Faulty Alternator / Voltage Drop? - PeachParts Mercedes-Benz Forum

Darinkald · #1 01-07-2004, 09:18 AM

Let me start by saying that this website is loaded with everything for the "do it for yourself" mechanics. The members provide support and the tricks of the trade for everyone.

I bought my 240D about four months ago and it ran great until a few weeks ago. The instrementd ash gage lights came on (Brake, Break Pad Sensor, Battery) and wouldn't go off. At that time I started investagating to see if any other members had the problem. I found that it might be the alternator. I called a buddy of mine at the local parts store and he sold me a rebuilt one at his discounted price. The alternator was a 55 amp. I proceeded to install the alternator, and it cured the dash light problem and the car ran great.

Two to three days later my wife stated that she was driving home and the lghts were really dim, and the heater wasn't blowing. My first thoughts were the blower motor fuse. I removed the access panel and saw that the fuse was blow and the metal connection was damaged. I put a by-pass fuse link in and the fan came back to life. I then proceed to recharge the battery and it tested fine 12.4 volts sitting. Becuase the alternator was under warrenty my buddy exchange it and upgraded it to a 65 amp one. I put it in for her and the car ran great again.

The same problem is occuring again. The light are really dim and the heater fan stopped working after a few minutes. I got out my multimeter and was testing the battery while sitting idle and while on a load. I noticed that the battery at idle went from 12.4 (engine off) to an average of 13.4 volts. When I put a load on the alternator for instance the lights the battery started to lose its voltage slowly. When I turned the heater fan blower on the multimenter readings dropped faster. I then turned off the blower and the light and let the alternator recharge the battery.

I the volts came back up to around 13.4 on the battery and then I disconneted the battery terminals to see what would hapen. As I thought the alternator couldn't keep up with a load put onto it. I stopped a few minutes into the test. I have verified all gound connections are clean and even replaced the ground cables becuase I noticed some corrosion on them. I also cleaned the battery terminals.

I read that the glow plug realy might be putting a amp draw on the system and I preformed that test by disconnectig the plug while the engine was at ide. Cheched my multimeter and there was no voltage change. I also verified that the voltage regulator and brushed attached to it were in good shape. The brushes appear to be new with no excessive wear.

Is there anything else that I am missing or not testing? Or did I get another faulty alternator. My 240D doesn't have any upgrade options on it.

I don't want to give up on the car. The car has been great but it has been cold up here in the north and the heater is needed badly. Any insight on this problem would be appreciated.

Thanks in advance.

Zeus · #2 01-07-2004, 01:22 PM

Hey Darinkald,

I just went through a bunch of alternator testing and repair on my 1989 300E. Similar symptoms. My car comes with a stock Bosch 70A alternator. I had dimming lights, slow wiper and weak wiper motor.

I would first verify that you have a good battery. Get it tested to make sure it is holding a good charge. Also check your serpentine belt (or poly belt, whatever your engine has) for tension and wear. Since you've checked your electrical connections already, I would suspect the alternator.

I found out that the stock 70A alternator is kind of underpowered for my car and Mercedes upgraded to an 80A alternator in 1990. The 70A just doesn't crank out enough current at low RPMs. So I replaced my ailing stock 70A with a new Bosch 80A alternator. Car runs perfectly now. You may have a similar problem with your car. Did you buy a Bosch alternator or a different brand?

Your voltage readings seem kind of low. The battery should have a resting voltage closer to 12.5-12.8 if it is in good shape. The charging voltage should stay above this even with a strong load being applied (mine doesn't dip below 13.5V even with a full load).

Good luck!

Darinkald · #3 01-07-2004, 03:54 PM

Thanks for your input.

I just got another 65 amp alternator from my friend at the parts store. I will see if it was the alternator. Hopefully this will solve the problem. I will post the results later tonight.

Does anyone know if there is a larger amp alternator for a 82' 240D? The parts store only has one up to 65 amps.