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Old 08-29-1999, 01:34 AM
Robert W. Roe's Avatar
Robert W. Roe Robert W. Roe is offline
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Join Date: May 1999
Location: Lehigh Valley PA
Posts: 1,321
Thanks to all who've helped me understand my car so much better. Now I am having the same exact problem with my 1984 SD. Wondering if it's a jinx with me and S-class electricals; anyway...

I've had the car four weeks. Two weeks ago, while vacuuming the car, I found the trunk lid light bulb and reinserted it into its socket. I'm happy because now everything *works" (except for my power antenna only going up a foot or so; my car is in pretty good shape for an '84.)

About one week ago Friday, my battery voltage died, below 10 volts. Recharge the battery, dies Monday. Replace the battery Tuesday (with a red top Optima 800U), now on Saturday it too is dead (below 10 volts.) In effect, I replaced a good battery; I charged my old one so now I have a spare/jumper battery. Anyway, what I'm trying to figure out is where to begin looking for an obvious problem before I turn it over to a mechanic.

Since replacing the battery, I've had a little DC voltmeter on the console, plugged into the lighter socket. I've had over 12 V all week while driving, sometimes going as high as 13. Revving the engine made it jump .3 to .5 volts. Even with the glow plugs preheating, I still had 11 volts. All week long I'm over 12 volts.
This morning, dead battery. I notice the amount of time for the fully charged battery to die was about 5-7 days. The first charge was only a two hour quick charge, and it died in 3 days.

Previous replies suggest that I check the following:

1. preglow relay (where is this thing?)
can it stick "on" and will it get hot if it's stuck?

2. Benzmac suggested to check to see if any of the door entry lights are contacting the inside of the door frame. Do I need to pull the door panels (ack!) or do they pry out from below? Can I spot problems from below?

3. Perhaps the trunk lid bulb is staying on. I haven't looked at this yet. Is there a mercury switch inside the trunk lid? Thinking maybe it moved a bit and is staying on or got pushed enough to short something out.

4. Possibility of a short (worn insulation?) on a wire near the left trunk hinge.

If I can't find an obvious problem I can foresee checking the ignition-off current draw from a (charged) battery, then if it's too high (over what, maybe .25 to .5 amperes at the *most*?) I'll debate whether to quit at this point and turn it over to a pro or pull every fuse and check each branch. (My last SD would blow the fuse for the dome light circuit and I never did get to troubleshoot that.)

I'm hoping it's a gradual depletion rather than an intermittent (yikes). Is the 126 notorious for electrical problems like these? Any thoughts? Thanks to any and all.

Robert W. Roe
1984 300SD 165,333 miles

[This message has been edited by Robert W. Roe (edited 08-29-1999).]