So my alternator failed yesterday in my 2002 Honda Accord 4cyl. I took it apart and it needs new brushes, slip ring, and I'm at the point now where I am testing the diodes. I set my dvom to diode test and proceeded to test the diodes in both directions. There are 8 diodes. 6 of the diodes tested .5 volts in one direction and OL in the other indicating the diodes are good. The last 2 diodes tested 0 volts in both directions. Does it mean those diodes are bad?
Question about testing diode in alternator
- Thread starter joegreen
- Start date
- Status
I have never replaced diodes in an alternator . Think I replaces bearing and brushes , when I was a teenager . But , I would bet those 2 diodes are bad .
For years , I have just replaced alternators .
My advice is to add up the cost of the parts vs. the cost of an alternator .
Best of luck to you ,
For years , I have just replaced alternators .
My advice is to add up the cost of the parts vs. the cost of an alternator .
Best of luck to you ,
Where you getting a large AC ripple before it failed?
You test protocol seems sound.
+ to anode = no current
How much is repair kit vs reman?
You test protocol seems sound.
+ to anode = no current
How much is repair kit vs reman?
Last edited:
Sounds like you have two shorted diodes...I would normally expect them to blow open, but apparently that did not happen in your case!
I don't know the internal configuration of an alternator very well and I suppose there is some chance that the last two elements were actually switches used for diode emulation (to reduce power loss) and you somehow got the switches into the on state when you tested them. I think this is very unlikely, but it might be worth making sure that all the elements you tested were actually the same...maybe you got a good visual when you did your testing? Someone who really knows the guts of an alternator might step in and say my concern here is foolish...I am very familiar with diodes and switches (and DMMs), but not alternators.
I don't know the internal configuration of an alternator very well and I suppose there is some chance that the last two elements were actually switches used for diode emulation (to reduce power loss) and you somehow got the switches into the on state when you tested them. I think this is very unlikely, but it might be worth making sure that all the elements you tested were actually the same...maybe you got a good visual when you did your testing? Someone who really knows the guts of an alternator might step in and say my concern here is foolish...I am very familiar with diodes and switches (and DMMs), but not alternators.
Three phase 6 diodes.
Is one end of each diode disconnected when you test? It needs to be.
Most places will replace the bridge, not individual diodes.
Most places will replace the bridge, not individual diodes.
Where can you actually buy the brushes and diodes for alternators nowadays? Some years back the electric power steering pump in my nephews Subaru was intermittent. Needed brushes, I drug the slots on the comm and cleaned the scorch marks with crocus cloth in anticipation of the new brushes only to discover that a set of brushes cost within a few cents of a rebuilt pump. Hard to figure. Ended up just buying the rebuilt and used the old one as a core.
I recently bought some brushes off ebay. I think amazon might have had them too. Some of the dealerships run online stores (not under their name though) and you can enter in your VIN and search for OEM parts. Get the OEM P/N and hit up ebay. You might find OEM, or you might find a cross.
I recently did this but either a diode opened up or the regulator did. The regulator would have been another $40. At that point, on a beater, it seemed wiser to go with a cheapo reman. After taking in and out a couple of times, I could do a full swap in about 10 minutes, so I'm not too worried about "lousy" remans as a result.
I recently did this but either a diode opened up or the regulator did. The regulator would have been another $40. At that point, on a beater, it seemed wiser to go with a cheapo reman. After taking in and out a couple of times, I could do a full swap in about 10 minutes, so I'm not too worried about "lousy" remans as a result.
Originally Posted by mk378
Yes there will be six big diodes, the other two devices are likely not diodes.
Diode failure is rare in a modern alternator. The problem is almost always the brushes or the regulator.
Usually 6 diodes but not always. some stators are STAR. They have found that you can add diodes to the center connection and gain about 10% current and lower ripple. Two diodes for 10% current is way cheaper than more copper.
Meaning 8 diodes is pretty common.. Most all fords. Many Mitsubishi, and others are common.
Rod
Yes there will be six big diodes, the other two devices are likely not diodes.
Diode failure is rare in a modern alternator. The problem is almost always the brushes or the regulator.
Usually 6 diodes but not always. some stators are STAR. They have found that you can add diodes to the center connection and gain about 10% current and lower ripple. Two diodes for 10% current is way cheaper than more copper.
Meaning 8 diodes is pretty common.. Most all fords. Many Mitsubishi, and others are common.
Rod
Originally Posted by ARCOgraphite
Three phase 6 diodes.
That is what I thought , also . Wired as a 3 phase bridge rectifies ?
Three phase 6 diodes.
That is what I thought , also . Wired as a 3 phase bridge rectifies ?
Originally Posted by mk378
Yes there will be six big diodes, the other two devices are likely not diodes.
Diode failure is rare in a modern alternator. The problem is almost always the brushes or the regulator.
I see it quite often in Denso alternators on the various manufacturer boards I frequent - I grant it's rare that they fail prematurely, but we are talking about a 2002 model Japanese car and it's absolutely the time they start to fail. Replacement voltage regulators in the mass-rebuilt alternators from Mexico and China that you get at O'Reillys, etc. fail at a horrible rate right out of the box.
Looking for AC voltage before disassembly is the easiest way to diagnose, as mentioned.
When I need brushes, bearings, and a VR, this is a good source for quality parts:
https://www.romaineelectric.com/
You would be MUCH better off rebuilding your original alternator with decent components from them than getting an Autozone or O'Reills, etc. replacement.
Yes there will be six big diodes, the other two devices are likely not diodes.
Diode failure is rare in a modern alternator. The problem is almost always the brushes or the regulator.
I see it quite often in Denso alternators on the various manufacturer boards I frequent - I grant it's rare that they fail prematurely, but we are talking about a 2002 model Japanese car and it's absolutely the time they start to fail. Replacement voltage regulators in the mass-rebuilt alternators from Mexico and China that you get at O'Reillys, etc. fail at a horrible rate right out of the box.
Looking for AC voltage before disassembly is the easiest way to diagnose, as mentioned.
When I need brushes, bearings, and a VR, this is a good source for quality parts:
https://www.romaineelectric.com/
You would be MUCH better off rebuilding your original alternator with decent components from them than getting an Autozone or O'Reills, etc. replacement.
Originally Posted by Linctex
The local pick-n-pull style of auto wrecking yard sells used alternators for $25 (with a guarantee to test OK or money back)
That might be worth considering.
I did consider that. The wrecker around me is super expensive for everything. They want $75 for an alternator. I can find them much cheaper on eBay but I am going to rebuild with the parts I bought and see how it goes
The local pick-n-pull style of auto wrecking yard sells used alternators for $25 (with a guarantee to test OK or money back)
That might be worth considering.
I did consider that. The wrecker around me is super expensive for everything. They want $75 for an alternator. I can find them much cheaper on eBay but I am going to rebuild with the parts I bought and see how it goes
Originally Posted by joegreen
I already bought a new brush assembly, slip ring and the whole diode bridge thing. I have always rebuilt my own alternators. Parts total came to $50. The bearings felt prefect so I'll let them ride for now
I'd probably do the bearings at the same time as it's often only $10, and then you don't have to potentially yank it out again. But then, it's not such a big deal to do so if you know what you are doing
I already bought a new brush assembly, slip ring and the whole diode bridge thing. I have always rebuilt my own alternators. Parts total came to $50. The bearings felt prefect so I'll let them ride for now
I'd probably do the bearings at the same time as it's often only $10, and then you don't have to potentially yank it out again. But then, it's not such a big deal to do so if you know what you are doing
Check the resistance across the 2 suspect diodes, that should verify a shorted diode quickly.
Originally Posted by Or
Originally Posted by joegreen
I already bought a new brush assembly, slip ring and the whole diode bridge thing. I have always rebuilt my own alternators. Parts total came to $50. The bearings felt prefect so I'll let them ride for now
I'd probably do the bearings at the same time as it's often only $10, and then you don't have to potentially yank it out again. But then, it's not such a big deal to do so if you know what you are doing
I thought about doing the bearings but they feel really good. It is only one bolt and one nut to remove the alternator and it comes right out without anything in it's way having to be taken off
Originally Posted by joegreen
I already bought a new brush assembly, slip ring and the whole diode bridge thing. I have always rebuilt my own alternators. Parts total came to $50. The bearings felt prefect so I'll let them ride for now
I'd probably do the bearings at the same time as it's often only $10, and then you don't have to potentially yank it out again. But then, it's not such a big deal to do so if you know what you are doing
I thought about doing the bearings but they feel really good. It is only one bolt and one nut to remove the alternator and it comes right out without anything in it's way having to be taken off
Last edited:
So I received my parts. I tested the new rectifier and all 8 diodes tested the same, open in one direction and .450v in the other direction. This means that the old rectifier has two bad diodes. I put the alternator back together and it's been working great. Pretty happy with the $50 fix.
![[Linked Image]](https://www.bobistheoilguy.com/forums/attachments/usergals/2019/06/full-55200-33681-img_20190618_124337396.jpg "[Linked Image]")
![[Linked Image]](https://www.bobistheoilguy.com/forums/attachments/usergals/2019/06/full-55200-33682-img_20190618_135211720.jpg "[Linked Image]")
![[Linked Image]](https://www.bobistheoilguy.com/forums/attachments/usergals/2019/06/full-55200-33683-img_20190618_141612042.jpg "[Linked Image]")
![[Linked Image]](https://www.bobistheoilguy.com/forums/attachments/usergals/2019/06/full-55200-33684-img_20190618_143512194.jpg "[Linked Image]")
- Status
Similar threads
