W123 1984 300D Alternator Experts, Intermittent Charging
 

See the Pic.

This is from over on another Forum. The short story is the Charging Light seems to work OK but Alternator does not want to charge until RPMs are up.

I am just wondering if the corrosion on the Metal Contact Spring inside of the Alternator could cause the Symptoms in the thread.

Yup, another alternator thread. Stumped. - Mercedes-Benz Forum

I also thought that this is a area that someone might not check.

Answer
 

Corrosion on the contact will not help.

However:

The most common cause is that a previous owner replaced the alternator.

Often the replacement unit pulley is larger = low or no charging at idle or low RPM.

Check with a local salvage yard or alternator re-builder for a smaller pulley.

Have a great day.

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My 'new' rebuilt/upgdraded alternator did this sporadically for a few weeks when I first installed it late last fall. Starting with the very first day.

The behaviour simply stopped after a while. The unit was not 'fresh', having sat on my shelf for about three years after purchase.

I had swapped in the original pulley when I installed it (my '85 has the dual belt setup), so it was not related to that.

An idea that worked for me on my 123, was measure the voltage at the battery WITHOUT the engine running. Start the car and measure the battery voltage again at the terminal. If the voltage is different and has risen since before you started the engine, then the alternator is charging. Now how much current, I can't answer without an amp meter, but it gives you an idea if the alternator is working. Whunters comment is also of relevance because if the pulley is the wrong size, the alternator is not putting out what it was designed to.

With the amount of hair I've pulled out on alternator problems, grounding problems (?), battery problems (?)...the little bit of money it costs to replace the alternator is often well spent.

Except that some People exchanged their Alternators and still had the same problem or got a faulty rebuilt Alternator. Both resulting in more Hair pulling and labor.

Quite a few Years ago the Bearing went bad in My Alternator and I replaced it with no Further issue.

The total Cost for both Alternator Bearings was about $8.

The bulk of the Labor was in removing and re-installing the Alternator.
But, I have done the above and more on a lot of US Car Alternators.

If the inside of that Guy's Alternator has that corrosion on that Contact I am guessing the exposed ends of his Wires might also be badly corroded.

I was just hoping someone would have had experience with the same problem.
In My case I have had no Charging issues with My Mercedes with the exception that when I changed the Chassis to Battery Ground Cable I gained another charging Volt!

What should the alternator pulley diameter be and at what RPM should the alternator kick in and start charging? When I get a chance I'll measure my 85.

Start out with a trip to Autozone. Let them check out your charging system. They will diagonose: battery voltage with load/no load, amp draw, battery drain, alt output, and give you a nice little printout with graphs and numbers.

My problem ended up being a bad wire connector (at the alternator) AND the insulation was worn off of all three wires running from the alt to the battery. The bare wires were rubbing on the chassis.

Some ideas
 

Sometimes, a bad belt or a loose one will cause intermittent charging problems. This is often the case when a bad or wrong pully has been used or if the belt was not tensioned properly when installed.

Might be good to get one of those voltmeters that plugs into the ciggy lighter so you can keep an eye on the voltage. I believe it should be between 13 and 14VDC - closer to 13.5V.

I've had a few voltage regs lately that were not up to the task as well.

Answer
 

If the belt is tight, and this is intimately related to engine RPM, odds are it is an over size pulley.

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On the other hand if the alternator is older and worked fine for at least a reasonable time. Then it should not be the pulley size.

Just for fun put a battery booster cable between the negative battery terminal and the engine as a test. Just verifies the grounds are really good.

Just as follow up, the OP over on Benzworld notes that their glow plug relay is staying on after the car starts.

At an unknown time during the drive it will eventually turn off, but it's not cutting out when the engine starts as it should be.

Some aftermarket W123 glow timers are designed to run for about three minutes following start. I have one of those types; I have to wait three minutes before donating a jump.

what a crusted mess. corrosion like that is unserviceable. I say, run tests, write them down. pull the failed parts and go to the alternator shop in your area. or, go see the kids at the parts store. whatever.

Please note that this is not My Alternator. I was just curious to see if someone had the same issues and if they could relate it to the corroded Contact Spring.

Concerning the Alternator Pulley. People have said that when you get a rebuilt Alternator it comes without the Pulley.

I don't know if that is true as I have not bought a Rebuilt Alternator for over 40 Years.

And, back then in My case it would have been one from a US made Car and I believe it would have had the Pulley on it.