Barn Find

[1991V3500]

Hey guys- just bought a 1991 V3500 Crew Cab with 35k miles, 454, 4L80E, 205, D60, corp 14b, but has been sitting for 14 years and the wiring is shot among other things. It was completely rhino lined in great condition, w hunting racks on front and rear, running boards and parked in a barn. I got it home replaced the fuel lines, tank, pump, changed oil, etc, to try and get it running. Rats or mice have chewed most of the exposed insulation off all wires. I did squirt some gas in TB and it fired right up w/50 psi oil pressure.

Might first issue is I get no power to the fuel pump, even after direct wiring 12 V to the fuel pump, i get fuel to TB but no fuel squirting. There is a wire coming from an oil sensor by the starter and its not hooked to anything and I think it runs the TBI or something.

These old square body crew cabs are super hard to find in TX- please help !!

 

[1987 GMC Jimmy]

Hey guys- just bought a 1991 V3500 with 35k miles, 454, 4L80E, 205, D60, corp 14b, but has been sitting for 14 years and the wiring is shot among other things. It was completely rhino lined in great condition, w hunting racks on front and rear, running boards and parked in a barn. I got it home replaced the fuel lines, tank, pump, changed oil, etc, to try and get it running. Rats or mice have chewed most of the exposed insulation off all wires. I did squirt some gas in TB and it fired right up w/50 psi oil pressure.

Might first issue is I get no power to the fuel pump, even after direct wiring 12 V to the fuel pump, i get fuel to TB but no fuel squirting. There is a wire coming from an oil sensor by the starter and its not hooked to anything and I think it runs the TBI or something.

These old square bodies are super hard to find in TX- please help !!

That down there by the filter is an oil pressure switch. I believe it supplies power to the fuel pump in case the relay fails. Mine ran just fine without that for a while. What I would do is start with the relay and the wiring to it. Just trace wires and see what's broken. There is a literature section on here with the service manual and wiring diagrams for your car. If the injectors aren't squirting, it's more likely than not that they're not getting electricity. Again, trace those wires. I believe they go directly to the ECM. You can also verify the injector pulse using what's called a noid light. If the noid light flashes when you plug it in, your injectors are bad or filthy.

 

[RetroC10Sport]

These old square bodies are super hard to find in TX- please help !!

Are you kidding? There's 6 square Suburbans and 2 K5s in my neighborhood alone and I am in an affluent section of Austin! Now K30s (or V30s or V3500s) are another matter and you got a good one.

Where in Texas are ya? I am a wiring guru and if you're in the Central Texas area, I can help you. We also have full wiring diagrams in the library.

 

[1991V3500]

That down there by the filter is an oil pressure switch. I believe it supplies power to the fuel pump in case the relay fails. Mine ran just fine without that for a while. What I would do is start with the relay and the wiring to it. Just trace wires and see what's broken. There is a literature section on here with the service manual and wiring diagrams for your car. If the injectors aren't squirting, it's more likely than not that they're not getting electricity. Again, trace those wires. I believe they go directly to the ECM. You can also verify the injector pulse using what's called a noid light. If the noid light flashes when you plug it in, your injectors are bad or filthy.

Thanks for the response and I guess im gonna have to replace the harness, I just hope the ECM isn't fried. I am only running the drivers side tank and trying to bypass the selector switch and valve. I get an accurate fuel gauge reading, just no power to pump. No key on 2 second burst or continuous when cranking.

 

[1991V3500]

Are you kidding? There's 6 square Suburbans and 2 K5s in my neighborhood alone and I am in an affluent section of Austin! Now K30s (or V30s or V3500s) are another matter and you got a good one.

Where in Texas are ya? I am a wiring guru and if you're in the Central Texas area, I can help you. We also have full wiring diagrams in the library.

Sorry I meant crew cab one ton 4wd's. I am about an hour east of Dallas on I30. I wish you were closer, never diagnosed elec issues with an ecm, fuel injection and electronic od trans. The power windows, locks, ws wipers and turn signals work, no headlights, horn, park lamps or ac controls. All fuses are good too.

 

[chengny]

Don't give up yet. It may be something simple:


With the ignition in RUN and engine off do you get a SERVICE ENGINE SOON warning?

That is the indication that the ECM is powered up. Without at least an SES light it's not going to fire. Even if there is an SES showing there are still a few more power and sensor inputs that the ECM must see before it will operate the injectors.

Start with confirming an SES light. If that's okay the ECM is powered up.

But before it will operate the injectors, there are two permissives that must be met:

1. Spark. It needs to see a pulsed reference signal from the ignition module on the 430
PPL WHT circuit.

2. Fuel. It also needs to see voltage to the fuel pump on the 120 GRY circuit (confirms
fuel is available).

As far a spark, I think it's safe to assume that - since you got it to run by supplying an source of fuel - there is a viable source of ignition (i.e. a good spark at the plugs). That means the ignition module in the distributor is good and (unless the mice chewed through the 430 lead) the reference signal it produces should be getting over to the ECM .

But about fuel pressure confirmation...while the ECM can sense the module's operation directly - because the input is electric - it isn't that simple with the fuel pressure input. There is no sensor/switch that closes to supply voltage to the 430 circuit when sufficient fuel pressure is developed.

The way the ECM determines whether there is a pressurized fuel supply available at the injectors is by the presence of 12 VDC in the 120 circuit. The 120 is the power to the fuel pump. It just kind of assumes that the pump is running and supplying pressure. It doesn't have any way to tell if there is a broken wire to the pump, if the the pump is even functional, that the fuel lines are intact, whether the fuel filters are fouled, etc.

The power to the 120 fuel pump feed can come from 3 places:

1. The normal way - the ECM pulls the FPR in, it's contacts close and the fuel pump
receives power

2. The oil pressure switch (the one down by the oil filter). That switch closes 4 psi and
supplies the fuel pump with power

3. The fuel module. In your case - because of the LE8 engine and or the 10,000+ weight
rating - there is an additional power supply. For 20 seconds after the key is moved to
the START position, the module will also supply power to the pump.


But wherever it comes from, as long as the ECM senses power in the 120 circuit at pin
D7 - it's happy - and will allow injector operation.

The point is; the way you powered the fuel pump externally may be important. Look at the wiring diagram below and I'm sure you'll see what I mean:

You must be registered for see images attach

If you disconnected the normal power lead to the fuel pump when you jumped to it, the permissive to the ECM was lost and it won't allow injector operation.

 

[1987 GMC Jimmy]

Thanks for the response and I guess im gonna have to replace the harness, I just hope the ECM isn't fried. I am only running the drivers side tank and trying to bypass the selector switch and valve. I get an accurate fuel gauge reading, just no power to pump. No key on 2 second burst or continuous when cranking.

I wouldn't runt to replace the harness just yet unless it was just gnarled completely. You have a consistent connection going from your cluster to your fuel pump which is potentially a good sign for the wiring going back that way. Here's what you should do about the pump. One, check the fuse to make sure it's not popped. Two, if it isn't, apply 12V to Terminal G on the ALDL connector (bottom left corner). If the pump runs, you have a good wiring. Three, if it does run, but it won't run by just turning the key, try just replacing the relay. I think that's a good place to start with the fuel issue. As far as the injectors go, you can trace wires, try a noid light test, or you can take the ignition module out and have it tested at your local auto parts store. The injectors won't spray if that thing doesn't work right. It's very likely gonna be a bad ignition module, a lack of ECM reference, or gunked up injectors. I don't know everything. In fact, lots of people know way more than me on this forum, but I just wanted to help you get started. Oh, I almost forgot! The ECM interfaces with the fuel pump relay. Stick a paper clip between Terminals A and B (side by side on the top right corner) with the key off and when it's in there good, turn the key and read the codes. Code 54 is a fuel pump relay issue. There are two sets of flashes. You count up the first set and then the second set. For instance, it'll flash three Code 12's before it'll tell you anything else. That's one flash, a pause, and then two flashes. After it does the three 12's, it may tell you some stuff that was wrong when you were trying to start it. If it ran for a minute, it may have been able to process some information. If you already know this, I'm sorry. I wasn't sure if you were new to TBI or not.

 

[chengny]

Below are some diagnostic charts/diagrams that may help you get started. They are all specific to trucks equipped with the 4L80E transmission.

Cranks - no start :

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No SERVICE ENGINE SOON light:

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Low or no voltage in the fuel pump supply circuit:

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No fuel injection:

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Fuel pump circuit interface with the ECM:

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[1991V3500]

Wow- Great stuff guys and thanks for the info !!! I'm gonna hit it hard this weekend and see what I can find out......

I have had a couple of K30 CC's but they both had carbs and were very simple. I will also splice and tape the chewed wires to see if that helps.

BTW- is this a good tranny and will it hold up turning 35's with a weak ass 454 ? RPO advises it has 3.73 gears -GT4

 

[1991V3500]

trying to upload photo

 

[yevgenievich]

Wow- Great stuff guys and thanks for the info !!! I'm gonna hit it hard this weekend and see what I can find out......

I have had a couple of K30 CC's but they both had carbs and were very simple. I will also splice and tape the chewed wires to see if that helps.

BTW- is this a good tranny and will it hold up turning 35's with a weak ass 454 ? RPO advises it has 3.73 gears -GT4

4l80e is equivalent to th400 with overdrive.

 

[1991V3500]

Don't give up yet. It may be something simple:


With the ignition in RUN and engine off do you get a SERVICE ENGINE SOON warning?

That is the indication that the ECM is powered up. Without at least an SES light it's not going to fire. Even if there is an SES showing there are still a few more power and sensor inputs that the ECM must see before it will operate the injectors.

Start with confirming an SES light. If that's okay the ECM is powered up.

But before it will operate the injectors, there are two permissives that must be met:

1. Spark. It needs to see a pulsed reference signal from the ignition module on the 430
PPL WHT circuit.

2. Fuel. It also needs to see voltage to the fuel pump on the 120 GRY circuit (confirms
fuel is available).

As far a spark, I think it's safe to assume that - since you got it to run by supplying an source of fuel - there is a viable source of ignition (i.e. a good spark at the plugs). That means the ignition module in the distributor is good and (unless the mice chewed through the 430 lead) the reference signal it produces should be getting over to the ECM .

But about fuel pressure confirmation...while the ECM can sense the module's operation directly - because the input is electric - it isn't that simple with the fuel pressure input. There is no sensor/switch that closes to supply voltage to the 430 circuit when sufficient fuel pressure is developed.

The way the ECM determines whether there is a pressurized fuel supply available at the injectors is by the presence of 12 VDC in the 120 circuit. The 120 is the power to the fuel pump. It just kind of assumes that the pump is running and supplying pressure. It doesn't have any way to tell if there is a broken wire to the pump, if the the pump is even functional, that the fuel lines are intact, whether the fuel filters are fouled, etc.

The power to the 120 fuel pump feed can come from 3 places:

1. The normal way - the ECM pulls the FPR in, it's contacts close and the fuel pump
receives power

2. The oil pressure switch (the one down by the oil filter). That switch closes 4 psi and
supplies the fuel pump with power

3. The fuel module. In your case - because of the LE8 engine and or the 10,000+ weight
rating - there is an additional power supply. For 20 seconds after the key is moved to
the START position, the module will also supply power to the pump.


But wherever it comes from, as long as the ECM senses power in the 120 circuit at pin
D7 - it's happy - and will allow injector operation.

The point is; the way you powered the fuel pump externally may be important. Look at the wiring diagram below and I'm sure you'll see what I mean:

You must be registered for see images attach

If you disconnected the normal power lead to the fuel pump when you jumped to it, the permissive to the ECM was lost and it won't allow injector operation.

Well so far pretty easy- There is no SES light firing up when the key is turned to the run position. I just bought a new battery, cleaned all ground connections to cab and chassis and replaced terminals at battery. I also taped and soldered all bad connections I could see that were chewed up. I have been studying the charts everyone has posted and getting familiar with each, but if the PCM is working I guess i'm cornholed... I don't want to change intake and throw a carb on this truck, I have had 2 or 3 TB trucks and they were always dependable. we have no smog laws in Texas for this old of a veh and I have already got some longtubes, flows, salad bowl, and TB spacer on the way. When my buddy told me about this truck, I had no idea it was a 91, with the TBI and 80 E and I think I got pretty lucky. The old lady is already pissed so Im ordering shackle flip kit and 6 inch springs from ORD next week.

 

[Jrgunn5150]

Slow down, take your time, and work through the flow charts. There's no reason to carb swap it or buy a whole new harness if you would prefer the TBI.

Not to mention, carb swapping it would require a standalone trans controller if you have the 4L80E.

 

[1991V3500]

Well so far pretty easy- There is no SES light firing up when the key is turned to the run position. I just bought a new battery, cleaned all ground connections to cab and chassis and replaced terminals at battery. I also taped and soldered all bad connections I could see that were chewed up. I have been studying the charts everyone has posted and getting familiar with each, but if the PCM is working I guess i'm cornholed... I don't want to change intake and throw a carb on this truck, I have had 2 or 3 TB trucks and they were always dependable. we have no smog laws in Texas for this old of a veh and I have already got some longtubes, flows, salad bowl, and TB spacer on the way. When my buddy told me about this truck, I had no idea it was a 91, with the TBI and 80 E and I think I got pretty lucky. The old lady is already pissed so Im ordering shackle flip kit and 6 inch springs from ORD next week.

3. The fuel module. In your case - because of the LE8 engine and or the 10,000+ weight
rating - there is an additional power supply. For 20 seconds after the key is moved to
the START position, the module will also supply power to the pump.


chengny - have a question, reading your response you advise of an LE8 engine and my truck has the L19. Is #3 still applicable ?

 

[chengny]

I think we may have a disconnect as far as the SES light coming on with the key in the RUN position. When I read this:


There is no SES light firing up when the key is turned to the run position. I just bought a new battery, cleaned all ground connections to cab and chassis and replaced terminals at battery. I also taped and soldered all bad connections I could see that were chewed up. I have been studying the charts everyone has posted and getting familiar with each, but if the PCM is working I guess i'm cornholed...

From the two sentences highlighted in boldface, I get the impression that your understanding of the SES light is that:

If the SES warning is not illuminated with the key in RUN, it means the ECM is powered up and ready to operate the engine management system.

That's not correct, it's the other way around.

If you move the ignition switch to the RUN position and the SES warning illuminates and stays that way, that is the indication that the ECM is powered up and ready to operate the engine management system. You need to get an SES light when the ignition switch is in RUN - if you don't get that light, find out why.



Assuming the ECM (AKA - PCM) itself is not fried, the only things needed to get an SES light are:

1. A charged up battery

2. Intact wiring from the battery positive to the fusible link at the firewall junction block.

3. Intact wiring form the fusible link to the ignition switch

4. A functioning ignition switch

5. Intact wiring from the ignition switch to the SES lamp in the IP

6. Intact wiring from the SES lamp to the ECM

7. Proper ground for the ECM

You can skip checking the battery, fusible link, ignition switch and all the interconnected wiring by checking for 12 VDC at the line side of the ECM 1 & ECM B fuse clips. If you get voltage at those two fuse clips and assuming the SES lamp is good, check the ground for the ECM.




On this:

have a question, reading your response you advise of an LE8 engine and my truck has the L19. Is #3 still applicable ?


Ignore that, it's my mistake, you don't have an engine with an LE8 RPO code - that's a carbureted 454. The RPO for a 454 with TBI is L19. The info above is from the 1991 Driveability & Emissions Manual. In 1991 carbureted engines were just a distant memory, so you can be confident it is all applicable to your power plant.

Substitute the LE8 with L19 or 454: