[jww]

Working on my brothers’ 98 GMC p/up.. 5.7 gas, auto, 3/4 ton,, etc. 91k miles.

He drove a short distance a month or so ago. Went into a store and on leaving truck would crank, but no start. On coming home from FL couple wks ago I find it in my driveway waiting for me! LOL !

No hum from in tank pump and a little spray in throttle body made it wanna fire. I’m thinking new fuel pump.
Checked for power before raising tank/new pump back in place and no power on the gray pump wire.

Changed to a new fuel pump relay but no bueno.
Cleaned a bunch of grounds, still no juice to pump.

Checked under hood fuse block and I have constant 12V power on pin 87, ( key on and key off) jumping pin 87 to pin 30 (gray wire) and have power to the fuel pump with key on and off.
Pin 86 constant ground .

I believe pin 87a has to close with pin 85 to make pin 30 hot only with switch on. Of course with the jumper I have now 87>30 pump has constant 12V.

I’m electrically challenged but can anybody explain in simple terms what it takes to close the electro magnet inside this relay to make pin 30 hot as it should??? Thanks in advance,,,,Jerry

Please reference pins 87, 87a, 86, 85, 30. I have a standard 5 pin relay………. Thanks

[roger1]

No.
85 and 86 control the relay. 87 connects to 30 when the relay is fired.
87a is connected to 30 when the relay is not fired and disconnected when it is fired. Many times 87a isn't used at all on a circuit like this.

[jww]

Thanks Roger….See if I got this right….
87 constant hot
86 constant ground
85 receives power from ECM to close circuit and energize 30? If this is correct I think that’s not happening with switch on.
Appreciate yur comments!

[roger1]

Without actually seeing the service manual schematic, I would say that's true.
How did you determine 86 was a constant ground? Can you see where the wire goes?

[jww]

I could certainly be wrong, but with a test lite I can probe 87 to the block and it lights. If I probe 87 to 86 it also lites, makes me think 86 is a ground. If I set digital vom to continuity and probe 86 to block I get continuity beep and assumed ground.?? I cannot see the wires. Unbolted the fuse block and turned it upside down but has a plastic cover on bottom that does not wanna come off.

Read on internet the starter relay can also cause fuel pump relay not to do it’s thing. Have not checked the starter relay but will tomorrow.

Lastly with switch on, removing and inserting the fuel pump relay should cause the relay to click…… this one has not done that since I started work on it. Something is not powering up this relay.

Thanks, Roger…… my bedtime!

[roger1]

Making assumptions based on your the way you are testing can play tricks on you sometimes. Your testing shows that wiring for 30 and 87 are good and the fuel pump is good.
But to find the issue, you really need to get a hold of a factory schematic. You really need to know how both 85 and 86 are wired.

One more thing you can do is unplug all the wires to the ECM and then check to see if you still get continuity from pin 85 to ground.

[jww]

Good morning, Roger…………

Thanks for the help. I did unplug the ECM and I do NOT have continuity to ground..
Plugged ECM back in and do NOT have continuity to ground…….
Bro. is just gonna have to take it somewhere for diagnostics. I’ll jump 87 & 30 and get it running so it can go to someone else. I do body work this stuff is for somebody more versed than me!!!!

Really do appreciate your suggestions.
Jerry

[roger1]

So the 86 isn't constant ground. At this point I believe that the ECM provides the ground when it determines the fuel pump should be running.
So the question to me is how the wire to 85 is wired to provide the +12V.
Is it just wired to the ignition circuit? Where is the fuse for that circuit? Does some other system come into play before that circuit can get activated? I could only answer those questions by looking at the schematic for the vehicle.

[jww]

You’re right….. I read last nite that 86 is not a constant ground like I thought. Yes, I believe 85 is the problem and I’ll research to see what it takes to get 85 to play ball.

Other stuff comes into play as you said: ign switch, crank sensor, starter relay etc.

[jww]

Update…….
Spent most of today trying to figger out what it takes to bring 85 into play. Decided to install tank, button things up jump pin 87 to 30 and get this thing outta here! Started right up and ran fine.
Removed jumper wire and replaced original relay expecting nothing and it primed, started and ran like new!!! Could not believe it was running!!!

My whole “brain dead” concept was that since the relay was not making an audible “click” then it was not powering or making contact and pin 30 had to be dead!!!! Well learn something new every day. Can’t believe I spent 3 or 4 days looking for a problem that didn’t exist.
On the plus side it did make me find 2 broken ground straps. The body ground strap and engine to frame ground strap. Plus fixed a bad ground wire from fuel pump to frame. Lessons learned, I guess

Thanks, Roger

[72projectbird]

Originally Posted by jww

Update…….
Spent most of today trying to figger out what it takes to bring 85 into play. Decided to install tank, button things up jump pin 87 to 30 and get this thing outta here! Started right up and ran fine.
Removed jumper wire and replaced original relay expecting nothing and it primed, started and ran like new!!! Could not believe it was running!!!

My whole “brain dead” concept was that since the relay was not making an audible “click” then it was not powering or making contact and pin 30 had to be dead!!!! Well learn something new every day. Can’t believe I spent 3 or 4 days looking for a problem that didn’t exist.
On the plus side it did make me find 2 broken ground straps. The body ground strap and engine to frame ground strap. Plus fixed a bad ground wire from fuel pump to frame. Lessons learned, I guess

Thanks, Roger

That body style truck can be finicky with grounds. My 98 had those same ground straps broken, resulting in a bunch of random electrical issues. I ran new heavy duty straps, and made a second ground from the battery to engine, and it's perfect.

Glad you got it figured out!

[beemergary]

Old chevy trucks can loose body ground to fuel pump. May not loose all ground but enough for low fuel pressure thinking pump. Run ground to a neg. battery location and know pushing that 500K. 1996 wt1500 vortec 4.3. Only replaced distributor gear and intake gaskets last fall. Trusty but rusty.