300TDT Not Charging Properly
 

I have an '82 300TDT that does not seem to be charging properly. It shows 10.5v at idle. If revved, the voltage increases linearly. I didn't totally floor it but revved it pretty well and it went up to 13.5V. Is there any chance that the poor voltage could be caused by the regulator or is it for sure the alternator?

Sounds like the alternator is working. Why else would voltage increase with revving?

You can check the voltage regulator, here are instructions:
http://www.dieselgiant.com/mercedesvoltageregulatorinstall.htm

There's a picture of one you can compare yours to, it will give you a general idea of how worn your VR is.

what else is using power from the battery? Is this when you first start it ?
At 10.5v the car may have trouble starting. Are your GP's still on?

The battery showed 12.5V prior to touching the key. That dropped down to 10-10.5v by the time the glowplug light cycled off and the starter cranked. (It did indeed have a hard time starting and has been having a hard time starting after a couple starts after the battery has been fully charged.) The voltage then stayed down to 10.5 unless I revved the engine up and once the engine returned to idle, the voltage dropped back down as well. I didn't have any accessories or lights on.

Are you waiting for the glow plug relay to go off? It takes about 2min for the relay to go off, once it does the voltage should jump up to ~13.5 or higher if there is no drain on the battery.

You can drain the battery by driving for short hops <2min. when you turn the car off the cycle starts over irregardless of the engine temp.

I did not run it for more than 2 minutes. I will try that tomorrow.

It can take a while to charge the battery right up to over 13.5v. When you go for a good long run some place, say over 20 minutes on the highway, you will see it is right up where it should be. Dont forget things like the rear demister & the fan use power.

Good Luck with your car.

Was the battery fully charged with a battery charger? Maybe you have a weak/bad battery on top of alternator issues?

If you want to keep tabs on what your charging system is doing without installing a voltmeter in the dash, you can get on of these(see thumbnail). It plugs into the cigerate lighter and cost about $10 on eBay. Search "car voltmeter".

I would suggest that you first replace the voltage regulator. It is inexpensive and easy to do. Then go from there but I have a suspicion that it will cure your ills.

I tested it again today, but let the car run for more than 2 minutes. After that time, the idle voltage jumped up to 14v. It think the charging system is operating as it should, I just wasn't aware that the glow plugs were on after the start and pulling the voltage down so dramatically.

Sounds good to me, although a spare voltage regulator is a great inexpensive item to have on hand in case the alternator goes out away from home (it does happen).

Glad you found there wasn't a problem. When I thought I had the same problem, I traced the electrical system looking for a short. I didn't find one in the charging system but found problems with the climate control and the fog lights. So my journey was not just chasing a ghost.

If you want to keep tabs on your charging system get one of the plug in voltmeters. They sure beat guessing about what is going on.