1999 S70 alternator diodes

asummers · Post by **asummers** » 10 Apr 2015, 04:53

My alternator has seemed to have died yesterday. After putting in a new battery and getting it tested at my local Oreilly, it said the diodes were bad and the regulator was good. Are the diodes something that is built into the replaceable regulator, or internal to the alternator themselves? Trying to figure out if it a complete alternator replace, or something else. Thanks, alan

draser · Post by **draser** » 10 Apr 2015, 05:36

They're internal to alternator. Did they give you any data or just the verdict?

Post by **precopster** » 10 Apr 2015, 05:48

To test diodes you would need to fully disassemble the alternator and check each one with a meter OUT OF CIRCUIT. They don't just ALL go bad. Hmm

Post by **cn90** » 10 Apr 2015, 07:51

Checking diodes is an academic challenge etc. In other words, not easy.

The practical thing to do is to get an alternator...

Bosch AL0805X (X = rebuilt) = $150
Bosch AL0805N (N = new) = $210 or so

Herb Goltz · Post by **Herb Goltz** » 10 Apr 2015, 07:53

I think you can check for bad diodes by measuring AC voltage across the battery terminals with the car running. I seem to recall that >0.135VAC was an indication that unconverted AC voltage was passing through.

I think you should measure the DC output of the alternator with some load (lights, defroster grid), measure the AC output, and then check for voltage drops on the pos, neg and B+ cables. Report back with your results and people here will help!

Post by **precopster** » 10 Apr 2015, 15:47

Even if there is excessive AC component if car still has 14V or more with engine running the battery will continue to charge. You would get some weird electrical gremlins with high level of AC voltage content.

I would throw in a regulator ($45 or so) and see if the mud sticks. Wouldn't hurt to do above tests.

draser · Post by **draser** » 10 Apr 2015, 18:06

If you're getting the 14volts across batt posts chances are alternator is okay. Not quite a fan of the alternate component across batt, since each module has it's own power supply regulator and filter - but to an extent. I'd try to find out how many volts does the alternator output, and how much alternate component there is. They might as well be correct, but before spending the money I''d get some detail.

Post by **abscate** » 11 Apr 2015, 05:24

Diodes will usually fail open circuit imhe. You don't get DC or AC component through them in this case.

However, YVMV the detection of rectification protection.