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Old 03-04-2008   #1
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How to test alternator diodes?

Hi,

battery went empty over night, and the battery control light was flickering just before. I suspect the culprit is a faulty alternator diode (meaning the battery gets drained by the alternator over night). How to test them?

Are they located in the alternator? Can they be changed? A power diode typically cost almost to nothing, whereas an entire alternator would be rather expensive...

thanks,
Zardo
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Old 03-04-2008   #2
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Re: How to test alternator diodes?

to check diode (with multimeter) you need to take them out of the circuit, i.e. unsolder them from diode pack if you can.

probably better getting secondhand alternator.
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Old 04-04-2008   #3
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Re: How to test alternator diodes?

which alternators will fit?

it's a 1,4 SX 1997 model with 80 horsepower without clim
Alternator does 14 V 65 A

do I need to search for the same modell and age?

There plenty of wrecked bravos available around here, but they are newer and more powerful, almost nothing here which would be that old and feeble.
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Old 04-04-2008   #4
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Re: How to test alternator diodes?

charge battery, fit to car, start engine, get multimeter across battery terminals. voltage should be 14.4v. now switch everything electrical on (demister, lights, radio etc.) and the voltage reading should be 13.99v minimum. if your voltage readings are less then the alternator output is low.
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Old 04-04-2008   #5
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Re: How to test alternator diodes?

seems gone are the days of renewing diode packs,brushes & slip rings
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Old 04-04-2008   #6
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Re: How to test alternator diodes?

Quote Originally Posted by T14086 View Post
seems gone are the days of renewing diode packs,brushes & slip rings
I was lucky and found the diode test and alternator overhowl described in the manual... hm but those diodes seem to be hard to find.
Anyway, they would use a metal cutter to get to the diodes, but they didn't explain how to put the thing back together again. Soldering wouldn't be a good idea, or would it, since those diodes might get hot?!
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Last edited by Zardo; 04-04-2008 at 20:08.
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Old 04-04-2008   #7
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Re: How to test alternator diodes?

Quote Originally Posted by Zardo View Post
Soldering wouldn't be a good idea, or would it, since those diodes might get hot?!
use a heatsink.......I use a pair of medical forceps
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Old 05-04-2008   #8
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Re: How to test alternator diodes?

So today the battery is fully charged again, I had to do it in my room since I don't have a power line long enough to get the charger out to the car, charging behavior was normal, there was some light gas smell the first hours when amps stayed at 5 but then amps finally went down now it shows almost zero amps, the smell is completely gone now even with the charger still connected and voltage's well over 13 V - so I think I can exclude it as being the culprit.

I started checking the car:

I got all the power fuses out, the 80A, the 60A, the 40A, the 30A, and even the 40A for the radiator, out out out. So no power fuses left. I then connected my ohmmeter to plus and minus to the battery cables of the car (the battery is still in my room). Surprise: it shows 8,5 Ohm. Ignition was off, of course (doesn't actuelly matter since all the fuses were out), and there was no power source connected to the car for about two weeks.

So I think there is a parasitic drain present, and 8,5 Ohm would correspond to 12V/8,5Ohms = 1,4 amps! So if I would connect the fully charged battery now, it would get drained fast and tomorrow it would be almost empty...

The plan is to disconnect the alternator and measuring ohms without it connected. If it's 8,5 Ohms, then it can't be the alternator...

But how? The cable is quite difficult to access, as the car is still on the roadside. Will it be enough to pull heavely on it? Or would I have to screw it off? It's a 1997 Fiat 1,4 SX.

I tried checking the cable and I squezzed my hand in but all I could feel was some plastic down there and some cable dangling.

how to get the cable off?
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Last edited by Zardo; 05-04-2008 at 16:12.
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Old 05-04-2008   #9
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Re: How to test alternator diodes?

UPDATE

ok the cable seems to be screwed on to the alternator... couldn't get if off... I believe I will have to crawl under the car to get it off.


I took the battery to the car and measured the amps it drains with ALL power fuses taken out... 3,7 ampères! wow that was quite a spark

Strange that it's 3,7 A now with the multimeter in circuit, instead of the calculated 1,4 A. I would have prefered the later...

I put the power fuses back in, radiator, 30 A, 40A, 60A - still 3,7 Amps drain. When I put the 80 Amps fuse back in, the parasitic drain increased to 4,1 amps and I could hear some small electric motor working for some fraction of seconds. Ignition was off!!!

I left the battery disconnected. What now?
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Last edited by Zardo; 05-04-2008 at 18:43.
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Old 07-04-2008   #10
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Re: How to test alternator diodes?

UPDATE 2

Quote Originally Posted by jug View Post
charge battery, fit to car, start engine, get multimeter across battery terminals. voltage should be 14.4v. now switch everything electrical on (demister, lights, radio etc.) and the voltage reading should be 13.99v minimum. if your voltage readings are less then the alternator output is low.
I tried this today.

First, I checked the voltage of the battery. After having it fully charged, I let sit it in the car disconnected for at least 24 h. When I checked voltage today (before reconnecting it), it showed 12,85 Volts which would mean that it is still 99,99% charged...

So I connected it and started the motor. It came on immediately, but when the motor was idling, the charging light was flashing wildly. When I put on gas, the motor did rev, but not as smoothly as normaly. I revved the motor for about 10 seconds just to look if the flashing would stop, which it did not. I then I approached the engine compartment with the motor still running. I noted that the battery was sputtering out acid at a fast rate (it's a wet acid battery). In seconds, the engine compartement fast wet from acid spill. I tought it was about to explode, it did smell very strangely indeed. So I jumped to the ignition key and took it of, then I disconnected the battery. When I remeasured the voltage of the disconnected battery again, it had 13,8 Volts!

So there is definitely something heavely wrong with the alternator charging circuit.

As it is quite difficult to access the thing, I wonder if anyone ever did take of the diode plate with the alternator still in place. Is it possible? I believe replacing the diode plate with a new one could solve my problem, so there would be - theoretically - no need to get the entire alternator off the engine.

Will the diode plates of the other magneti marelli alternators (75 A and so on) fit?

Hm, as the car is still on the roadside, I fear I will have to finally get it jacked up now and crawl under it.
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Last edited by Zardo; 07-04-2008 at 18:23.
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Old 08-04-2008   #11
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Re: How to test alternator diodes?

You'll find it difficult to get replacement diode packs or parts for alternators - best to get a second hand or reconditioned unit - its not worth the hassle trying to replace them - soldering is very difficult. If your battery also threw out acid, it has most likely been damaged and will also need to be replaced. you're lucky it didnt explode. make sure you wash down wherever acid got spilled..
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Old 08-04-2008   #12
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Re: How to test alternator diodes?

Quote Originally Posted by T14086 View Post
seems gone are the days of renewing diode packs,brushes & slip rings
you cant buy the bits any more, that is the only reason i no longer recon alternators. shame really. same thing has happened to caliper overhaul kits.
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Old 08-04-2008   #13
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Re: How to test alternator diodes?

Quote Originally Posted by Zardo View Post
I got all the power fuses out, the 80A, the 60A, the 40A, the 30A, and even the 40A for the radiator, out out out. So no power fuses left. I then connected my ohmmeter to plus and minus to the battery cables of the car (the battery is still in my room). Surprise: it shows 8,5 Ohm. Ignition was off, of course (doesn't actuelly matter since all the fuses were out), and there was no power source connected to the car for about two weeks.

So I think there is a parasitic drain present, and 8,5 Ohm would correspond to 12V/8,5Ohms = 1,4 amps! So if I would connect the fully charged battery now, it would get drained fast and tomorrow it would be almost empty...
to measure the drain you must disconnect the battery negative lead and then connect the meter between the battery negative lead and the car chassis, and set it to measure amps. you have not measuired drain, you have only measured the resistance between the positive and negative lead.

Quote Originally Posted by Zardo View Post
So I connected it and started the motor. It came on immediately, but when the motor was idling, the charging light was flashing wildly. When I put on gas, the motor did rev, but not as smoothly as normaly. I revved the motor for about 10 seconds just to look if the flashing would stop, which it did not. I then I approached the engine compartment with the motor still running. I noted that the battery was sputtering out acid at a fast rate (it's a wet acid battery). In seconds, the engine compartement fast wet from acid spill. I tought it was about to explode, it did smell very strangely indeed. So I jumped to the ignition key and took it of, then I disconnected the battery. When I remeasured the voltage of the disconnected battery again, it had 13,8 Volts!

So there is definitely something heavely wrong with the alternator charging circuit.
alternator is overcharging dangerously! fit a scrapyard replacement, it is not as difficult as you would think.
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Last edited by jug; 08-04-2008 at 13:05.
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Old 08-04-2008   #14
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Re: How to test alternator diodes?

Quote Originally Posted by jug View Post
you cant buy the bits any more, that is the only reason i no longer recon alternators. shame really. same thing has happened to caliper overhaul kits.
used to use washing machine brushes filed to fit on paris-rhone alternators when there was none locally
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Old 09-04-2008   #15
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Re: How to test alternator diodes?

Quote Originally Posted by jug View Post
to measure the drain you must disconnect the battery negative lead and then connect the meter between the battery negative lead and the car chassis
erm, what was a thinking, i mean between the battery negative terminal and chassis.
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