Terminator X Conversion - 1991 TBI

Over the last ~2 years I've had a general auto repair shop make some repairs to the engine harness and throw parts at a code 42 issue. This issue appears to be mostly fixed *knock on wood*, but I fear the harness repairs will lead to other issues down the road.

A local auto wiring/fab shop that has rewired another older vehicle for me in the past has given some options and I'm at a crossroads...

1. Sell the vehicle and move on - the downside is sentimental as the 91 burb has been in the family since new, and for all original, it's in really good shape inside and out.
2. LS swap - hate to do this on an all original vehicle. Also not cheap to have a shop perform the swap $$$, assuming the original 4L80E will bolt up and survive the power increase.
3. Terminator X system to replace the factory TBI/PCM setup. Less money, keep the original drivetrain for maintaining "matching numbers". In theory the terminator can control both the engine and trans.
4. New engine harness - I have scoured the interwebs and no one sells a replacement factory harness. The wiring shop in theory might be able to make a new one, but how much labor would it take to accomplish that? Probably as much or more than the Term X conversion. I've checked some salvage yards and what few squarebody's I find, their older models that don't have the same harness or they have been mutilated over the years. Unfortunately the only harness that would work for my burb is the 1991 year due to it being the 5.7/4L80E combo. Even if I found the unicorn harness, it would be a 35 yr old harness that may or may not have issues that aren't visible to the naked eye.


Let's choose #3 for arguments sake:

New computer and associated harness for engine/transmission control. Questions:

A. 1991 has the DRAC module for converting speed signals for the factory ECM, cruise control, ABS, speedometer. The Term X does not have provisions for cruise control operation. Can the DRAC module stay in place and function for cruise/ABS/Speedo even though the factory computer would not be there anymore?
B. If the DRAC module cannot stay, what would be needed to make the factory cruise/ABS/Speedo function? I know Dakota Digital makes a new cluster that would be able to pull all the necessary info from the Term X computer (including Speedo), but that's an added ~$2k and still doesn't address the cruise/ABS items.
C. Has anyone attempted this kind of conversion? I'm not looking for more HP/torque. I just want a reliable drivetrain control system and wiring I'm not worried about every time I physically touch the harness.


The deeper I go down this rabbit hole and reading various forums, the more my brain hurts. Open to thoughts, suggestions, and guidance on option #3 if that's a viable route.

#1... don't sell it because someone can't figure out the issue.

#2... I don't see the need to go fishing with dynamite.

Imho, worst case, a harness like Rick posted should suffice. Also, the Terminator X is for multi port FI systems with a cam and crank sensor. I don't think it can run a TBI.

Ricko - I looked at the same harness late last week. I should probably reach out to Speedway to see if there is any issue with my year having the 4L80E vs other trans in previous years. I know that may change the pin outs on the computer side of the harness. Worth exploring a little deeper. I have a set of GM service manuals, including the one with all of the wiring schematics for all systems. Maybe between this particular harness and the wiring schematics, the local wiring shop could make it work.

Bucket - the Term X option would include the Holly “TBI” unit.

I’ve had a couple shops in the Raleigh area work on it over the last two years when trying to resolve the code 42 issue. Unfortunately getting hard to find someone familiar with these older computer/TBI systems.

It may be worth mentioning, there's a couple members here that run the Edelbrock Pro Flo 4 and swear by it. Iirc, it's @SquareRoot and @Frankenchevy

Although I'm unsure if it offers transmission control.

You COULD run a terminator and turn the distributor into a cam and crank sensor, its how I used my HEI on my big.block to send the holley crank pulses... problem is none of the terminator ECMs have the tooth wheel count the tbi distributor has. You'd be better off with a holley sniper to replace the TBI.


At best you could attach a crank wheel on the outside of the crank shaft for your tooth count and fab up a cam sensor too. Nada a good idea. No trans control either. Assuming you have a 700r4. No trans control obviously with a termX non "max" option. At best you can control when lockup happens based off a ground signal.

The LS thing just is a poor solution. Like he said dynamite to go fishing. Dont cut up the 91. I have a strong love hate for LS though. Dont mind that.


Code 42 is usually ignition module, the coil, coil grounding, or the lame wiring from coil/distributor to the ecm/ground or what not.

Me and @gmbellew are both TBI suburban owners and still have our same 350/700 combo, still keeping the original ecm and management. If you need help we can probably help you. Both of us have obd1 diagnostic tools. Both of us have faced code 42. I personally live in salisbury not far from you OP.

Vbb199 said:

Me and @gmbellew are both TBI suburban owners and still have our same 350/700 combo, still keeping the original ecm and management. If you need help we can probably help you. Both of us have obd1 diagnostic tools. Both of us have faced code 42. I personally live in salisbury not far from you OP.

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yes, code 42 is tricky bc only a few things will set it and those can be hard to diagnose because they are usually not obvious.

if the shop was repairing and splicing the harness properly, and everything works right now, it dont know that I'd just rip out and replace the harness as a preventative measure.

Vbb199 said:

Me and @gmbellew are both TBI suburban owners and still have our same 350/700 combo, still keeping the original ecm and management. If you need help we can probably help you. Both of us have obd1 diagnostic tools. Both of us have faced code 42. I personally live in salisbury not far from you OP.

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I'm hoping that the code 42 issue has been resolved *knock on wood*. There were two shops working on the code 42 issue (sister shops but different techs). The first shop focused on the parts side - distributor, ICM, computer. They went through multiple AC Delco units because they felt some of the replacement parts were not functioning properly. Efforts then switched to harness between the engine/computer when the second shop thought it was more harness related.

gmbellew said:

yes, code 42 is tricky bc only a few things will set it and those can be hard to diagnose because they are usually not obvious.

if the shop was repairing and splicing the harness properly, and everything works right now, it dont know that I'd just rip out and replace the harness as a preventative measure.

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100% agreed after the experience of the last two years. The last tech to work on it was a 70 yr old retired GM tech now working for a local shop part time. No code recently. Runs like a swiss watch when cold. After getting up to temp (switching from base file to sensor readings) and idling for a long period of time at a light, the idle gets slightly erratic. Most recently, when driving for awhile then shutting off for 5-10 mins, when starting back up it will struggle to stay running at idle. Stumbles/sputters and if I rev it up once/twice, the the idle returns to normal. The only thing I have done since October was replace the EGR valve when the light came on and replaced the vacuum line from the TBI to the MAP sensor because it was the original dry rotted line. I checked the EGR this weekend and it's holding vacuum. Line is clear and the EGR solenoid was bench tested to make sure it's working properly. As far as I can tell the EGR is staying closed at idle like it's supposed to.

The most recent idle issue may be completely unrelated to the past code 42 issues. I currently have an appointment with the shop next Monday to take a look into the idle issue.

bucket said:

It may be worth mentioning, there's a couple members here that run the Edelbrock Pro Flo 4 and swear by it. Iirc, it's @SquareRoot and @Frankenchevy

Although I'm unsure if it offers transmission control.

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The generation I have does not. Not sure if they have something newer. Also not sure if they have a TBI pattern manifold.

This will do 4l60e or 4l80e

Ricko1966 said:

This will do 4l60e or 4l80e

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Thanks for sending Ricko. It was a brief moment of excitement. I just spoke with Howe and the person on the phone was adamant that their harnesses are only for TBI engine transplants and not for factory replacements. She mentioned that they have been getting many phone calls from folks looking for a factory harness replacement but they're not aware of anyone in industry currently making one.....I wanted to say "Maybe you should make one!?!". She suggested salvage yards but I've found that is a needle in a haystack for 1991 burb/blazer.

Looking at the harness, it looks like it has all of the correct plug ends. I wonder if it could be retrofitted to an existing square body.

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.

creich68 said:

Thanks for sending Ricko. It was a brief moment of excitement. I just spoke with Howe and the person on the phone was adamant that their harnesses are only for TBI engine transplants and not for factory replacements. She mentioned that they have been getting many phone calls from folks looking for a factory harness replacement but they're not aware of anyone in industry currently making one.....I wanted to say "Maybe you should make one!?!". She suggested salvage yards but I've found that is a needle in a haystack for 1991 burb/blazer.

Looking at the harness, it looks like it has all of the correct plug ends. I wonder if it could be retrofitted to an existing square body.

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I have no doubt I could make it work. But we all have different skill sets. All the under hood 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 for 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.

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.