kenhartz

I have an 87 300D and the central locking system does not work. I have been trouble shooting this, and I think my problem is electrical. To date I have done the following:

-All vacuum circuts hold vacuum
- X30, all connections look perfect, no corrosion, etc.
- the vacuum pump is good, I have orthopedic seats, and they work fine
- The connection at the drivers door actuator looks good, and two pins have voltage, albeit, very low (less than 1 volt). I thought the wiring between the body and door might be bad. I inspected the wires, and no signs of external breakage, but I have read that the wires that were used had strands that wern't fine enough, and were sustepable to breaking. I checked the wires that go to the vacuum acctuator inside the cabin, and they test the same as they do inside the door at the acctuator. The Blue wire and red wires both check at .62 volts, and spec is anything greater than 10 volts. I am really perplexed.

None of the locks operate when using the key in either the drivers door, passenger door, or trunk. I have to lock and unlock them all manually. I thought maybe there was a relay?? But I cant find any info on one.

Any help is appreciated.

Also, my alarm will go off after I unlock the drivers door, and open it. If I shut the door, lock, and unlock it, it normally cures it with one try. Sometimes, I have to do it a few times. I only mention in case the problems may be related.

Thanks
Ken

PaulBennett

I am slightly familiar having installed a wireless fob remote some years back on my 1993 190e with the ECU under the back seat. Since I happened to have my MB repair CD right here as I'm changing the radio-looking for speaker info. So I checked for quick help on your problem.

Don't despair, 0.62v is often good. The troubleshooting section on the central locking system is 74 pages long. Most of them are the troubleshooting tree of logic tests after measuring >10v and <1v here and there and the likely causes depending on the answers.

The vacuum system and electrical switches require simple but extensive and thorough testing to pin point the problem and you have a good start but without getting meaning from your measurements. So, can't say exactly what the problem is but I can help you with MB's treatise and have PMed you with 74 pages of hope.

kenhartz

Hey Paul;

Thanks for the reply and encouragement! I have a suspiscion that the .62v is from a bad/missing connection eother at the trunk, or passenger door, and the .62 volts is just trace voltage. This is my guess, as I know the vacuum portion of the system is good. When I depress the rear door lock, the drivers door wants to go down, as the vacuum is acting on it, but the vacuum pump does dot actuate. Anyway, I will keep on looking, and post progress, and any further thoughts are welcomed.

Thanks
Ken

PaulBennett

I wanted to reinforce that seeing .62v is a normal condition, not especially a bad or missing connection and generally indicates a good ECU. While there is undoubtedly a bad component in the mix, don't read that .62v as a bad sign...it's simply a voltage condition which is important for the circuit.

Semiconductor computer electronics produces a hi (not high) condition and a lo (not low) condition to the circuit. A silicon transistor typically produces an output or input to either to the battery or to ground, called the rails. Whereas a mechanical switch with results in NO voltage loss, a transistor driver looses about .7v across it's elements, from emitter to collector. So, it's common to see 12v ( a hi condition-not high but hi) which is .62v below battery 12.6v. And a mechanical switch to ground would give 0v but a transistor gives .62v. A lo...not a low but a lo. Hi and lo are measureable computer conditions which are .7v from the battery and ground supply rails.

Please tell me if you have sucessfully downloaded the support manual so I can remove the file. It isn't large, under 5mb. GL

kenhartz

Hey Paul;

Very helpful stuff. I have looked at everything under the rear seat, and all connections look good. What other electrical componets can I look at related to this circut? I was able to view a W124 manual, and I can see the diagram for the central lock system, and I also checked the connection again at the x10 junction block, and the connections are like new. The only thing I can think of now are the actuators them selves. Acording to the manual Hella (they give a few different madel numbers) were problematic. Since I don;t know of any electrical control device in the circut, maybe thats it?? But I wonder if one bad one would create the condition I'm experencing with the .62v??? I also checked the connection at the ignition that operates the central locks and buzzer, and it looks good, and my buzzer for the lights works, so I believe thats OK too. Could this be caused by something in another circut/system of the car?

Thanks
Ken

kenhartz

In searching the mercedes w124 manual, it states as part of the test proceedure of the electrical system of the central lock circut, that if the car is equipped with the anti-theft system, to disable it at the vacuum pump, and test to see if the central locks work. I do have a problem with my anti theft, as sometimes the drivers door lock does, or does not engage the anti-theft. If these two circuts are tied together, I would like to know how to isolate the anti-theft, as I can not see any connectors around the vacuum pump. Any suggestions where to look??