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Patrick Eaton 03-23-2000 10:23 PM

My 1988 300CE has an intermittent problem with power to the fuel pumps. At first it would stop running but restart immediatly. Now it has stopped altogether. I can supply direct power to the pumps and the car will run. I suspect thet the fuel pump relay may be at fault. If this is the case where is the relay and what will it look like when I find it? Any other suggestions welcome.
Thank you. Patrick Eaton. Seattle WA.

stevebfl 03-23-2000 11:02 PM

The fuel pump relay is behind the battery in the area beneath the windshield. There are two large control units KE-Jet and ABS and there are two black boxes a little smaller than a pack of cigarettes. One will say Klima it is the A/C comp control relay. The other one is the fuel pump relay. It usually will have an rpm rating on it and possible say kickdown on it.

Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

Benzmac 03-24-2000 10:47 PM

The relay is problematic. If you have not used the partsshop, we sell the relays.

1981 280GE SWB
1987 16V

Patrick Eaton 03-24-2000 11:06 PM

Thanks for the replies. Unfortunately I tried replacing the relay with a used one today with no success. I could have been extremely unlucky and got a bad part or there is something wrong with the trigger supply. I am not familiar with all the pin designations. I do know that terminal 15 comes live with the ignition and that 87 and 30 supply current to the pumps, I can connect these and have the pumps running constantly which will at least allow me to move the car.
I am now at a loss as to what is causing the problem. Any help is much appreciated.
Thank you. Patrick Eaton

[This message has been edited by Patrick Eaton (edited 03-24-2000).]

stevebfl 03-25-2000 11:46 AM

You will need a wiring diagram.

Basically the fuel pump relay looks for one of two conditions for the relay to engage. The first is the starter on signal #50. This is the same as the starter gets at the solenoid to engage the starter. If the relay sees the starter signal it will turn on during that signal.

The second condition is engine speed. This signal is converted from a standard primary ignition signal into a square wave pulse signal by the ignition control unit. If the relay sees 300rpm it turns on.

The relay has some other functions depending on the year. It limits engine speed by cutting the fuel pumps at a certain RPM and it controls tranmission kickdown limits similarly.

Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

Patrick Eaton 03-25-2000 02:03 PM

Thanks Steve. Given this information I would expect that the relay would attempt to supply power when cranking the motor. I wonder is this telling me that I got a bad replacement relay. I have driven the car with the direct power to the pumps and the kickdown seems to work fine. I will try to get myself a wiring diagram and work out the remaining relay contacts. In the mean time do you see any problem with driving the car with the pumps directly wired?

Patrick Eaton 03-31-2000 09:57 PM

Thank you Steve for your help with my injection problem. It turned out that the used relay I got was faulty, I should have known better. I got a new relay plugged it in and all is well. Your help saved me a lot time.
Patrick Eaton

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