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Old 01-10-2019, 01:12 PM   #1
chevykid65
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GMT400 Wiring Woes

1992 Chevy silverado k1500. 350, 5 speed, 4x4. Truck started idling rough the night before as I was pulling in to park it. Next morning I had to feather the throttle to keep it running. Got it warmed up and started backing up and it died. I knew the fuel pump was older so I replaced the pump and filter. Replaced fuel pump relay, ignition module in the distributor, oil pressure sensor. I found the injector A and B fuse was burnt. Replaced and hasn't blown since. With a test light the grey wire on the fuel pump relay goes hot for about 2 seconds when the key is turned. At the plug under the master cylinder where the grey and black/white wires go back to the fuel pump I get a 2 second shot with the key. With the key on at the pump the purple wire for the gauge is hot but nothing from the grey. The fuel pump just won't kick on. This has been tried with the old pump and the new. The best I can figure is a short somewhere but I can't find an exact wiring diagram to trace all the fuel system wiring. At this point I'm ready to find another truck to transplant my powertrain into. Anybody have any helpful advise?
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Old 01-10-2019, 02:41 PM   #2
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Re: GMT400 Wiring Woes

Here is all I have on the 88 to 98 wiring diagrams.





Name:  88 wiring diagram.jpg
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Name:  88 wiring diagram.gif 2.jpg
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It doesn't look like the wire colors match with yours too well but it may help.
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Old 01-10-2019, 04:43 PM   #3
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Re: GMT400 Wiring Woes

thanks Vettevet. I have the top one. I'll look over the bottom one better this evening. Once I get caught up with everyone elses cars here I'll get back to mine. I've got feelers out so I can upgrade to an extended cab but until then I still need something to drive.
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Old 01-13-2019, 03:37 PM   #4
Andy4639
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Question Re: GMT400 Wiring Woes

Most late models run a oil pressure switch to the fuel pump so if loss of oil pressure the fuel will shut off before engine damage. Have you worked on the oil system of late. Maybe bumped are knocked a connector loss.
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Old 01-13-2019, 11:12 PM   #5
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Re: GMT400 Wiring Woes

I haven't had to do anything since I put the motor in a couple years ago. I pulled the oil pressure sending unit. Noticed there was some oil clean back on the plug so I went ahead and replaced it. As well as the fuel pump, fuel pump relay, fuses, ignition module, and distributor. As far as I can tell it's a short somewhere. I didn't have time to play with it this weekend. Had to finish up a toyota head job and a transfer case in a tahoe.
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Old 01-14-2019, 12:11 AM   #6
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Re: GMT400 Wiring Woes

Have you verified that both injectors are getting power and spraying fuel? I had a similar issue in my 89 suburban that I just sold. After wasting $200 on injectors and an ignition control module, it was the injector driver in the ecm burned up. Open up the ECM case and look for anything burned. Should be behind the glove box.
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Old 01-14-2019, 10:12 PM   #7
chevykid65
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Re: GMT400 Wiring Woes

Carolina HD I am getting injector pulse. I can't get the pump to kick on to see if the injectors are spraying or not. I have another computer out of a 91 with an automatic. Tried that computer for the hell of it with the same results.
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Old 01-15-2019, 11:04 PM   #8
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Re: GMT400 Wiring Woes

Quote:
Originally Posted by Andy4639 View Post
Most late models run a oil pressure switch to the fuel pump so if loss of oil pressure the fuel will shut off before engine damage.
I don't know about the later ones, but the early tbi's(87-ish) used the oil switch a bit differently. If I read the diagrams correctly, the oil switch was simply a second path to the fuel pump.

The fuel pump would be energized through the fuel pump relay, but if that quit, it could also be energized through the oil switch. Thus, a customer might complain that he had to crank his truck for quite a while before it would start. This would indicate the relay had burned out, and simply needed replacing.

Since the oil switch was in parallel with the relay, and not in series, it could open from lack of oil and the engine would continue to run on the relay.

Evidently GM didn't trust their relays...

Edit to add: The "diagram 3" posted above agrees with me on this. Look closely at the oil pressure/fuel pump sensor and the fuel pump relay. You'll notice they are in series.

Also, you likely don't have that hot fuel module mentioned. That was a work-around for heavy duty trucks or big blocks. These were more susceptible to vapor lock due to underhood temperatures, and needed to keep the fuel pump running longer than usual in a hot-start situation to expel the vapor lock.

Last edited by kipps; 01-15-2019 at 11:16 PM.
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Old 01-15-2019, 11:14 PM   #9
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Re: GMT400 Wiring Woes

Note the "fuel pump prime connector" just below the fuel pump relay. It should be an empty terminal on a red wire protruding out of the harness mere inches away from the relay on the firewall. Connect a wire from the positive post of the battery(probably should fuse that wire, but I never did) to that terminal, and it will energize the pump. If you don't hear the pump run, the problem has to be downstream. Either the wiring, the switching valve, or the pump itself.

Edit to add: looking at the diagram I posted, that test terminal might not be where I said it was. On my '87 v20, it was at the fuel pump relay, but it may have moved to the ALDL by the time your truck was built.

(ALDL -- Assembly Line Data Link -- also known as the obd1 port)
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Last edited by kipps; 01-15-2019 at 11:27 PM.
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Old 01-16-2019, 10:54 AM   #10
chevykid65
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Re: GMT400 Wiring Woes

Kipps, I'll have to try it that way. I've probed right at the fuel pump relay and I get like a 2 second pulse of power. I don't have an extra set of hands to test at the pump.
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Old 01-16-2019, 11:03 PM   #11
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Re: GMT400 Wiring Woes

If you're getting a 2 sec pulse at key on then the ECM and fuel pump relay are working the way they should.

Here's the 1992 GM CK Wiring manual.
mediafire.com/1992_ST-375-92-EDD_Chevrolet_CK_Wiring_Manual.pdf

The rest of the 1992 service manuals from my library are in my signature.

Sounds like you have it pretty well nailed down to an open in the fuel pump power wire between the relay and the fuel pump. Start looking for damage along those wires. Look at the pump ground too. It'd be obnoxious to get the pump power fixed and have a dodgy ground hang you up.
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Last edited by hatzie; 01-16-2019 at 11:09 PM.
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Old 01-17-2019, 09:36 AM   #12
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Re: GMT400 Wiring Woes

If you didn't notice there is an inline fuse (on the firewall) for the fuel pump between the relay and the pump.

Yes, the oil pressure switch is just a redundant path to power if the relay fails.

Pretty simple circuit. It should be easy to fix
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Old 01-20-2019, 11:39 AM   #13
chevykid65
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Re: GMT400 Wiring Woes

Thanks you guys. I have checked the fuse. swapped the relay out, swapped distributor with ignition module out from my spare parts. Interior fuses. injector fuse was blown but hasn't tripped again. I just haven't had time to mess with it as of yet. Boss still has me swamped. I have to track down a speed sensor issue on a tahoe and put an oil pump in a 07 pontiac today. Maybe by next month I'll have time to do something on my own truck.
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