88 420SEL Alternator and idiot light Question

[mercmerc]

While driving home this evening from work my Mercedes 420 SEL done something really strange, I stopped at a stop sign and it almost died, about 1/4 mile up the road it did!!!, I tried to start it and it was dead, no lights no turn signals radio quit, CD changer stopped, I opened the hood, the neg. battery terminal was loose, I tightened it up, tried to start her again and no go, I had a volt meter in the car, battery voltage was 8.5 volts, not near enough to start it. Called my dad to come and give me a jump, the car started right up, I drove it another 1/2 mile home, pulled in the drive way and let it idle, to charge the battery, it ran about 2 minutes and died again.
I pulled a battery out of another car and put it in, she started up again and ran fine. I went to a job site about 12 miles away, drove the car there, it ran fine all the way. I was there about 3 hours, got back in and started it back up and drove 12 miles back home. As I was driving home I saw a very very dim light from the battery idiot light, I thought it was a reflection from the dash lights as it was dark when I came home, I turned the head lights off and could still see a dim light, could I have a bad alternator or a loose alternator belt or other issue, any help would be appreciated, I drove the 12 miles home with head lights on, high beams on and the dim light never changed, and the headlights stayed bright all the way home.
Tnx, Tim :?

 

[jaques2k]

My guess is certainly the alternator/belt. You should start the engine and then put the volt meter on the battery. It should rise up to about 14 volts or something in that area when it´s charging good. If it does go up to that level, the alternator is probably fine, or at least it will work for a while... =) If it doesn´t, try to rev the engine somewhat to see if it helps. It could also be some bad connections to the alternator, battery or any other "main" electrical connectors (e.g. not lights or so). The alternator could also have bad brushes/coals. Good luck!

PS! Nice car by the way!! =)

 

[Buick8]

88 420 SEL Alternator

Hi, it's a beauty (same color as my '86!)
My alternator belt was slipping badly; worn out. I replaced it and all is well. Sometimes older pulleys are worn to the point even a new belt won't work properly.
There is an overvoltage protection relay you have to keep an eye on, it is behind the outer firewall, and it has a fuse in the top.
Sometimes when a battery fails to hold a charge, it is being drained. If the circuit which disconnects the altrnator field with the engine off fails, the alternator field will remain energized and drain the battery. you will find this with the engine cool; the alternator case will be warm-hot 9at least until the battery drains). if you have ever repaired your climate control, let me know, as I am about to tackle mine.
Regards, Randy