No power to seats and telescoping steering wheel on 1987 300TDT W124 - PeachParts Mercedes-Benz Forum

swogee · #1 08-15-2009, 03:20 PM

Recently the power seats and telescoping steering wheel on my 1987 300TDT W124 stopped working. As far as I can tell there seems to be no power to any of them. I checked for 12V at fuses E and F in the fuse box and there is no voltage. I also checked for 12V at pin 1, and pin 3 of seat relay K5 and there is 12V at both pins.

According to the wiring diagram the red/blue stripe wire from pin 1 of relay K5 goes to the fuses E and F in the fuse block and it doesn't show anything else between the two but there must be since wire runs inside the passenger compartment and then back out to fuses E and F. (Unless I missed a page or I am looking at the wrong schematic.) So what is between the pin 1 of relay K5 and fuses E and F that is preventing the power from getting to seats?

Any thoughts or ideas would be appreciated.

Thanks in advance.

-Steve

swogee · #2 08-17-2009, 11:33 AM

I think I may have found the cause of the problem. For reference I'll post it since I'm guessing this a problem that not many people are familiar with since there have been no replies.

It would appear that there was corrosion inside the wire leading to pin 3 of the power seat relay K5 located at position A inside the fuse/relay box.

Originally after looking at the wiring diagram and measuring 12V at pin 3 and pin 1 of the relay K5 while relay was installed in its socket I concluded that the problem must be with the red/blue stripe wire that went to the fuses E and F for the power seats. I couldn't find anything wrong with either ends of the wire and that was the mystery. One end has a plug that goes on a pin in the fuse block. The other end has a socket that the relay pin fits into. I cleaned the socket and tightened it just to make sure the relay was making good contact. Also at a wrecking yard I checked the routing of the red/blue stripe wire and it seemed to go in a loop that was bundled with wires that went into the passenger compartment then it looped back out to the fuse block. When I first saw the wire routing I wondered why the wire didn't just go straight to the fuse block, and I am guessing an extra length of wire was added for serviceability and manufacturability.

After observing the voltage at fuses E and F, I noticed the 12V would come and go. I then started to move wires around looking for a bad connection. I then discovered that pressing on power seat relay K5 would cause the 12V to return to fuses E and F. I also noticed that relay K5 was getting warm. (normally relays don't heat up very much.) I pulled the relay out and touched the pins and pin 3 was really hot. I also looked inside the relay for evidence of burns but there was nothing. I then examined the pin 3 socket and it was clean, but I looked at the wire going to the socket and seemed to have trace amounts of corrosion. I cut the insulation back and it looked like the corrosion was limited to the area near the socket. I then cut the socket off and then desoldered the old wire from the socket and soldered the socket onto the freshly stripped wire. I then reassembled everything, and the power seats had power on a consistent basis.

As best as I can figure there must have been corrosion in the wire that was acting as a resistor to heat up the wire and relay pin. The flexing of the wire which occurred when pressing on the socket must have caused the resistance of the wire to drop and current would then flow. My 1987 300TDT was kept in Florida near the coast for a while so there was some corrosion on certain parts.

I believe the corroded wire was the cause of the problem, but time will tell for sure. I hope this helps someone.

sixto · #3 08-17-2009, 04:38 PM

I've read where failure of a diode in the fuse/relay box will result in power to the seats only when a front door is open.

Sixto
87 300D

swogee · #4 08-17-2009, 05:15 PM

I was hoping that was case here, but there was no power to the seats/telescope steering adjust in all cases. It would have saved me a lot if time it was just the diode. Thanks for the suggestion though. We'll see if the corrosion was the cause of the problem.