85 380SE fuel pump relay testing - PeachParts Mercedes-Benz Forum

Kyle Blackmore · #1 12-11-2011, 07:40 PM

On Friday night, after driving for about 5 minutes, my car stalled and restarted right away. I drove another 5 minutes and it stalled again. This time it took about 10 minutes of no starting and then it fired up without a problem. I got it home after a couple more times of it doing this, I pulled the relay and tried bridging across terminals 7 & 8 and the pump worked.
I checked in the manual and went through the steps in 07.3-165 checking fuel pump relay.
I have 12v at jack 8 - good
Pump runs when I bridge jack 7 & 8 - good
Ignition on, I have 12v at jack 9 - good
I have 12v between jack 8(pos) and jack 11(neg) - good
I have dwell angle of 13.4 at jack 10 while cranking - good
I have almost 10v at jack 12(pos) and jack 11(neg) while cranking - not good, should be 12v, but the test shows 0v as repair interrution to wiring.
The last step is checking the kickdown shutoff which I can't do because it's not running.
According to the wiring diagram in 07.3-165, jack 12 goes to 'Cable connector engine terminal 50'
Should I be checking to see why the voltage is only 9-10v at jack 12 by tracing back towards the ignition switch? I haven't had any other issues with components that are on that circuit. The circuit is only hot during cranking.
Thanks in advance.

mbdoc · #2 12-12-2011, 08:24 AM

Sounds like the pump is "heating" up the solder in the relay & it is going open circuit.

Very common failure of the pump relay.

Make sure that the pump draws approx 8A.

Kyle Blackmore · #3 12-12-2011, 08:47 AM

Thanks mbdoc, I tried resoldering the relay but it still won't start. The pump is drawing 6.5 amps, that's within spec.
I'll get another relay for it and let you know how it goes.

Kyle Blackmore · #4 12-13-2011, 12:27 AM

I picked up a new relay today from my indy. He asked if I had tested it to make sure it was the relay

.
I installed it tonight and she started right up. I went for a short trip and it seemed to be running smoother too, I've had a warm rough idle for a year or so. I've changed plugs, rotor, cap, coil and control unit to try and cure it.... hopefully this does.

mbdoc · #5 12-13-2011, 08:37 AM

Good feedback!

Hope your car stays running.

Kyle Blackmore · #6 12-13-2011, 08:45 AM

Thank you mbdoc for your help

Adler · #7 12-13-2011, 08:55 AM

Quote:

Originally Posted by Kyle Blackmore

Thanks mbdoc, I tried resoldering the relay but it still won't start. The pump is drawing 6.5 amps, that's within spec.
I'll get another relay for it and let you know how it goes.

on a 300se i reresoldered the joints ;worked for a year then required a replacement relay as the intermittent cutting off returned. Guess end of relay lifespan.