Engine does not stay running, possible fuel pump relay issue?

SHJON

MEMBER
1999 CHEVROLET TAHOE
5.7L
V8
2WD
AUTOMATIC
160,000 MILES

I'm having a problem with the truck staying running. It starts then dies. Note: pass lock has previously been removed. I installed a new pump around a year ago, but I often run it at the bottom end of tank and I haven't changed out the filter since, but I do show pressure at the Shrader valve, although I do not have a gauge to show if I'm getting around 60 PSI. So, I'm trying to understand what's going on with the relay and I'm by myself. With a test light key off, the 87 pin is the only pin that lights up. The ECM b fuse also lights up. I've bench tested the relay, and it appears good/clicking. So, to check ground I hooked the test tight to + now and the 30 lights up when it should be 85, right? 30 is power, right? So, with the key on now, and with the light connected to ground it's the same, only 87 lights up. With the light on the + to check ground, the 86 pin lights up, which is the signal from the PCM, right? So, my question is, what is this telling me? OBD shows no codes. Should I be able to jump a wire from a fuse, like the ign -e, ecm-1, or the ign-1, all small fuses in and rows that only turn on with the key, to the 87a.

3 Replies

JACOBANDNICKOLAS

MECHANIC
109,689 POSTS

Hi,

As mentioned, pin 87 has power at all times and that is correct.

When you apply power to pin 30, the test light should turn on because pin 30 provides a ground path through the circuit. Actually, if you are applying 12v to pin 30, the pump should turn on.

Pin 85 should only have power when the vehicle control module (VCM) sends power to the primary side (pin 85).

Pin 86 should be a ground for the primary side.

Let me try this again. LOL When you turn the key on, at first, pin 87a signals to prime the system. Once you turn the key, power from the VCM is sent to pin 85, and pin 86 provides a ground path to power the primary side and actuate the relay.

Once the secondary side of the relay is actuated, power flows through pin 87 (which always has power) out of pin 30 to the pump.

I hope that makes better sense.

Actually, here is a link you may find more helpful:

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

I attached the wiring schematic below, so you have a reference.

Let me know if this helps or if you have other questions.

Take care,

Joe

See pic below.

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Tuesday, September 20th, 2022 AT 8:53 PM

SHJON

MEMBER
2 POSTS
1999 CHEVROLET TAHOE
5.7L
V8
2WD
AUTOMATIC
160,000 MILES

Just wanted to add, if the ECM b and the 87-pin light up with the key off like they should, shouldn't the 30 also light up, cause power should be going through the 30 pin to an inactivated 5 pin relay to the 87 pin, right? When activated the power switches to the 87a the way I understand it.

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Wednesday, September 21st, 2022 AT 2:57 PM (Merged)

JACOBANDNICKOLAS

MECHANIC
109,689 POSTS

Hi,

Pins 87 and 30 are on the secondary side of the relay. Pin 87 should have power at all times as you mentioned. However, for pin 30 to receive power, the primary side needs to be completed to actuate the relay and send power to pin 30.

Pin 87a gets power when the relay is not activated.

Is that what you are asking?

Take a look at this link. It explains things more in-depth.

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

Let me know.

Joe

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Wednesday, September 21st, 2022 AT 2:57 PM (Merged)