No power to fuel pump relay when key switch on?

Tiny
MRCMACK5
  • MEMBER
  • 1998 GMC C1500
  • 5.7L
  • V8
  • 2WD
  • AUTOMATIC
  • 250,000 MILES
I have a 1998 GMC. I put a 1997 engine and transmission in. (And trying first start) If I jump the relay pump runs engine runs coincidence tach does not move on dash and volt gauge is low like 10 or 11 volts. But that is what I noticed. Back to main problem ECM not sending power to relay I thought immobilizer but security light on dash is not on. So any suggestions would help me a lot.
Monday, January 20th, 2025 AT 6:03 PM

26 Replies

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 54,137 POSTS
I would make sure you have the battery to block ground and then the block to chassis ground strap, because the engine and transmission are seated in rubber you must have the proper grounds. Do you have the MAF plugged in? The PCM must see engine rotate and air movement before it will turn the fuel pump on. We should run the codes to see if any are stored as well.

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

Also check the IGN E fuse in the fuse panel under the hood, I have seen the fuel pump balance module go out on these trucks, here is the fuel pump wiring diagrams so you can see how the system works. Check out the images (below). Please let us know what happens.
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Tuesday, January 21st, 2025 AT 1:06 PM
Tiny
MRCMACK5
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The big 3 grounds are attached MAF is plugged in. When I bypass the relay, she runs good no check engine light etc. But also, RPM gauge does not move. Figured if the crank sensor was bad it wouldn't run at all. That is what is troubling the computer is not even sending a signal for the 2 sec prime. What all has to be satisfied for computer to do the 2 sec prime and of course send signal to engage relay.
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-1
Tuesday, January 21st, 2025 AT 1:47 PM
Tiny
KEN L
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  • 54,137 POSTS
The computer must see engine rotation which it sounds like it has part of that, the tach needs to work though, I would try unplugging the cluster. The tach might be shorted out. Also, I would unplug the MAF sensor so the comp will use the default value. If the MAF is not working, it will not show air flow when the engine is cranking so the computer will not turn the fuel pump on. Let me know.
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Wednesday, January 22nd, 2025 AT 8:48 AM
Tiny
MRCMACK5
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Hooked up scanner computer shows 182 RPMs when cranking dash gauge still not moving but at least the ECM sees RPMs.
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Saturday, January 25th, 2025 AT 1:32 PM
Tiny
MRCMACK5
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  • 17 POSTS
Also pin 30 on relay goes to fuel pump, right? If I ohm it, it goes to ground. The guy replaced the fuel pump, I wonder if it’s wired wrong at the tank.
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Saturday, January 25th, 2025 AT 3:11 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 54,137 POSTS
Yes pin 30 goes to the pump which will ohm out to ground while still connected to the pump. While cranking does pin 30 have 12 volts, that is the test I would try.
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Sunday, January 26th, 2025 AT 9:41 AM
Tiny
MRCMACK5
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No and I do not have power from the green/white PCM.
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Sunday, January 26th, 2025 AT 9:43 AM
Tiny
KEN L
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  • 54,137 POSTS
This would be power from the fuel pump balance module on the frame inside the left frame rail. (Pink wire - while cranking).
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Sunday, January 26th, 2025 AT 9:48 AM
Tiny
MRCMACK5
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  • 17 POSTS
What exactly would you like me to do?
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Sunday, January 26th, 2025 AT 10:14 AM
Tiny
MRCMACK5
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  • 17 POSTS
I do not see a fuel pump balance module inside the frame rail near tank I see fuel filter.
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Sunday, January 26th, 2025 AT 11:18 AM
Tiny
MRCMACK5
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  • 17 POSTS
Car Scanner ELM OBD2
Version: 1.114.11/1.114.11/iOS
DTC report
Connection profile: GMC OBD-II / EOBDDate: 1/26/2025 3:06:47 PM
============================
OBD-IIOBD-II
DTCs: 7

P0740 [0x0740]
Torque converter clutch (TCC) solenoid - circuit malfunction
Status: Confirmed

P0785 [0x0785]
Shift/timing solenoid - circuit malfunction
Status: Confirmed

P1860 [0x1860]
TCC PWM Solenoid Circuit Electrical/ Transmission Transfer Case Contact Plate "D" Short Circuit To Battery
TCC PWM Solenoid Circuit Electrical
Status: Confirmed

P1810 [0x1810]
TFP Valve Position Switch Circuit/ Transmission 4-Wheel Drive Low Indicator Short Circuit To Battery
TFP Valve Position Switch Circuit
Status: Pending

P1718 [0x1718]
TFT Sensor In Range Failure High
Incorrect Neutral Gear Ratio
Status: Pending

C2000 [0x6000]
Status: Pending

P0099 [0x0099]
Intake air temperature (IAT) sensor 2 - circuit intermittent/erratic
Status: Pending
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Sunday, January 26th, 2025 AT 12:07 PM
Tiny
MRCMACK5
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Any of these keep the PCM from firing the fuel pump relay?
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Sunday, January 26th, 2025 AT 12:08 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 54,137 POSTS
I don't think so. Terminal 30 at the fuel pump relay, while the engine is cranking do you have 12 volts?
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Monday, January 27th, 2025 AT 9:26 AM
Tiny
MRCMACK5
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No I do not have power at 30.
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Wednesday, January 29th, 2025 AT 7:55 AM
Tiny
KEN L
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Does this truck have two tanks or just one? Sorry there is some confusion with the wiring if you have duel tanks. There is a fuel pump pressure switch behind the distributor I would bypass to see if you get power to the pump. Org to gry wires together. Check out the images (below). Please let us know what happens.
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Wednesday, January 29th, 2025 AT 8:49 AM
Tiny
MRCMACK5
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Only one tank just a 98 c1500 extended cab 2-wheel drive.
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Wednesday, January 29th, 2025 AT 9:08 AM
Tiny
MRCMACK5
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It does have the oil pressure switch did it change from 97 to 98 because I put the drivetrain from a 97 in this 98 if it is a different pressure switch, I can take the one out of the old motor.
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Wednesday, January 29th, 2025 AT 9:11 AM
Tiny
KEN L
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I would just jump the connector to see if that is the problem.
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Wednesday, January 29th, 2025 AT 11:03 AM
Tiny
MRCMACK5
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I will try that this evening. Thanks for your help.
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Wednesday, January 29th, 2025 AT 11:22 AM
Tiny
KEN L
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Sorry for the confusion with the dual tanks, let me know.
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Thursday, January 30th, 2025 AT 11:51 AM

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