Terminator X Conversion - 1991 TBI

[creich68]

Most of your options are way more expensive, complicated, and difficult than fixing what you have.

Microsquirt can run a TBI for a lot less, but again, harder than fixing what you have.

The simple, time consuming thing to do, is sit down on a nice day with your current harness and ohm meter, and checking every wire for continuity. Take vbb1999 up on his offer, fix what you have, learn about your truck, don't let these shops piss your money into the wind with their guesses.

That's what the second shop allegedly did with the harness - ohm wire and look for any breaks in continuity. I may have to go back and check myself on the engine harness specifically.

I have no doubt I could it work. But we all have different skill sets. All the underwood and transmission wiring is there,so use it like a stand alone on a retrofit car. That takes care of engine and transmission. Now use your remaining harness fir lights horn etc. Again just like this was a retrofit. There's also the possibility since all the under hood is there to re pin their harness to mate to your bulkhead.

Good point. Depending on how the next couple weeks ago, the Howe harness may be the last resort that would be the least amount of pain/work. The other vehicle harnesses appear to be functioning normal. The only CEL's I've had the last two years are engine related. Helps narrow the focus.

yeah, I think that harness could also work. it might have a few other things you dont need. and you might have to add a few things that are missing. so it is not a OEM replacement, but looks to be a solid 90% solution.

but, I'd still work with what you have. make sure plug wires aren't on any of the ECM harness wires, especially those going to the ICM. make sure the ECM wires, especially those at the ICM, don't have any rub through. even if they aren't touching anything, it seems that it induces EMI.

if you dont have diagnostic capability with a ALDL cable and Tunerpro or winALDL, or if you can find a used Bluetooth cable and ALDLDroid phone app, I would get it.

if the shops dont have the ability to read the ECM, you and them are going to have a tough go of things. you can do a lot with a DVM on TBI, but you can not check everything. plus you need some other things like a timing light, fuel pressure gauge (and adapter to tap downstream of the filter), etc.

anytime you need a sensor or something engine management related, only use AC Delco. and routine maintenance is important. get some high dollar wires that will last. use AC Delco plugs, cap, rotor and change them every 30k miles. change the oil every 3k and lube the steering and drivetrain components. drain and fill the transmission a couple oil changes in a row every 15k and replace the filter every 30k (assuming you have a drain plug).

TBI is, generally, pretty simple and reliable if you keep showing it some love.

I think y'all have talked me out of any drastic measures (i.e. Term X or LS) at this point. Going to focus on the existing harness again. I moved some plug wires further away from the ICM over the weekend as an added measure.

The shop has a way to observe ECM readings and when they did replace distributor/ICM it was AC Delco. I didn't want to take a chance on less reliable brands. I'll check into the diagnostic tools you mentioned. Seems like a must have for these TBI systems at home.

Maintenance has been religious. I change the oil once a year, even if I don't eclipse 3k miles. Father was the same way so fortunately I know the vehicle was maintained in the same manner since new, with records.

 

[Bextreme04]

Are you SURE you have a 4L80? I have never seen a 4L80 in a 1500... I was sure they were 3/4 and 1 ton only in 1991. The 91 4L80's are also unique in several ways and not necessarily desired. If you do end up doing some kind of swap, the easiest would be to take the entire engine/trans/harness/pcm from a 3/4 or 1 ton silverado/suburban/express van. Get an L31/4L80 or L29/4l80 and it would just drop right in with hardly any modifications needed. Bonus points if it's from a 2000-2002 Express van, because it would have the LS PCM already, but would be a Vortec 350. The only change you would need to do to the Suburban to support that is a few simple wiring splices cutting the stock engine/trans/pcm harness out of the system and then splice the whole new everything in. You would also need to just pull your stock sending unit from the tank and swap the fuel pump for an EP381(which you could also pull from the same donor vehicle). I think on my squarebody it comes down to something like 6 wires that need to be spliced into, everything else is internal to the engine/trans/pcm systems.

 

[creich68]

Are you SURE you have a 4L80? I have never seen a 4L80 in a 1500... I was sure they were 3/4 and 1 ton only in 1991. The 91 4L80's are also unique in several ways and not necessarily desired. If you do end up doing some kind of swap, the easiest would be to take the entire engine/trans/harness/pcm from a 3/4 or 1 ton silverado/suburban/express van. Get an L31/4L80 or L29/4l80 and it would just drop right in with hardly any modifications needed. Bonus points if it's from a 2000-2002 Express van, because it would have the LS PCM already, but would be a Vortec 350. The only change you would need to do to the Suburban to support that is a few simple wiring splices cutting the stock engine/trans/pcm harness out of the system and then splice the whole new everything in. You would also need to just pull your stock sending unit from the tank and swap the fuel pump for an EP381(which you could also pull from the same donor vehicle). I think on my squarebody it comes down to something like 6 wires that need to be spliced into, everything else is internal to the engine/trans/pcm systems.

100% positive. It was an option in 1991 and it’s confirmed on the window sticker, and underneath the vehicle.

EDIT - I was wrong . See post 19-21

I won’t call mine a unicorn but it was in the transition period of being the last year of the square body suburban with a mixture of old and new tech.

 

[Bextreme04]

100% positive. It was an option in 1991 and it’s confirmed on the window sticker, and underneath the vehicle.

I won’t call mine a unicorn but it was in the transition period of being the last year of the square body suburban with a mixture of old and new tech.

Not according to the GM 1991 Suburban Vehicle information kit.
4L80 was only available for R models under 8600GVW. 8600GVW and above, and ALL V models got a 700R4 only. They had the same RPO code:
https://www.gm.com/content/dam/comp...on-kits/chevrolet/1991-Chevrolet-Suburban.pdf

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Also, according to the info kit... you couldn't get the TH400 in the suburban either. No option to select any different trans, only standard equipage based on GVW and whether it is R or V. Which seems crazy to me too be honest. I would have thought the 4L80 and 700R4 equipage would have been opposite, but maybe they didn't have the adapters ready yet... no idea honestly.

 

[gmbellew]

they did change the 700r4 nomenclature to 4L60 around this time frame for the 1500s. that is how my 1990 v1500 transmission is named. then a few years later it became the 4L60e when it became electronically controlled.

I think the R/V 2500 could get the 4L80 (might have even been 4L80e?) around the same 89-91 timeframe. be weird if they used 4L60 in any 2500 applications, but maybe they did?

but I am not sure a 4L80 of any sort was a factory option on a R/V 1500?

 

[creich68]

Not according to the GM 1991 Suburban Vehicle information kit.
4L80 was only available for R models under 8600GVW. 8600GVW and above, and ALL V models got a 700R4 only. They had the same RPO code:
https://www.gm.com/content/dam/comp...on-kits/chevrolet/1991-Chevrolet-Suburban.pdf


Also, according to the info kit... you couldn't get the TH400 in the suburban either. No option to select any different trans, only standard equipage based on GVW and whether it is R or V. Which seems crazy to me too be honest. I would have thought the 4L80 and 700R4 equipage would have been opposite, but maybe they didn't have the adapters ready yet... no idea honestly.

Well I'll be dipped (Derek Bieri voice). I may have been wrong my entire child and adult life with this Burb. The RPO code in the glovebox is MXO but that means either one. I dug the window sticker out of the files and sure enough it only says 4 Speed with Overdrive, but doesn't specifically say which. Goes to show my memory is slipping now.

Also crawled back under the vehicle and noticed the electrical plug is only 3(?) wires, not a larger bundle that the 4L80E should have. Set the trans tag on the window sticker for reference. Rebranded 700R4 as 4L60? Keep in mind GM replaced the transmission in May 1996 under warranty (at 51k miles).

It does have an electronic speedo which I think is the cable on the other side? Who knows, my life has been turned upside down this afternoon lol

You taught me something today...

 

[creich68]

Bextreme04 - 16 bolt pan. Not an 4L80E. You sir were correct.

I feel like this simplifies things if it comes down to replacing an engine harness as a last resort.

 

[Bextreme04]

Bextreme04 - 16 bolt pan. Not an 4L80E. You sir were correct.

I feel like this simplifies things if it comes down to replacing an engine harness as a last resort.

The three wire connector is for TCC lockup. You can probably leave all of that in place and run a holley sniper system... but personally I wouldn't even mess with any of it unless you have a major failure. At that point I would do an L31/4L60E swap including all factory wiring and LS PCM. It will be far more reliable than any aftermarket EFI system. The real benefit here is that since you have a 91 4x4 already, you have the NP241 passenger drop T-Case that has a VSS sensor in it that makes such a swap a direct drop in replacement. Just change the fuel pump and everything else will bolt right in place. Strip out the old TBI PCM and wiring harness and splice in the new factory stuff. It should even be super simple to find a 97-2002 5.7/4L60 4x4 donor locally in almost any junkyard.

 

[creich68]

The three wire connector is for TCC lockup. You can probably leave all of that in place and run a holley sniper system... but personally I wouldn't even mess with any of it unless you have a major failure. At that point I would do an L31/4L60E swap including all factory wiring and LS PCM. It will be far more reliable than any aftermarket EFI system. The real benefit here is that since you have a 91 4x4 already, you have the NP241 passenger drop T-Case that has a VSS sensor in it that makes such a swap a direct drop in replacement. Just change the fuel pump and everything else will bolt right in place. Strip out the old TBI PCM and wiring harness and splice in the new factory stuff. It should even be super simple to find a 97-2002 5.7/4L60 4x4 donor locally in almost any junkyard.

I'm going to tinker with the factory harness a little more, but good info to know in case the current driveline bites the dust in the future.