Troubleshooting Fuel Relay Issue on 1990 Chevy Cheyenne

[CasualTruck]

Hi everyone,

I am a young adult that is self taught and taking after his father and grand father. Bear with me if you can.

I’m looking for some guidance on a frustrating issue I’ve been trying to solve with my 1990 Chevy Cheyenne W/T 1500 (6-cylinder, 4.3L). The fuel relay doesn’t activate when I turn the ignition key, and I’ve gone through several troubleshooting steps to figure out what’s wrong, but I’m still stuck.

Here’s what I’ve done so far:

I started by checking all my fuses and cleaning the connections to ensure they’re solid. My inline fuse link appears to be healthy. I’ve confirmed that I’m getting 12 volts running from the ignition to the ECM, which tells me the ECM is receiving an ignition signal. Additionally, I verified continuity between the fuel relay and the ECM through the green/white wire, so there’s no break in that line.

To rule out grounding issues, I confirmed there’s no floating ground on the ECM. I also checked the ECM fuse, which shows 12 volts when the ignition is on. The ECM itself is getting 12 volts through the orange hot wire as expected.

When I unplug the fuel relay and turn the ignition to the "on" position, my multimeter shows 0 ohms and 12 volts. When I turn the ignition off, the readings shift to 0 volts and 15.30k ohms. However, when the relay is plugged in, and the ignition is on, the green wire leading to the relay reads 0 volts and 550 ohms.

As an additional test, I hot-wired the fuel pump, and it powered on without any issues. I also checked the ground wire at the fuel pump and found 0 ohms, so the ground there seems solid. To rule out a faulty relay, I replaced it, but unfortunately, that didn’t resolve the problem.

I feel like I’ve covered a lot of ground, but I’m not sure what else could be causing the issue. Has anyone dealt with a similar problem, or does anyone have ideas on what else I should check? I’m open to suggestions or insights into what I might have overlooked.

Thanks in advance for your help!

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[evilunclegrimace]

Did you check the inline fuse that is under the "T" shaped cover on the firewall?

 

[CasualTruck]

Seeking clarification: Are you mentioning the inline fuel pump fuse next to the fuel relay on the firewall. I did check that, its good. Perhaps there is there another inline fuse I am not ware of?

 

[docstoy]

Have you downloaded service manuals under sticky thread above that is titled 88-98 service manuals also download the fuel and emissions manual in it you should find information to diagnose your fuel pump relay , I think it's chart A-5, which is titled fuel pump relay diagnosis. Hope this helps

 

[CasualTruck]

Have you downloaded service manuals under sticky thread above that is titled 88-98 service manuals also download the fuel and emissions manual in it you should find information to diagnose your fuel pump relay , I think it's chart A-5, which is titled fuel pump relay diagnosis. Hope this helps

Thank you. I used this to do some further testing.

 

[CasualTruck]

Update: I have come to find that the ECM is sending 12 volts to the fuel relay via a green wire, but the 12 volts is not able to withstand load. When I connect the 12 volts green wire to a test light that runs to ground, the multi meter reads 0.4 volts. I opened the ECM today and noticed a pungent smell of sulfur. I want to believe this is the smell of something having burned, but I am unsure if this may be due to confirmation bias. I checked my ECM grounds, all of which are healthy. I have not been able to find a circuitry diagram for this ECM, which is disabling me from further diagnosing the ECM.

Edit: I have learned what i am experiencing is a voltage drop in the fuel relay coil circuit.

 

[docstoy]

There wasn't a electrical manual in the thread you looked at ? What were the charts/electric diagrams in your first post ? Odors from circuitry can often be bad or corroded connections or loose electrical connector terminals

 

[evilunclegrimace]

Seeking clarification: Are you mentioning the inline fuel pump fuse next to the fuel relay on the firewall. I did check that, its good. Perhaps there is there another inline fuse I am not ware of?

Yes, that was the fuse I was referring to.

 

[Road Trip]

Update: I have come to find that the ECM is sending 12 volts to the fuel relay via a green wire, but the 12 volts is not able to withstand load. When I connect the 12 volts green wire to a test light that runs to ground, the multi meter reads 0.4 volts. I opened the ECM today and noticed a pungent smell of sulfur. I want to believe this is the smell of something having burned, but I am unsure if this may be due to confirmation bias. I checked my ECM grounds, all of which are healthy. I have not been able to find a circuitry diagram for this ECM, which is disabling me from further diagnosing the ECM.

Edit: I have learned what i am experiencing is a voltage drop in the fuel relay coil circuit.

Greetings CasualTruck,

Welcome to the GMT400 forum! You've joined a virtual community scattered around
the globe who share your enthusiasm for keeping some of the best trucks/SUVs that
GM ever made on the road & earning their keep.

FWIW I like your troubleshooting approach. If you are purely self taught, that's impressive.
If you have been under the tutelage of one or more mentors, that's cool too -- a good mentor
will save you a lot of time with the learning curve having to do with troubleshooting/fixing stuff.

****

Anyway, I wanted to chime in on your observation above. The short answer is that when the
ECM is in charge of controlling an electrical load, the majority of the internal control logic is only
capable of driving a load that only needs a few milliamps to be signalled/driven successfully. (!)

But in order to interface with the real world electrical loads in the engine bay, the ECM designers
made use of current drivers that can supply up to 1000 milliamps (1 amp) in order to do real work.
(think the control coil of the fuel pump relay, TCC solenoid, etc)

What I have just described is referred to as a Driver. (The Drivers in our automotive computers
are similar in function to the output transistors on audio amplifiers.) So, when the chip designers
fit 4 of these individual output drivers on a single IC, they are referred to simply as 'Quad Drivers'.

Since the General's FSMs treat the computer as a single swappable part, the manuals don't go
into a lot of detail on these internal components. But here's a blurb from the '89 Driveability & Emissions manual:

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(p. 1154, Computer Command Control, '89 Driveability & Emissions FSM)

If you have your computer open for a visual inspection, you might be able to find
the Quad Driver that burned up and let out the stanky magic smoke? Possibly
you could cheat and follow the internal trace from the pin that connects to the
green wire connected to the fuel pump relay.)

Unfortunately, since we're in 2024 & interested in a late 1980s electronic components,
whatever information that was on the interwebs about all this is disappearing before
our eyes thanks to the aging out of the guys who knew this stuff, which leads to link rot.

But I'll share what I was able to dig up after a brief search:

1) At least one of the GM part numbers for a Quad Driver was #16042368.
And a lot of the links surrounding this part number is a dead end / 404 error.

2) In order to get a general idea of the component we are looking for inside your ECU,
I found a thread where they were discussing a TI (Texas Instruments) part number that
was either compatible or just similar to the GM quad driver # above. Of course the
page on the TI official website was a 404 error, but by skulking around websites that
keep this kind of stuff around, I found the following data sheet:

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(note the date on the bottom of the sheet - this is prehistoric info in the land of electronics/logic.)

3) I have also been looking for the schematics for the GMT400-era ECM/PCM/VCMs with no luck.
There are a few promising threads in some of the ECM-oriented hacker websites, but all the links within
those 20+ year old threads are all dead ends so far.

****

Where do we go from here?

1) What's been emphasized in troubleshooting threads surrounding the failure of these
Quad Drivers is that they normally don't fail by themselves, but are taken out by either
a chafe/short to ground in the wiring harness -or- the component being driven suffers
an internal short.

In English, if you decide to replace your ECM, first verify that the control coil in the fuel
pump relay is still within specification. (Offhand, at least 20 ohms of resistance?) Otherwise,
a melted/shorted relay coil will proceed to also take out the new ECM Quad Driver in short
order. :-(

Of course it goes without saying that the wire between the ECM and the fuel pump relay
needs every to be inspected the full length for any signs of chafing/compromise.

****

By the way, IF you locate where the magic smoke was let out of the IC and/or evidence
of a short circuit, please take sharp photos and upload them to this thread. Electronic
theory is only somewhat persuasive, but photos of real world failure is a big crowd
pleaser / effective tutorial for others facing similar issues.

Hope the above helps clarify what you are troubleshooting.

Looking forward to your next status update.

Cheers --

 

[CasualTruck]

I do not have significant progress to report on @Roadtripper . But I do have a diagram that might be worth while the community. Its a simple layout of the chip relationships inside the ECM. I believe this is for GM vehicles between 1986 and 1989. This ECM diagram is practically true for my 1990 ecm as well. My next job will be to trace my green wire to the quad processor.