground creating a dead short inside the alternator than can drain your battery in about an hour, or they fail in an "open" position that prevents the alternator from producing full output. Just an fyi, a bad diode in the alternator can drain the battery even when the engine is not running. Rebuilder says the alternator has a bad diode which will give fluctuations and sometimes give DC and sometimes ac power. Step 2: Connect positive test lead (+) to alternator AC post (TP-1) and negative test lead (-) to alternator B+ post. I hooked up the alt exactly like the last but something isnt right. If it's below that, charge the battery with a battery charger before you conduct the test. A new replacement Valeo alternator for the Porsche 356 Speedster pictured above comes in around $600 to $800 depending on the year. We had a cold snap recently and one morning the van wouldn't start. I had the same problem with a 1993 Oldsmobile 88. My Alternator Light came on and I went to the auto part store where I had bought the alternator and it was tested on the car and wasn't charging so we thought the voltage regulator had gone bad within 24 hours so I took it off right there i the parking lot and they took it in and did a bench test on the Alternator and it passed the bench test. He says still shows excellent alternator and battery showed 98 % and in excellent working order. Testing the Alternator. I would get it fixed. Caution AC voltage in the car electrical system can lead to running problems and damage to sensitive electronic components, namely the DME control unit. The diodes are part of the rectifier assembly converting the alternator's AC output to DC. with your alternator and charging system in general, the first thing you need to rule out is a bad battery. If light goes out, diode is defective. Of course, an electrical problem doesn't have to be caused by the alternator itself… even if the alternator isn't charging properly. There is more to an alternator than watt output. I took it to advance to have them test it and I started asking about leaky diodes and the dood said alternator is either good or bad, he tested and he said it was good. Or if you want to, see if you can find a rebuild kit and give it a try if you want. Rebuilding an alternator is a fairly simple procedure, which anyone with basic automotive knowledge can accomplish. As an inexpensive alternative to permanent magnet - generator can be turned into a car alternator for wind turbine generator. The test is for the externally regulated alternator, but the same test works for the internally regulated alternator too except the terminal is marked I and the test won't tell if the regulator or alternator is bad, which doesn't matter because they are one replaceable unit. If the dashboard gradually dims, the alternator is likely at fault. A multimeter will come in handy when checking the battery and alternator on your Briggs and Stratton engine. Bad diodes in the alternator definitely do not cause bad cells in the battery. Start the motor. The real problem when testing a diode using the diode test function of a digital meter is that an open or leaky diode, the meter sometimes reads okay (0. Current is allowed to flow from the anode to the cathode, but is blocked in the other. They will explain the extent of the repair and answer honestly and. How to Check if a Diode Is Bad Diodes are semiconductor devices that conduct current in one direction only, and are commonly made from silicon or germanium. I copied it into Word and saved it. Starting Problems? How to Tell If It's the Battery or Alternator. If you do, you've got a bad diode and you need a new alternator. 8T Quattro and the car died on me the other night, first the radio cut out, the interior lights went dim, headlights dimmed, and it eventually died. Usually the brushes go bad, and sometimes the core gets worn out from too much heat over time or too much current drawn. simazoran/ThinkStock The next three symptoms are less obvious than a signal from a warning light; however, these signs of alternator trouble can tip off a driver that there's something going on that is out of the ordinary. If the alternator tests bad then you should remove it from the vehicle and bring it to a parts store that has the ability to bench test the alternator. If you only want the bad diode replaced, then ask them about just that repair. Diodes are responsible for converting alternating current (AC—what the alternator produces), into direct current (DC— which is what the vehicles battery and other electrical components run on). I need to know where I can get diodes and how to rebuild an alternator. Alternator Rectifier Bridge. Failed diodes are indicated by AC voltage in DC electrical system. Touch the big post on the alternator (the positive terminal post on the alternator) with the red lead of the multimeter, and touch the other lead on the alternator case. A standard 12V alternator is delta wound, and uses 6 diodes in the rectifier. If you are experiencing problems with the electrical system, the alternator itself might be fine because the problem could be with one of the belts connected to the alternator. Many of us assume that our battery is dead when our car doesn't start. I took it to advance to have them test it and I started asking about leaky diodes and the dood said alternator is either good or bad, he tested and he said it was good. It is possible that you had both problems and they were unable to verify the bad starter but did find the bad alternator. Anyone have info on fixing a bad diode on an alternator for 5. For others that might be searching for this potential problem, the diode that I purchased at Radio Shack is an epoxy rectifier diode, 3amp - 400PIV, part number 276-1144. Diodes, like any other electrical component can be purchased in grades, a good, better, best if you will. Any amount of AC voltage indicates a bad diode - you'll need to replace the alternator. Can I just replace the diodes? Or do I have to replace the entire alternator? I have the dimly lit brake light thingy and low voltage. If it does, this means you have a bad battery. Diodes have two terminals -- an anode and a cathode -- with the cathode being marked by a line painted on the body of the diode. I need to know where I can get diodes and how to rebuild an alternator. I had it tested at my local auto parts store, they said the alternator was fine, but the battery was low on cranking amps. I took the alternator home, put it on the motor, grabbed a neighbor to go with, and we ran the boat down to the lake. Sell Your Car As It Is. I replaced my bad alternator with the new one and started the car. Car batteries can (and do) fail on their own. 5VAC, if you get more, your rectifier bridge on your alternator. bad alternator diode. Set meter on ohm scale. A bad circuit can also cause the above. The inline plug looks like a great temporary fix but if I could rebuild the bearings and the electrical components I would like to do that. Switch your multimeter to the lowest AC setting and reattach the test leads to the battery. A bad ground connection will significantly reduce the amount of current produced by the alternator to a point where the engine could stall. Took it off, took it apart, and tested all the diodes only to find a bad one in the tri-diode pack. Symptoms of bad alternator Battery light. The Solar BP9 showed 180mv AC ripple at the alternator. If the alternator fails the tests, your customer needs a new or remanufactured replacement alternator. The alternator is comprised of a voltage regulator and three main components: The stator, rotor, and diode. (First diagram) To check alternator diode leakage, connect the multimeter in series with the alternator output terminal when the car is not running. to check, you can test the rest. Electric failures could present themselves in a variety of ways. I replaced my bad alternator with the new one and started the car. Yes it is replaceable , the only thing I would look at too is the brushes if the are worn real bad along with the commutator then replace the alt. Alternator repair with pics Here is how I did it: Since my Alero only had 27,500 miles on it when the alternator went, and I could assume that the bearings were OK (spun good), and because I've always wanted to do it, I decied to try to repair it. Even though the alternator is supplying ample DC power, a little AC current escaping from it will cause sensors to have erratic readings. Recheck as per Step 02. If the diodes are exposed and very easy to get to I could get it fixed here also, I'm just afraid to take it out of car without a fix in the works or parts in hand to swap it with. Know the warning signs of a failing alternator and what to do if it goes out. Bob further explained that one of the Diodes inside the Alternator could be shorted, causing the drain on the Battery, but the Alternator would still put out 12 Volts, and therefor pass the Voltage Output Test, however it could not put out full Current, and would fail the Current Load Test, IF the Shop actually ran that Test. The alternator gets hot at the rectifier and whines. A bad ground connection will significantly reduce the amount of current produced by the alternator to a point where the engine could stall. Another important aspect to consider is the alternator's diodes, which can in turn affect the vehicle's voltage performance. Page 1 of 3: How to test a BAD Alternator on Honda 2. An alternator already contains diodes before the output terminal that will prevent reverse current flow, in most automobiles the alternator output is wired directly to the battery. Accord, Odyssey, Prelude Alternator troubleshooting tests. CAN be used to verify a diode is bad in a specific application after a Diode Test indicates a diode is bad. If the bushings that the alternator is mounted on have gone bad, a noise will be produced. If you have a friend with an oscilliscope have hime look at the output of the alternator and if there is a bad diode , which I am pretty sure you will find , he should see the noise on the output trace. The rectifier is made up of a series of diodes which together convert AC power from the charging coil to DC power used to charge the battery and run other electrical appliances on the mower such as the headlights. Although the construction of some brands will vary slightly, the basic anatomy of an alternator consists of the same components. These can include the power windows, power locks, dashboard lights, air conditioning, even your car radio. , one-half a volt) with the engine running. If it still tests bad then you need to replace it. If it goes bad, unless you're pretty knowledgeable about such things, rather than try to take the alternator apart and fix it yourself I would recommend you replace. , if the alternator has a bad diode, replace the alternator. Yet when powered up, control panel appears to be working correctly, full resistance when standing on pedals ( the slowly lower to the ground with full body weight) and resistance does not change. Or you can get it tested at an autoparts store. * To check for a possible bad alternator diode, switch your voltmeter to a low setting on the AC (alternating current) voltage scale. A car alternator recharges the battery and powers certain electrical systems like lights, radio, air-conditioning, and automatic windows. There is more to an alternator than watt output. The purpose of the three extra diodes is to make the alternator self-exciting, without using the battery as the source of the rotor current. The alternator will have to come off for repair. When the entire trio assembly burns open, the alternator won't charge at all. Having bad diodes in the alternator will drain all battery power even when power is Off! This condition will eventually destroy the battery, not the alternator! Car engine cannot be started with dead battery… even when battery is replaced, the cyc. 5 A or more, check the diode condition on the display. Autozone says bad diode in the alternator So, I have had hard starting for a few days, so I went to my local Autozone and the battery checked out okay. How-To: Replace an Alternator: Strapped for cash this holiday season? Replacing an alternator can cost you several hundred bucks at the local repair shop, but it's one of the simplest repairs you can do at home. The alternator is responsible for keeping your electrical system alive. If the bushings that the alternator is mounted on have gone bad, a noise will be produced. A standard 12V alternator is delta wound, and uses 6 diodes in the rectifier. There are certain signs of a bad alternator to look for: While running the car, note the brightness of the interior lights. Use of the terms "open diode" and "closed diode" refer to the flow of current through the diode. I recently replaced my alternator with bad diodes with one of the shiny SoS alternators. Regardless of the reason why your alternator fails, in most cases, the warning signs are there before an alternator fails. Also, I didn't test the alternator before putting it in. Bad diodes can also allow current to leak from the battery back through the alternator to ground, causing the battery to run down overnight. 4V price $ 7. The bad alternator diode can cause the circuit to charge even when the engine is shut off, and you end up in the morning with a car that won't start. With the multimeter set to measure resistance, it should measure more than 550 ohms if the alternator diode is any good. Bad diodes can also allow current to leak from the battery back through the alternator to ground, causing the battery to run down overnight. A few dollars later the problem was fixed. You simply push a few buttons on the tool and follow the prompts to determine if the alternator is good or bad. How to Check if a Diode Is Bad Diodes are semiconductor devices that conduct current in one direction only, and are commonly made from silicon or germanium. A multimeter will come in handy when checking the battery and alternator on your Briggs and Stratton engine. Because your car often will not start when the alternator is dying, an alternator that's on its way out can easily be confused with a dying battery or starter. As an inexpensive alternative to permanent magnet - generator can be turned into a car alternator for wind turbine generator. Diodes are like one way check valves that allow current to flow in only one direction. Checking the alternator, he said the output was fine, but his little checker said an internal diode was bad inside the alternator. How to Test the Alternator on a Craftsman Riding Lawn Mower A multimeter allows you to test the current, voltage and resistance of numerous types of equipment. Bad alternator diode causing a no start condition. It is possible that you had both problems and they were unable to verify the bad starter but did find the bad alternator.