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Fuel Pump Relay not working properly

Tiny
3CARPRO
  • MEMBER
  • 1997 GMC YUKON
  • 5.7L
  • V8
  • 4WD
  • AUTOMATIC
  • 140,000 MILES
Fuel Pump Relay won't energize when key is in on or crank positions. Tests on the relay, ECM B fuse, jumper to fuel pump and fuel pump relay slot B1 all check out operable at 12v. Is the EMC/VCM easy to test? What is left to test?
Monday, January 17th, 2022 AT 9:50 PM

29 Replies

Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
Good morning,

I attached a wiring diagram for you to view. This system may have two fuel pumps. There may be a second on the frame rail and need to verify.

The fuel pump relay has 2 feeds. Pin 30 is a constant battery voltage. Pin 85 is power from a fuse as well. Pin 86 is a ground to the ECM. The ECM should close the relay for 2 seconds with the key on for priming. It will close all the time while cranking and running.

https://www.2carpros.com/articles/how-to-check-wiring

https://www.2carpros.com/articles/how-to-check-an-electrical-relay-and-wiring-control-circuit

The ECM test is performed with special equipment. We can only verify that the ECM is not grounding the relay. If that is true, then the ECM is a failure.

Roy

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Tuesday, January 18th, 2022 AT 2:40 AM
Tiny
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  • 15 POSTS
Thanks.
Single fuel pump, in tank.
With multimeter:
Slots 86 and 87 drop to 9v when cranking.
Slot 85 with Key off is 12v, 0v when cranking.
Slot 30 with key off is 12v, 9v when cranking.
Fuel Relay energizes for 2-sec prime, still won't start.
Fuel pump starts when jumped. 87, 30
Inside cab fuse box found, #8 fuse (crank) circuit open, but 10A fuse not blown. Found 73 ohms to ground top half of fuse slot.
Seems to be another problem?
Thanks again.
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Wednesday, January 19th, 2022 AT 12:49 PM
Tiny
ASEMASTER6371
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When the relay energizes, does pin 30 have power for that 2 seconds?

If it does not then the relay is bad.

Roy
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Wednesday, January 19th, 2022 AT 1:12 PM
Tiny
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With test light it also energizes 2 seconds and deenergizes including by touch and sound. Both the old relay and new relay.
Does the #8 fuse (crank) circuit seem okay?
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Wednesday, January 19th, 2022 AT 1:36 PM
Tiny
ASEMASTER6371
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That is for the crank circuit.

You need to test the gray wire at the pump connector for power surging the 2 second prime.

Check that voltage.

Roy
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Wednesday, January 19th, 2022 AT 5:31 PM
Tiny
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Thanks.
Can't get to pump connector now but Slot 30/GRY/A3 is12v, key off.
Fuel pump draws 2.8A, no crank.
Strange occurrence putting #8 fuse back, truck tried to start in that instance without key.
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Thursday, January 20th, 2022 AT 3:14 PM
Tiny
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Wasn't #8 but some other fuse/s I touched.
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Thursday, January 20th, 2022 AT 3:18 PM
Tiny
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Fuse 8 is for cranking only.

Should have nothing to do with the fuel pump.

How did you measure the current draw on pin 30 of the relay to the pump?

Roy
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Thursday, January 20th, 2022 AT 4:55 PM
Tiny
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DC Ammeter, pin 30 to pin 87 or A3 to B1.
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Thursday, January 20th, 2022 AT 5:10 PM
Tiny
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No, not at all.

You need the relay installed and energized and then test the current draw. Hopefully, you have an amp clamp for the test as it is never good to cut the wire and put the meter in line.

Roy
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Thursday, January 20th, 2022 AT 5:15 PM
Tiny
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  • 15 POSTS
Tested amps without fuel relay installed. When relay installed it only energizes for 2 seconds then off, with key on/crank. Can't get a stable reading with my clamp in 2 seconds.
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Thursday, January 20th, 2022 AT 5:30 PM
Tiny
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Take the top off the relay and manually energize it.

Roy
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Thursday, January 20th, 2022 AT 5:45 PM
Tiny
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Okay. If it stays closed or opens again, is the relay bad?
What should I expect?
Thanks
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Thursday, January 20th, 2022 AT 6:00 PM
Tiny
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You are just holding it closed for testing only.

Roy
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Friday, January 21st, 2022 AT 12:53 AM
Tiny
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After 2 second prime relay opens. Then manually close switch nothing happens.
However, turned key on again and after 2 seconds, kept turning key very slowly, relay closed again, kept turning slowly until crank, relay opens.
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Friday, January 21st, 2022 AT 10:05 AM
Tiny
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Used the remote start, it cranked, still won't start.
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Friday, January 21st, 2022 AT 10:19 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,878 POSTS
Hi,

I noticed it has been a couple of days since contact has been made. Have you made any progress?

If not, I have a couple of questions. I read through all the posts above and need to confirm this is a fuel pump issue only and has nothing to do with the starter.

Also, the orange wire from the ECM-B fuse can only reach the gray wire to the pump if there is power at the dark green wire with a white tracer and a ground is provided via the black wire with a white tracer. Are these two wires working properly? Note, the power supply to the relay is from two different sources. It seems the red wire for the prime is working, based on what I read. I need to know if the dk green/white provided power to actuate the relay.

Let me know.

Joe

PS: Sorry to jump in. You may have already done all of this.
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Sunday, January 23rd, 2022 AT 5:40 PM
Tiny
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Thanks.
Don't think it's the starter but wondering if the Crankshaft position sensor is bad causing ECM a no start. Jumped the fuel pump at relay box to check pressure and got low 18PSI at the fuel rail. Also got 0.8 ohms on the pump at outside tank connector. A new fuel pump is 2.8 ohms. But yes, 12v on control and load sides but will check green/white and black/white tomorrow.
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Sunday, January 23rd, 2022 AT 6:33 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,878 POSTS
Wow, that is low. It should be upwards of 60 PSI.

As far as the crank sensor, if you have a live data scan tool, check to see if there is an RPM signal when cranking. If you don't have the scan tool and the vehicle has a tachometer, see if it moves a little when cranking.

Let us know.

Joe
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Sunday, January 23rd, 2022 AT 6:53 PM
Tiny
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Fuel pump used to be louder when starting. Tachometer slowly moved about 1mm while cranking.
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Sunday, January 23rd, 2022 AT 7:56 PM

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