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Old 09-02-2003, 08:52 PM
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Join Date: Jan 2002
Location: Coeurd'Alene Idaho
Posts: 48
fuel pump relay mystery

Howdy all,
1989 300E 180K.
Car died comming to a stop light. Pushed her off the road and found that it was not getting fuel, allthough I could here the pumps working......Infamous pump relay!
Replaced the relay with new and everything was fine...for about 30 days then she died again. Again no fuel. Refering to this handy dandy site I learned how to jump the relay....connecting pins # 30 to 87 on the new relay. (saved on towing. Thanks mercedes shop).
Today the car saw my tech, he pulled the pump cover and both pumps seem to be operating fine. When we pulled the relay apart the new one had a burn spot in the exact same place as the old one. He re-soldered the break and now of course she starts right up without my jumper cable. My Question is what could be burning out my relay?
Could there be a poor or no connection on one of the pins (fe-male end)?
With #30 and #87 jumped my pumps run without a key in the ignition. Is this normal?
By the way the ovp is new and the fuse is good.

Any help will be greatly appreciated.

Thanks, Brian

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Old 09-02-2003, 09:18 PM
Gilly's Avatar
Registered User
Join Date: Mar 2001
Location: Evansville WI
Posts: 9,616
Guess I'd start by checking the current consumption of the pumps. If they're drawing too many amps it possibly could overheat/burn out the relay. That'd be my first guess anyways. Everything else that I can think of (except maybe a ground) would be an input, and I would think that an input could do this.


Oops, another possible output of that relay is I believe it also controls the heating of the O2 sensor (I believe yours has the heated O2 sensor anyways) so maybe they want to check the current consumption of that as well. Unsure if a O2 sensor drawing too many amps would affect the relay in this manner (stalling problem due to fuel delivery problem) or not.

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