Alternator problem or not?
 

I have a 1986 300E; I have had to pump the gas to get it started. Then I noticed a few electrical issues; radio noise, dome light didn't work when I opened the drivers door, (it would work when I opened the passenger door), clock quit working. I checked the fuses and noticed a blown fuse for the clock etc. and I replaced it. Also went to the dealer and had them 'reprogram' the radio to get it out of "I'm being stolen" mode. Everything was working fine, except the dome light issue, then I went to start it last Monday and got nothing. The battery is 8 months old. The car will start if jumped. I checked the battery with a volt meter and only got 9.84V. I loaned my battery charger out about 8 months ago and still can't get it back. Anyway I could get one and charge my battery and then check my alternator. What should I do at this point? I have had to drive my Mini Van all week! Any help will be appreciated.
Thanks,
Bonnie :confused:

Recharge the battery and load test it....if access to the cells test them as well

After the battery tests ok

Then Test the charging system at the battery...test BOTH Voltage and AMP output

I Would also then test the vehicle for a draw on the system...

As It sounds like the car does have a draw in the system that causes the battery to go dead

Thanks, what should the AMPs be?

I charged the battery over night and checked it this morning; it had a reading of 13.78V. I put the things back on the battery posts but did not tighten them; I will check the battery when I get home. If there is a drop in voltage I will assume that there is a draw on the battery somewhere. Is there anything else that I can do after that?
Thanks for your help,
Bonnie

With a fully charged battery and runing the engine at fast idle the voltage should increase to near 14.2 volts. If not there is probly a alternator issue..

Thanks, I will try that when I get home from work.

Did you notice a spark when re-attaching the second battery cable?

Yes Larry, I did notice a spark. I think that I have located at least part of the problem. There are 2 black buttons, (switches?) on the drivers side front where the door closes. When I pushed on the bottom switch I noticed noise on the radio, as if it was trying to turn on, (the radio is now in ----- mode). Is this switch on the frame bad? If so how do I change it?
Thanks a bunch,
Bonnie

After a power shut down you will have to put the Radio Code in to get the radio to work again.. Hope you have it .. if not any Benz dealer can get it with the vin number..
What is the system voltage when the engine is running at fast idle?
That door switch may be just causing static in the audio system when you actuate it..Probly nothing wrong with the door switches..

If the door switch isn't the source of your problem, I would suspect the alternator brushes are worn down and not making good contact. The fact that your battery isn't charging properly points in that direction.

When the brushes failed in my '92 300e, the first symptom was radio static, then the windshild wipers stopped, then then the car died on the road and I had to have it towed home.

The good news is that the brushes can be easily replaced without removing the alternator. This is one of the few perks of owning a Mercedes. They come as an assembly that includes the voltage regulator for about $25 if I remember correctly. Working from under the car you can access the two phillips head attachment screws. Be sure to spray some penetrating oil on the bolts because they might be a bit corroded.

It takes about 5 minutes to remove the brushes from the alternator, I would check them first. I had a similar experience. If you have the original Becker radio then the only way to reset it is with a reset tool, there are no codes as in later years. The dealer or a Becker service facility have this special tool.

Thanks everyone for your responses. The voltage at fast idle was 14.02V. Would a problem with the altenator cause the battery to lose charge when it is just parked? I must admit that I am not a fan of being under a car; would I have to raise the car up to get to the alternator? What do you think a nice MB mechanic would charge to change the brushes? I have been to the dealer to have the radio reset and he used a plug in key thing. The weather is getting nice and I want to get out on the road with my baby 300E. I am afraid to drive it and cause damage if I have an electrical issue.
Thanks,
Bonnie

The alternator brushes can be accessed by only opening the hood, no need to go under the car or raise it. Any compentant mechanic can remove/replace the brushes in 15 minutes or less. In summary, a very simple, quick, easy, repair

Alternator?
 

You might want to check a switch on the dash. I have an '87 300E that gave me the same kind of problems. Turns out that the switch for the rear light, not the dome, was on. You don't notice it unless you drive at night. My wife had dusted the instrument panel and accidentally turned it on. It is enough draw to run the battery down over night.
Anziani

'87 300E 201K

'95 E420 110K

The original radio in an '86 does not have a code, it is plugged into a ground wire in the back and will go into lock mode if the power is cut to the radio while the alarm is on. Losing power should not cause this problem, but a bad ground might.

It sounds like your regulator & alternator are fine if it's showing 14vdc at the battery while running, turn on the lights, rear defog, and defrost blower to high and see if it still maintains 13.5vdc or better at ~1200rpm. If lower, the alternator is still suspect, if it maintains this then you have three possibilities:

1 - The brushes are worn (alternator/regulator), and making contact some of the time. I had this failure on my '91 and it was an easy change in a Fire Department garage along the freeway, on Christmas Eve, in a blizzard, with the family in the car, ... Michigan State Police car pushing my 2-year-old car into the bay, ... I pulled the regulator with an offset phillips screwdriver from above, stretched the wires a little to allow the brushes to extend further, put it back in and drove it until the next business day. Be sure you put the hood in the straight-up position!

2 - Bad battery / shorted cell. Batteries can self-discharge or might not even charge properly, it can be tested after a full charge ("load tested"). An Autozone or equivalent will probably do it for free hoping they'll sell you a new battery.

3 - Battery drain. If all else checks out, the light left on or equivalent can drain the battery overnight. A good battery will run the dome light overnight and still start the car, but not a weak one, ... and a proper system will still give a small spark when attaching the battery lead (charging capacitors). If you have access to an ammeter, connect it between the battery lead and post, if there is a constant current greater than ~100mA then start removing the fuses one-by-one to find which circuit is drawing the current.