Found source of battery (idiot light) issue finally - unreal
 

I posted awhile back that within a week on both my 300SD and 300TD the battery light would come on at startup and with some RPM would go away. The other thing that caused it to go away was releasing the emegancy brake.

I can't beleive the cause, but after alot of hours spent on it, here it is.

If one of the idiot light blubs is dead , it causes strange things to happen.

In the case of the 300TD, it was the high beam bulb (which I didn't know about as it is my wifes wagon) and in the case of the 300SD it is the brake wear light.

The stranger part is that if I swap the high beam with the brake wear on the SD everything is fine, which is the light that is causing the issue (I am assuming as I have yet to play with the TD) in the TD.

Wow, I didn't realize those instrument clusters were made by Lucas. ;)

BIGRED
Thanks for the tip. My '82 SD was doing the same thing. I would give it some fuel after starting and the alternator light would go out. I had recently replaced the alternator and thought it was something inherent in the new alternator. My wife was using the car yesterday and I noticed one brake light was out. I changed the brake light bulb and the alternator light no longer glows after start up.

Tom
'82 SD
'86 SDL

Often, on certain other unmentionable brands of cars,
 

such odd behavior of warning lights and taillights, etc. turns out to have roots in bad grounds to the chassis from the light units or dashboard.

Re: Often, on certain other unmentionable brands of cars,
 

Bad ground is right.

Look at where the battery neg cable is attached to the vehicle. Most older 123's have that whole area of the thin sheetmetal it attaches to rusted.

I'm thinking I'll run a ground strap directly to the engine and MORE than one to the instrument panel (already have an extra one to the back of the speedo housing - cured a weird electrical issue with the panel).

Interesting thread but I am not sure....are you guys saying that a bad ground and a bulb out are two seperate things? How would BIGRED changing a bulb fix a ground problem?

I have heard using non OEM bulbs in the tail lights can toast your cruise control amp and cause other weard problems. Anyone know about that?

The act of plugging and unplugging... tugging and moving wires etc. ends up crackling off some of the corosion that was preventing a good ground.

Sometimes a bulb in a complex lighting unit
 

can send its current to ground through another bulb (at the loss of some brightness), but when the other bulb burns out, they both quit lighting and sometimes a bulb can ground through a filament which is not in use, but can't when that other filament turns on -- ask any old codger like me who's fought with ground problems in '65 Chevy taillights in his youth -- remember the ones on which, when you turn on the turn signals one way, the _taillights_ on the opposite side of the trunk lid would start blinking on and off?