Help! Battery light on 240d 1976 115
 

After this last little cold spell (10's & 20's for 10 days or so) and the car only being driven 2-3 days a week instead of 5-6 the battery light came on yesterday morning but it was very dim and went out as soon as the rpm's came up as I started driving.
Last night the car cranked a little harder than normal and the light came back on but dimly, I drove home (20 miles) with headlights, radio and heater blowing and no battery light on but when I pulled in the driveway and put it in park the light came back and when I cut the headlights off the battery light glowed very brightly (that is the only reason I know it was dim before). If I turned the headlights back on the battery light would glow dim and brighten up again when the headlights were turned off.
The car started fine this morning but the light is still there (dim).
The battery is only 9 mths old and the correct size for the car.
I will of course check everything tonight or tomorrow but I am hoping that the battery light getting brighter when I turn the headlights off will mean something to someone with more Mb experience.
Thanks for the help
If this looks familiar, I had posted it in Tech help and didn't get an answer so I thought I might have better luck here.

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You probably have one or more diodes out on the alternator (this is assuming the belt is tight!). This reduces output to as low at 4 amps, but the voltage will be fine. You could also have a failing voltage regulator -- its inside the alternator on this model.

Unfortunately, the lamp works by balancing the voltage from the battery with voltage from the alternator output and doesn't measure amperage. You get sufficient voltage, most of the time, to keep the lamp off or dim, but you are not charging the battery.

Get the charging system checked.

Peter

>>>>Last night the car cranked a little harder than normal and the light came back on but dimly, I drove home (20 miles) with headlights, radio and heater blowing and no battery light on but when I pulled in the driveway and put it in park the light came back and when I cut the headlights off the battery light glowed very brightly......

Stop right there - now exit the car while leaving it running!!

Step inside and enjoy a cup of coffee.... then check on the car periodically to see if the warning light eventually dims and then goes out.

After you have tried this experiment...... even idling your diesel for an hour or two or three (as might be necessary for crummy, stone-dead battery) return to this thread and tell us the results.

Another idea is to fiddle with the idle speed adjustment to see if diesel rpm has any effect on whether bright warning light goes dim...... If it dims at higher idle, then you might just have a loose or slippery fanbelt.

By all means, check the dash lights and guages at 20-30 minute intervals during your evening diesel watch...... and dont worry about an idling diesel depleting your battery with no electric components operating - diesels do not rely on electricity for operation.

"Let it run"
 

Thanks for the replies.
Before I read 240dieseldog's reply I had put a battery charger on the car at two amps and did not get back to it for 24 hrs. The car cranked easily but the light was still on very,very dimly Sat. night when I tried it.
Sunday I drove the car and then left it running while I did odd jobs, I checked it every 45 mins. or so and everything was fine, temp 175, oil pressure pegged and the light still on. After 4 hours the car was smoking a little more than usual but everything else is the same and the light might be slightly brighter, I ran it up the road to blow it out and the light goes out as soon as I touch the pedal and comes back when idling and gets brighter when the headlights are turned off.
I am reluctant to change the idle since the light has not been coming on until recently.

help 240dieseldog
 

240dieseldog, do these symptons mean anything special to you?
:confused:

Back up to what psfred said a few posts earlier.... the alternator/regulator combo....

If you actually remove the alternator to check the contact bushings, etc - a pencil eraser makes an excellent tool for cleaning carbon build up where bushings make contact.

Hell, I'd replace the regulator as a matter of course - if not replace the entire alternator with a bosch rebuild.

Under the nefarious category of what else could be wrong - how solid is the grounding cable on the battery? Bright warning light with headlights off could mean that alternator/battery charging circuit is not completely tight.... It sounds like alternator sends plenty of power for headlights, but the volts are not completing their circuit with headlights off - or the regulator is sticking.

Thanks
 

I will take the ground off and clean and retighten it.
I can't remember if this alternator was put on when I revived the car, but I don't think so, so I will replace the alternator and belt.
I am going to replace the instrument cluster with one out of my parts car is there anything in particular I should watch out for?

Replacing instrument cluster.....

Place a towel over the wood trim to prevent it from getting scratched.... squirt some grease down the speedo cable while its unattached..... gently clean the contacts on instrument cluster light dimming switch with small brass brush or steel wool