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Fundamental of a relay. I am dealing with a five post relay for a fuel pump. According to the relay, post 30 is feed, 85 is grd, 87 is accessory, 86 is trigger (I have no idea what 87a is for). When I use a test light on the relay panel 87 is always hot with key on or off. I thought 30 would be hot? Since 87 is hot I jumped 87 to 86 and the test light came on at the female side of the fuel pump connector. But with the fuel pump “plugged” in and 87 and 86 jumped, the pump would not work. I think I need a fundamental lesson in the workings of a relay. One more thing. With the relay installed and the key turned on, the test light, on the female side of the furl pump connector, illuminates for a second then turns off. Will not come on again if key is turned off then on. I have to pulled the relay out and start all over again. I think this is a safety issue since it is a fuel system. My questions are: Why is 87 hot and not 30? Why does the test come on then off when 87 and 86 are jumped? I have not applied 12V to the fuel pump yet to trouble shoot it. I’m too engrossed in the workings of this rely. I must be one sick puppy.

It just so happens I have this relay.

Post 30 is ground. This makes the ground between post 87 & 87a. This is the primary side of the relay. So there should be voltage to 87a to ground for 1 -2 seconds when the ignition is turned on. This primes the fuel system. Posts 85 & 86 are the secondary side of the relay.

So here’s how it goes. The ignition is turned on, the voltage from pins 30 to 87a allow the fuel pump to run for a second or two and then shuts off. Controlled by the ECU. When the engine starts and the ECU see’s crankshaft rotation, it applies voltage to pin 85 and pin 86 is ground. That’s the secondary side of the relay. This pulls the contact in the relay to switch to post 87. That’s the dotted line between the primary and secondary side of the relay. And when the relay switches to post 87 it allows the fuel pump to run as long as the ECU see’s crankshaft rotation.


Thank you so much for your reply. I’m lost when you say 30 is a grd but when the ignition is turned on, voltage comes from pin 30(I assume this is battery voltage) to 87a. Why is there continuous voltage at pin 87 (the test light comes on when I plug into it)and not pin 30?

Pin 87 can be hot all the time because it supplies constant voltage to the fuel pump.

You say voltage from pin 30, and then no voltage from pin 30. Which is it?


Thanks again for your reply. Per your first reply, “The ignition is turned on, the voltage from pins 30 to 87a allow the fuel pump to run for a second or two and then shuts off.”

I have never received voltage from pin 30 whether the key is on or off.

Regardless of the relay, how would you proceed in trouble shooting the no run fuel pump? I will not be working on the truck till this coming Saturday. Thanks again for your help!

What is everyones score in regards to non functioning fuel pumps and the cause being a relay issue? (Hondas must sit this one out). I have not found so many through the years, those Bosch 4 and 5 pin relays seem to be a rock solid item. For GM I go with the pump itself unless we go back to “quad driver” days but those are not the relay in question.

Thanks for the reply oldschool. Myself, I want to know how and why something works along with how it is integrated in the system. I was taught to troubleshoot the most accessible and logical part of a system. For other people, I would say that a relay is the cheapest and least troublesome component to change; unless it doesn’t fix the problem. Myself,I hate shotgun maintenance I need to know the WHY! Would you agree to using test leads to troubleshoot the fuel pump with 12v from the battery? I have never tried it before and I’m worried about the amps coming from the battery.