PeachParts Mercedes-Benz Forum - Alternator failure diagnoses confirmation on 82 240d.

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Quote:

Originally Posted by toomany MBZ (Post 3180061)

Where are the diodes located and how can I test them? That round black thing on the regulator, or the separate black box? Or elsewhere perhaps.

Dioded are inside the alternator and you don't need to take it apart to test them.

Use a digital ohm meter and set it on k ohms scale. Put it on the D+ terminal (the blue wire terminal of the alt) and ground (alt case). You should get a reading of around a thousand ohms. If you don't get a reading reverse polarity of the leads. If you still don't get a reading, the diodes are blown open.

I used the block terminal and get a 14.3 ohm reading.

Quote:

Originally Posted by toomany MBZ (Post 3180091)

I used the block terminal and get a 14.3 ohm reading.

You are not following directions!

Quote:

Originally Posted by funola (Post 3180052)

When you turn the key on, the diodes within the alternator provides the path to ground to turn the charge light on . You said you were able to turn the light on by grounding the blue wire after removing the connector.

Possible causes:

1. Alternator diodes blown open circuit (all 9 of them).

2. Connector not making good contact.

3. Alternator not grounded well.

4. Cluster traces damaged. This can happen if the ground strap on the bell housing is not making good contact while you start the engine.

Thank you, I'm going to test ohm load between alt housing and battery negative terminal. That should prove out if the alt is grounded well or not. I know the connector is making good contact, as I've separated it and manually connected each lead. The cluster traces you speak of are the traces in the dashboard instrument cluster, yes? When I had the cluster out I noticed my oil pressure gauge is weeping slightly. Do you know if engine oil is electrically conductive? This could be a problem.

Quote:

Originally Posted by funola (Post 3180058)

"Alternator makes a rhythmic rattly banging noise during operation (only with wires connected)"

This is a weird one and raises a red flag. Did this happen with the new alternator? Did you find the cause?

The new alternator is quiet, I was quite happy when I first fired it up and tested. It gave me 14.4 on the battery with smooth quiet operation from the alt. The old one sounded like someone banging on crap with a hammer. It's still quiet now, it's just not charging....and yes it has a belt.

Okay, I'll crawl under the car again.

Quote:

Originally Posted by funola (Post 3180103)

You are not following directions!

Alright, the reading fluctuates between 8.7 and 9.6 ohm, no k.

Here's an important diagram I'd figure I'd post up. It's what I've been using to test and diagnose with.

http://i.imgur.com/4cDYnb6.png

Said diagram is just a ? inside a blue box here.

Diagram showed up, thanks.

An ohm meter to check the grounds may not always tell the truth. An ohm meter puts micro amps into the circuit under test. An alternator 50 to 100 amps. A starter motor 300 to 350 amps. The effort is better spent cleaning all grounds and electrical connections. Try a jumper cable from alt case to battery - terminal.

Now it fluctuates between 9.5 & 9.8 ohm.

Quote:

Originally Posted by jeffr0000 (Post 3180126)

The cluster traces you speak of are the traces in the dashboard instrument cluster, yes? When I had the cluster out I noticed my oil pressure gauge is weeping slightly. Do you know if engine oil is electrically conductive? This could be a problem.

Yes....I can't be positive, but I would imagine oil leaking on the traces can very well cause a short or cause the grounds to not connect....there by not grounding the cluster...

Quote:

Originally Posted by funola (Post 3180074)

Use a digital ohm meter and set it on k ohms scale. Put it on the D+ terminal (the blue wire terminal of the alt) and ground (alt case). You should get a reading of around a thousand ohms. If you don't get a reading reverse polarity of the leads. If you still don't get a reading, the diodes are blown open.

Sorry, I gave incorrect information above. The ohm numbers are not correct and may vary. You should not get a dead short or open (infinite) and should get some resistance reading. Reversing polarity of the meter leads should result in a slightly different reading.

Ok, I've tried grounding the alt housing to the battery neg terminal, and running an alternative source of 12vdc to the exciter terminal and still nothing. This alt is going back.

Did you try tightening the belt?

Quote:

Originally Posted by funola (Post 3180491)

Did you try tightening the belt?

Ha, you're a funny guy. I was about ready to tighten a belt around my neck, but as it turns out I should have more faith in my troubleshooting skills as the new alt was dead after all. Jeeze, what a pain. I guess there is a possibility my car has taken to killing alternators, we'll see. I should go take that battery to be thoroughly tested while I await the arrival of my second replacement alternator.