fuel pump relay ground and power

[greenturdyukon]

1997 GMC Yukon 5.7 vortec 4X4

This started 10 minutes or so after I left a car wash, it didn't spit or sputter just died on the road. After a fair bit of diag and changing the pump i discovered that pin 87 ( should just be constant power ) is also a ground to the body. I don't know how this is happening. I'm curious if anyone with one of the k chassis trucks has had anything like this before or have any ideas why its a power and ground in one pin???

 

[xXxPARAGONxXx]

Don't see how that is possible without it blowing a fuse as a result of a short to ground. I'm looking at my 99 service manual. Pin 87 on the five-pin fuel pump relay does receive constant B+. So, if there was a connection to ground, it would blow the fuse. Maybe you want to take a break and re-think it in a bit?

Also, are you sure you weren't checking pin 87A? It looks like that circuit goes through the motor to ground.

 

[mnewxcv]

If I recall, pin 87 receives power directly from the ECM-B fuse in the underhood fuse panel. Is that fuse blown? Not sure if pre-98 is different.

 

[greenturdyukon]

Don't see how that is possible without it blowing a fuse as a result of a short to ground. I'm looking at my 99 service manual. Pin 87 on the five-pin fuel pump relay does receive constant B+. So, if there was a connection to ground, it would blow the fuse. Maybe you want to take a break and re-think it in a bit?

Also, are you sure you weren't checking pin 87A? It looks like that circuit goes through the motor to ground.

I've checked all fuses and replaced anything to fuse that connects to anything with ties to the fuel pump+relay. There must be a wire or two that somehow got connected before the relay pin 87, I can't think of why it would just spontaneously happen with no prior warning or signs.

 

[mnewxcv]

you will need a multi meter I figure. Pull the ECM-B fuse and see if there is continuity to pin 87 of the fuel pump relay on one side of the fuse socket. The other side should be 12V hot all times. If you have 12V on one side as you should, the problem is between the other side and pin 87. If you don't have 12V, we can help you with some wiring diagrams relating to ECM-B circuit.

 

[greenturdyukon]

Alright, I'll have to do that tomorrow. it just baffles me that pin 87 is a positive with 12v and still have continuity with the body like a ground.

 

[PlayingWithTBI]

it just baffles me that pin 87 is a positive with 12v and still have continuity with the body like a ground.

Keep in mind a floating ground or a coil, like a relay or solenoid, can make it look like you're going to ground. Back in the day, we used a Simpson VTVM to troubleshoot these. You can also use a test light which will put a load on that circuit.

 

[Erik the Awful]

A little late to the party here, but in my opinion ohming wires can be very misleading. I do 99% of my diagnoses by checking to see which circuits have 12v, and on the rare occasions I have evidence of a short I use a test light to try and narrow it down. If your meter says you have a short, but you can't get enough draw to light up the test light, you don't have a true short.