1984 MB 300D electrical ghost - PeachParts Mercedes-Benz Forum

whunter · #1 01-20-2008, 11:50 PM

1984 MB 300D Turbo.

The wipers sweep once and radio comes on every time ignition switch is turned from "off" to "glow plug" position.
This happens even though neither the wipers or radio were on when the car was turned off..

patbob · #2 01-21-2008, 07:28 PM

Quote:

Originally Posted by whunter

Help please.

Is this a new problem, or a new car with the problem?

If someone installed the radio with the acc pwr & always-on (clock/mem) wires reversed, it might do what you describe. Depends on the radio though.

As for wipers, I haven't been inside the MB units yet, but don't they have some internal contacts that turn their power off when they get to return position? That's how others I've worked on are -- you pulse the power enough to get the blades to move off their return stop, and then the internal switch keeps the motor running until they get back into return position. If the switch were broken or out of adjustment, it might think it isn't all the way in return position and go through a cycle to get them there as soon as you turn the ignition on.

Then there's always the old standby "it's a grounding problem"

pawoSD · #3 01-21-2008, 07:36 PM

I don't believe a W123 has that feature on the wipers....I turned off our 300D the other day and the wipers stopped dead mid-sweep on the window....so it must be that "grounding problem"

I am not sure if the W126 has that feature....I am anal and always turn my wipers....lights...etc off before I turn off the car.

Maybe I will check it sometime. I am thinking it does return them to the home position when the car is turned off....but I could easily be wrong.

So far my biggest W123 annoyance is that you need to have the ignition key in Position #2/glow sequence in order to run the power windows when the car is off. On the W126 the first position of the ignition turns on the radio/power outlet/wipers/windows etc....on our 300D it only turns on the radio. Why? Unless there's just something messed up on ours.

Palangi · #4 01-21-2008, 09:05 PM

My wipers make one sweep like that, but only when it's cold. I strongly suspect the ignition switch, but I have not changed it yet.

There was a thread on this 2 or 3 months ago, offering some other theories.

Edit: One reason I think it is the switch is because I occasionally see the wiper position "bump" as the key is being turned off.

tobybul · #5 01-21-2008, 10:19 PM

Quote:

Originally Posted by pawoSD

I don't believe a W123 has that feature on the wipers....I turned off our 300D the other day and the wipers stopped dead mid-sweep on the window....so it must be that "grounding problem"

I am not sure if the W126 has that feature....I am anal and always turn my wipers....lights...etc off before I turn off the car.

Maybe I will check it sometime. I am thinking it does return them to the home position when the car is turned off....but I could easily be wrong.

So far my biggest W123 annoyance is that you need to have the ignition key in Position #2/glow sequence in order to run the power windows when the car is off. On the W126 the first position of the ignition turns on the radio/power outlet/wipers/windows etc....on our 300D it only turns on the radio. Why? Unless there's just something messed up on ours.

P, thats how the D's are...

barry123400 · #6 01-21-2008, 10:36 PM

If this is the original radio. I believe it has an electrical/electronic latching system that turns it on/off. Since I assume the memory for the preset station is retained It would almost have to be some reaction between the main power feed and the memory retention feed. That is they are interacting somehow because of a flawed ground perhaps.
Anyways the first thing I would try is running a temporary external ground to the radios metal case. In fact I might even run that temporary ground right back to the negative terminal of the batery. For the time it takes it might prove worthwhile.
I was just wondering if it could be done without pulling the radio. Although I cannot believe you are asking on this site if it where that simple.
Hopefully when the cause for one deficiency is found the other will stop as well. Pick your poison. The only other possibility might be the main power wire is going to a different potential for just a milli second as you turn the keyswitch. The potential difference between the always on memory retention feed and the main feed might just trigger that latching system in the radio on. Under normal circumstances there is no difference in potential between the two feeds or too little to matter.
One thing for certain is there are no little elves from the black forest stomping around inside that dash pushing your buttons. Hoping someone else on sight can give better ideals. You have helped so many people over the years I hope they can help.. If there is any difference like an aftermarket radio or whatever from normal it should be mentioned.

whunter · #7 01-22-2008, 12:27 AM

This problem is one year old.
When first noted, it was only on cold days.

It is a Factory radio.

Hmmm.
A bad ground would explain multi unrelated circuit involvement.

barry123400 · #8 01-22-2008, 02:14 PM

Bump, This will not be the first example of this model to develop the problem. Think and contribute guys.

patbob · #9 01-22-2008, 04:55 PM

Quote:

Originally Posted by whunter

This problem is one year old.
When first noted, it was only on cold days.

It is a Factory radio.

Hmmm.
A bad ground would explain multi unrelated circuit involvement.

Old relays sometimes get wierd when cold.. until they get wierd when it isn't so cold. I don't have my electrical diagram with me, so I don't know if there's any relays in the wiper circuit that might cause it to behave this way.

Oxidizing connections are also sometimes affected by cold.. until they become affected when it isn't so cold. With these 20+ year old cars, that could be the problem with your radio. Have you tried pulling it and disconnecting/reconnecting it? That usually scrapes off enough oxidation to cause the problem to go away or change behavior for a little while.

Palangi · #10 02-21-2008, 09:33 PM

Quote:

Originally Posted by whunter

1984 MB 300D Turbo.

The wipers sweep once and radio comes on every time ignition switch is turned from "off" to "glow plug" position.
This happens even though neither the wipers or radio were on when the car was turned off..

Roy

Did you find the cause of this problem ??

whunter · #11 02-22-2008, 11:42 AM

Quote:

Originally Posted by Palangi

Roy

Did you find the cause of this problem ??

No answer yet.