Mercedes-Benz 420SEL

1987 Mercedes-Benz 420SEL Electrical / Lighting Systems  Happens always Always

The vehicle currently has the following problems:

1. The front overhead light would stay on even when the key is out and all the doors are closed; it could be turned off only with the overhead control switch.
2. The master key would not engage the central locking/unlocking system.
3. The door lights, rear overhead and reading lights, as well as the trunk light, would not go on.
4. When the engine is running, the overhead seat belt warning light keeps blinking.

These are all resulted from replacement of the #15 fuse (8A). Last week the power antenna started not extending its mast, and the front overhead light would blink every time I turned on the radio. This afternoon, I check the fuse box and noticed the #15 fuse was burned out. So I replaced the 8A fuse and operated the radio a few times to observe the power antenna to extend and retrieve its mast. After a few times the power antenna start the same problem and the #15 fuse would go out again. So I unplugged the power antenna unit connector and replaced the #15 fuse. But the #15 fuse would burn out the moment it was in the slot. I then noticed that the front overhead light would not go off on its own when all the doors are closed, unless I turn the switch off. I then disconnected the (-) battery terminal for more than 20 minutes. When I connected the battery terminal back again, with the #15 fuse out and the power antenna connector disconnected, the vehicle seems to run just fine, except the 4 noticeable problems described at the beginning.

Please advise.

If the vehicle is so equipeted, does' a door ajar light come on? I have seen in my past where a door seems to be shut, but the latch electrical may have been gummed up and caused said problem.
Good Luck,
Paul

No door ajar lights come up.

The #15 fuse keeps burning out, while the power antenna is disconnected.

It's been a while since I did wiring on Mercedes so if memory fails you know the drill.

Somethings gone short or to ground in the #15 fuse circuit. Get the wiring diag out and find everything that feeds off of that fuse. Would be nice if you could pick out all the splice connection locations and disconnect each one to isolate the the fault.

Personally, I would pull out all the door.trunk/hood switches first, keep the wires from grounding, replace #15 and test from there. If any wiring or rewiring was done - like aftermarket stuff- that would
be the second thing to disconnect. The third thing is ground. You may have lost the ground on a subcircuit and this is backfeeding - or not. The fouth place to take a hard look is all the wire bundles from
the hole in the firewall to every tie down attachment point forabrasion.

You might also try using a breaker instead of a fuse. Loosen up the fuse block and on the back, pick out any wires (there may be two or more) that #15 fuse feeds. With the breaker in place, touch all feed wires to find the hot one. If nothing else this will help to isolate a subcircuit and a wiring diag should tell where the wire goes.

Not to insult anyone, but your customer has a glorified Studebaker that was built by machinists. All the parts that could be machined are a work of art. All the parts that could not be machined are an
afterthought. The wiring was designed by a retarded machinist who got the job of electrical engineer and who has no concept of basic wiring safety. Mercedes pulls full unfused battery power into the car on a #10 wire and then feeds everything off that wire.

Sounds like this car is about due for a wiring harness fire, like so many before it. Don't let your customer drive around with this problem uncorrected. It may cost him the car.

This is a repairable problem, but it will cost more than 7.50$.

Pete

Thanks for the reply.

When the car is not running and the #15 fuse is out, the lower fuse contact terminal has about 12v to the (-) battery terminal and the upper fuse contact terminal has about 0.6 hom to the body. These were measured with a digital multi-meter. According to the fuse identification card, the #15 fuse is for power antenna, central locking system, door and rear roof and reading lights, seat possition memory.

Where is the central locking control system located?

Thanks,

1987 Mercedes-Benz 420SEL Ignition System   

It's happened a few times now...but occasionally when I turn the key in the ignition the engine turns but it doesn't start. I've taken it to the mechanic shop on more than one occasion but they could not diagnose the problem, Eventually the car starts like nothing ever happened

Usually this will be caused by a lack of spark. It will need to be tested for spark during the no start period to confirm if there's a spark or fuel problem.

Is the distributor cap and rotor in good condition?

Yes....to your question ...although I figured out that my wife was flooding the carburator with her big feet while turning the ignition. Anyways we took off the air cleaner cover and let some of the excess gas evaporate and after 15 minutes it started rightr away.

Thanks for your quick response though.....

Joe

If the problem appears again hold the accelerator to the floor.

Has she just recently started driving the car?