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Car Forum / Mercedes-Benz Cars / August 200586 300E - battery light stays on
This car was tested recently and the results were: Battery is healthy No drainage from battery Alternator charging ok .... but the light stays on while driving. Any suggestions? Thanks for help JJ > This car was tested recently and the results were: > [quoted text clipped - 9 lines] > > JJ Forgot to mention that I disconnected the battery for an hour, then reconnected, but the light still stayed on. jj. i bet it is the alt regardless of there testing. its feeding back to ground the case, minus a few cans! > i bet it is the alt regardless of there testing. > its feeding back to ground > the case, minus a few cans! I agree with the pool dude. It's either your alternator or your battery. > > i bet it is the alt regardless of there testing. > > its feeding back to ground > > the case, minus a few cans! > > I agree with the pool dude. It's either your alternator or your battery. My thanks to PD and MJ, and RF for help with the battery.. That means a new or reconditioned alternator? Is there a good argument for replacing the battery first or is there any other alt test that would confirm the problem? thx for the help jj >>> i bet it is the alt regardless of there testing. >>> its feeding back to ground [quoted text clipped - 7 lines] > replacing the battery first or is there any other alt test that would > confirm the problem? Personally I would try a different battery first just to see what happens. Of course I have a couple of extra batteries sitting here, so that would be easy to test... It really is tough to say without having the car... Reinspect everything for any visual evidence, like a cable that looks poorly connected or dirty etc. Also re-examine the fuse box. Try pulling and reseating all the fuses, or even better replace them all... Good Luck, and let us know what it really turns out to be... We aren't psychic really (at least I'm not). Marty A good, fully charged battery should have at least 12v, usually around 12.5v. A good alternator should give about 14.5v at idle. So just take a multimeter to measure on the battery posts. Most modern cars have an integrated design and a bad battery will cause wrong reading from alternator output. So start from the battery. Don't forget to check wiring, especially at the connections. If in doubt, loose the connection, sand them clean before putting back. I once worked on a Chevrolet that died completely while on the road. It had no cranking, no lights, not even emergency flasher! A few minutes later, everything came back normal. It took me a few weeks, and a few complete deaths, until I found the culprit. On that car, the positive cable goes to the starter and everything else connects there. Even though the nut was very tight and looked clean, there was enough corrosion to kill the car. > This car was tested recently and the results were: > [quoted text clipped - 9 lines] > > JJ a bad diode the case, minus a few cans! Thanks everyone for the helpful answers I haven't been able to do anything with it yet but will have a try within a few days. Back soon. jj > a bad diode > > the case, minus a few cans! Is the diode the light itself that comes on or is it somewhere else in a circuit? In the first case, I assume that the panel with the lights pulls out and then I can get at the diode - soldered in or plugged in or? Hope these questions aren't too "ignorant" or is it "iggerent" ;-) jj > a bad diode > > the case, minus a few cans! there are 6 diodes in an alienator 3+ AND 3 - when working properly the current goes one way through them. if one opens it may feed to ground there on the alt itself. the diodes convert AC voltage to DC. beats the old generators. there are diode pacts but i have never my self replaced one some one here has though i am sure the case, minus a few cans! Thanks poolman for the diode info. I doubt I'll need to get that far into the electronics but I may need to. Today I had my first real look. 1 Rotated the fuses and used two spares to replace two that looked doubtful. In the next few days I'll buy a complete set, just in case. 2 Measured the battery voltage with engine off - 12.75v 3 Measured the battery voltage with engine idling - 14.1v 4 The battery terminals look clean and have no corrosion 5 Removed the alternator cable. It has a 3-slot plug that fits on 3 prongs on the unit and is held there with a spring clip - no corrosion. Tomorrow I'll probably be able to make a battery substitution, just for the test, or possibly a few days, if the light does go out. Thanks all for the help :-) PS the light is still on :-( jj > there are 6 diodes in an alienator > 3+ AND 3 - when working properly the current goes one way through them. [quoted text clipped - 8 lines] > > the case, minus a few cans! > Thanks poolman for the diode info. I doubt I'll need to get that far into > the electronics but I may need to. [quoted text clipped - 6 lines] > 2 Measured the battery voltage with engine off - 12.75v > 3 Measured the battery voltage with engine idling - 14.1v Hmmmm, That all sounds kosher. > 4 The battery terminals look clean and have no corrosion > 5 Removed the alternator cable. It has a 3-slot plug that fits on 3 [quoted text clipped - 4 lines] > test, or possibly > a few days, if the light does go out. That would be nice... > Thanks all for the help :-) > > PS the light is still on :-( Keep us up to date please, I love a good mystery. Marty > > Thanks poolman for the diode info. I doubt I'll need to get that far into > > the electronics but I may need to. [quoted text clipped - 25 lines] > > Marty Your help and interest are appreciated Marty ;-) I switched over a newer battery from a car that has no problems but the light still stayed on. Tomorrow I'll get new fuses. Next step? jj > > Thanks poolman for the diode info. I doubt I'll need to get that far into > > the electronics but I may need to. [quoted text clipped - 25 lines] > > Marty This is embarrassing. Originally my friend told me about this battery light and I did the inquiring for her. Finally, someone else was riding with her in the car when she mentioned the problem. He had a close look at the light and the symbol near it. It did look like a square and somewhat like a battery, but had some other squiggle on it. Finally he decided it wasn't the battery light. When he checked, it was the cooling water light. Dumped in some water and the light was gone. Oh dear, what idiots! Sorry about the wild goose chase. RF >>>Thanks poolman for the diode info. I doubt I'll need to get that far > [quoted text clipped - 45 lines] > > RF It's a good thing that the engine didn't overheat in the meanwhile. It always helps to READ the Operator's Manual. <snip> >> This is embarrassing. Originally my friend told me about this battery light >> and I did the inquiring for her. Finally, someone else was riding with her [quoted text clipped - 10 lines] > It's a good thing that the engine didn't overheat in the meanwhile. It > always helps to READ the Operator's Manual. That light actually only indicates the overflow is low, so it really is no threat to the engine temperature (at that point). Still it's nice to know what the warning light means :~) Glad to hear the mystery is solved... Marty |
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