I have an 87 300SDL turbo and I have a very strange electrical problem. When I take the key out of the ignition the all of the warning indicator lights turb on. When the key is in the ignition (and while the car is running) the lights function properly. The only light that does not function properly is the glow plug indicator, however the car starts, and runs, just fine.

I need to disconnect the battery if the car is parked for more than a few hours or the lights drain the battery. When the battery terminal is connected to the battery, there is a lot of sparking and there are quite a few relays clicking on and off. The water pump remain on as well when the battery is connected.

I have replaced the alternator and the ignition switch already and the problem has not been rectified. Also, by pulling fuse number 15 (interior lights, central locking, etc etc) the panel lights turn off and everything seems fine.

I have noticed the glow plug relay does a lot of clicking, and was thinking that could be reponsible for this.

I am at my wits end with this problem, please let me know if anybody has any ideas. I would greatly appreciate any ideas.

Thanks.

A wiring diagram would be handy, and I have none at the moment. Since you have replaced my first two choices I'll have to make a guess. Get a wiring diagram and find the diode that separates the function of the seat adjustment. The seats adjust with the door open but the key off!. Shut the door and one must turn on the ignition to move the seats. A diode separates the two circuits and a shorted one will provide power to the ignition from the door open position.

This may require the door to be open but thats what the wiring diagram is for....just a thought.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Thanks for the idea.

There is one thing I neglected to mention in my original post; The driver's side entry light (the small light under the door) started to malfunction, and eventually quit working, roughly at the same time the dash light problem started. Now, when the driver door is open the interior light is not activated, but the passenger door will activate the interior light.

Also, the analog clock in the dash (under the tachometer) stopped functioning when this problem started. At the incept, this problem was intermittent and although the lights would remain on in the panel they would turn off after roughly 3-5 minutes. The problem progressed to the magnitude described in the original post in roughly 4 weeks.

I came across another member who had experienced the same problem, but his problem appeared to clear itself up, I am not so lucky.

Please keep the ideas coming, and I will be sure to try the seat diode problem.

Two questions, however; Where can I find a wiring diagram for this partcular car, and do you happen to know where the diode for the seat is located?

Thank you for posting.

There is a circuit 15 relay that could be stuck. I have no info at home so I cant tell you where it is. I would lift the fuse box cover and start pulling relays 1 at a time and see if everything goes off. If it does than swap it with am identicle relay from the fuse box. If the problem stays gone, then you have a bad relay. If not then you have a problem with the circuit energizing that relay.

Brian

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Mercedes Benz Master Tech
ASE Master Technician
Diagnostic Technician
Owner of "Behind the Star Inc."
Raleigh NC.
www.behindthestar.com
brian@behindthestar.com

Thanks for the idea Brian, I am not sure how the electrical system works in that car, but it is certainly confusing!

One of the first things I did when the problem persisted, was pull the relays one at a time to see if it had any effect on the problem. Unfortunately pulling the relays did not seem to help. I pulled the relays one at a time, and the lights stayed on. I pulled every relay, connected the battery and the lights were on.

When you have more information I would really appreciate any more information you could give me in regards to the problem.

Thank you for your time.

If the problem is in the power seat diode it will be easy to tell. The diode is in the fusebox, next to the combination relay (one of the bigger black ones). It is slightly bigger than the new blade type fuses and will be black in color. From the top its about 3/4 inch by 3/8 inch.

Pull it out of the box and see if the problem goes away.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Thanks for describing the location, Stevebfl.

I just pulled the fusebox cover and pulled everything out of the fuse box that was not a european fuse. The seats still worked, and the lights stayed on. I must be looking in the wrong place! In my fuse box there are 5 silver relays (they look like standard bosch 30amp relays with a fancy cover); there is a large box labeled "Hella" with three molex connecters plugged into it at the upper righthand corner of the fusebox; there is yet another large black box (1" X 4-5"); and lastly there is a small box that looks like a standard fuse holder (I was assuming that was the diode) however when I pulled it out the seats still worked, and the panel lights remained on. (the key was out of the car at the time)

Sorry if I am not looking at the right part.

There is also some kind of electrical device situated to the left of the battery (right if you are looking at it from the front of the car) that is placed between the firewall and the silver heat blanket that appears to remain running with the key out of the car, it sounds as though some sort of electrical motor is running. I dont know what this device is, what it is supposed to do, and if should be running with the key out of the car.

Again, I am beyond perplexed with this problem. All of you help is greatly appreciated and I look forward to your responses. Thanks, everyone.

I think you might have gotten the diode. It will have two round connector terminals the size of the terminals on the relays.

The seats will work with the diode out, but not with the door closed. If the seat relay is stuck they would work without these circuits. If you have pulled the diode then this isn't the road to follow.

Fuse 15 is for the door locks system and the alarm system.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

How about the OVP relay? The engine turns off by vacuum but the relay is stuck.

EST, I am not sure what the OVP relay is...can you tell me where it is?

Stevebfl, I am sure I got the diode then. It looks like a standard fuse holder and is situated between the larger relay boxes and the standard fuses. Is that the one?
With the diode in the seat works with the door open and does not work with the door closed. The will work with the door closed and the key in the ignition. Is that the proper way for the seat to function?

The only real lead I have been able to find is by pulling fuse 15 the panel lights turn off, as well as all of the things that are running in the engine compartment with the key out of the ignition.

One technician told me that the glow plug relay might be bad allowing power to go throught the relay and fooling the car into thinking that the glow plugs are being energized and thus powering up the panel. I pulled the plugs from the glow plug relay (both plugs) yet the panel lights remained on. Does that eliminate the glow plug relay? There is a red wire connected to the relay (I assume it is a hot wire) that I did not unscrew. Could this potentially be the problem?

Thanks again, everyone, for you continued help.

Since fuse 15 shuts off the symptoms that is where you should go next. Try disconnecting the Alarm control unit. It is under the dash or carpeting on the passenger side. Also try disconnecting the pump for the door locks. See if either of these shut off the lights as the fuse does.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

stevebfl, where can I find the pump for the door locks?

Thanks for all of your help!

I think its inside the spare tire. It is on the early 126 cars.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Stevebfl, I looked inside the spare tire well and nothing stood out as a lock pump. I also looked for an alarm module in the passenger carpet area and I did not see one either. Chances are I just dont know what I am looking for, or I did not dig deep enough. I will keep looking until i find something.

If there is any more information that you can share with me, please let me know. Thanks, again.

Black rubber triangle (10" diag.) with black metal bracket in passenger side rear of spare wheel well. At least that's where it is on 300SD and where the EPC shows it for the '87 300SDL.

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)