500 SEC possible alternator/battery trouble - PeachParts Mercedes-Benz Forum

Marshy · #1 03-13-2007, 08:51 PM

Hello All!

I hope someone has been there, done that, regarding a problem I have with my 1985 500 SEC.

The car has been working perfectly. Today, I noticed that when at idle, there was a slight ignition miss. Irritated me, but so what. Shut the car off to do some shopping and the miss was gone. Great.

Sitting at an intersection red light, I suddenly realized that the radio station I was listening to was not there.

Looking down, I find the radio is "off". Tried to turn it on, but "nothing".

Then I notice that "miss" again.

Accelerating away from the light, it is only a couple of blocks till the engine started to lose power. It took near full throttle to get up to 50 kph.

I got off the main road and drove for a couple of miles trying to see if the engine would clean itself out or whatever. A couple of green lights help me find a place to pull over and stop.

The car idled, but at around 400 rpm instead of the usual 700. I shut it off. Tried to restart. Nothing. Exactly as if the battery was totally drained. I waited a few minutes and tried again. The starter bumped probably half a turn. No go. Better call for a tow.

It took me probably 15 minutes to walk to a pay phone to make the call. When I got back to the car, it started up perfectly and idled at 700 rpm with no miss. I waited for the tow truck.

When the driver arrived I explained what had happened. He said his guess would be that the alternator was not charging the battery.

"Was the idiot light on, on the instrument panel?" No.

OK, turn the lights on and see what happens.

I did and the engine stalled and would not start again.

We towed the car to my house. A 20 minute drive. Tried to start the car but things were mostly dead. Just enough voltage to make all the instrument panel lights blink and activate the alarm system which was very annoying.

With a battery boost it started fine and idled perfectly and I drove it into my garage.

I used an ohmeter to check the battery voltage. Just under 12V. Since then I have had a charger on the battery for 4 hours now. It has been drawing just over 4 amps the comlete time.

Does anyone have any ideas?

Thanks for your time.

jhodg5ck · #2 03-14-2007, 09:47 AM

why not check voltage @ the battery w/ the car running? You should have @ least high 13's when idling around 5-600RPM.

Jonathan

david s poole · #3 03-14-2007, 10:33 AM

perfect example of what the symptoms are when alternator stops charging and battery starts to run down.you may be able to get away with changing the regulator[2 screws at back of alternator] but usually when brushes worn out alternator not far behind.

Marshy · #4 03-14-2007, 10:53 AM

Thanks guys.

Jonathan: I connected the battery and started the car. Idling at around 900 rpm, I have just less than 12.5 volts across the battery terminals.

Sounds like the alternator I guess although this battery sure is taking a long time recharging. I haven't seen this before with other batteries. The battery is a MB unit replaced at M-B of Toronto in December 2001.

It doesn't look like too big of a job to get the alternator off. Getting replacement parts locally will be my problem.

Any tips on purchasing another. The closest MB dealer to me is a 3 hour drive. Would the alternator be rebuildable or is it simply an R and R job? Oh, and maybe I can get away with replacing the regulator.

Thanks again.

david s poole · #5 03-14-2007, 11:46 AM

that battery has been on borrowed time for 2yrs now.replace it.

Marshy · #6 03-20-2007, 06:58 PM

For the archives

Diagnosis:

Less than 13+ volts across battery terminals with engine runninig.

Disconnect the negative battery post with engine running and the engine quit.

Fix:

Took the alternator off and took it to an auto electric shop.

They tested the alternator, replaced the regulator and retested.

Total charge $39.47 Canadian.

The person that did the work told me that the alternator was original and in great shape.

I installed the alternator. Everything is working perfectly under all the load I can give it.

Thanks for the help people !