@#$#$ Wiring harness at the firewall.

[57chevy]

Can anyone explain how to unhook the (94), engine wire harness at the firewall? Ready to drop motor and cannot get the frig'n thing apart!

Thanks in advance!
Chevy

[1BADAIR]

the ls stuff you unplug under the glove box
not sure about the older stuff

[ICEPICK]

disconnect5 the plug ins from the computer and then pull it out the wires from the dash out stay plug into computer on one side and the wires to the motor plug in the othere side, the computer is held in with a 10 mm. bolt. 2

easy as cheesy

[57chevy]

We've got all the wiring loose from the car up to the point of pulling it at the firewall right beside the brake booster. Doesn't appear to come apart at the firewall as we assumed, which would mean tearing the dash apart to unhook the harness at the fusebox and wherever it goes from there. Beginning to look like a nightmare here!

Thanks,
Chevy

[1BADAIR]

the engine harness should all be on the pass side
unbolt from pcm, pull the pcm and bracket out then remove the right kick panel and unplug 3-4 connectors

[meissen]

We've got all the wiring loose from the car up to the point of pulling it at the firewall right beside the brake booster. Doesn't appear to come apart at the firewall as we assumed, which would mean tearing the dash apart to unhook the harness at the fusebox and wherever it goes from there. Beginning to look like a nightmare here!

Thanks,
Chevy

Hope I'm not too late to the thread.

Before I start, an FBody is made up of 3 harnesses that can be disconnected from each other:

• Engine Harness - all the wiring that runs to all the sensors on the engine and INCLUDES THE PCM PLUGS.
  • The engine harness connects to the engine bay harness on top of the passenger side shock tower. In 1994, there was only 1 plug. In 1995 to at least 1997, there are two plugs.
  • The engine harness connects to the interior dash harness through the passenger firewall grommet. 3 plugs are behind the passenger kickpanel. There are wiring differences so while the plugs WILL plug into eachother, the wires on each side of the plug might not line up or be the same on both sides.

• Engine Bay Harness - this wiring is mostly just the air bag sensors, turn signals, headlights, horn, engine bay fuse block, cooling fans, etc.
  • The engine bay harness connects to the interior dash harness through the driver's side firewall under the brake booster. The connection at the firewall is misleading and is not the disconnect location unfortunately. The main connection is C200 and is located somewhere around the interior fuse block.

• Interior Harness - Everything from the dash, doors, and taillights.

The harness that goes through on the driver's side by the brake booster is the "engine bay harness" which does NOT have to be removed to drop the engine.

On the 1994, you only will have one plug on the passenger side shock tower, and then three plugs that go into the passenger side firewall grommet and then connect behind the passenger side kickpanel inside the car. That's it! You can leave all the wiring in the engine bay alone and drop the engine out after you disconnect those 4 major connections.

Now, whether you need to replace the engine bay harness or not is up to you. Honestly, I was able to get everything working while leaving the engine bay harness in place. I had to make 4 wiring changes so far - 2 wires for the battery feed to the PCM, power to half the injectors, and the data serial cable.

[Phantom409]

If you unplug everything you can get the harness out of the way to drop the motor with out having to go through all that hassle.

I pull mine from the top and never do anything but unplug all the stuff from the motor.
Just set the harness off to the side and your gold.

[57chevy]

Never too late to get in on one of our help requests! Thanks for taking the time.

This whole project is quickly becoming a nightmare. You folks offering advice is very much appreciated.

Here's what we've found so far as real problems -or perceived problems, that are piling up to the point -we are about ready to turn it into a stock car!

94 TA donor car (running)
99 V6 Firebird (project car)

Feel free to correct any points I make -please!

Our original intent was to use the computer from the 94, the engine harness firewall forward and change gauge cluster if not compatible with 99.

The ABS system has different plugs and computer system, appears that we'll have to swap it with the complete engine side harness (including the one going through the firewall), to keep it functional.

The AC also appears to be completely different at least from the evaporator out. Hoping we can use all lines and system from 94 in 99 using the original 99 evap.

GM factory alarm and key chip and computer issues. The computer I think would have to be reprogrammed to use the key from the 99 with the 94 computer. Or we'd have to swap the ignition switch and possibly the column as well.

At the end of the day yesterday, we'd pretty much decided we would have to pull the complete harness from one end to the other from the 94 and swap it out in the 99 to keep everything working. Pretty depressing thought since it would mean disassembling both cars.

The other option since I'm an old hot rodder was to replace all the gauges with custom units, rework the ignition switch to go old school on the the ignition switch eliminating the GM alarm and computer tie in. Using the computer only to run the fuel injection (likely a nightmare all by itself), ABS and AC -not really what we had in mind at the start of the project.

Any help and advice is more than welcome! At the rate we are going with this -I may have to give someone a Florida vacation!

Chevy

[meissen]

This is a real hard one - I've never really researched 98+ v6 to 93-97 LT1. Unfortunately lots of changes and differences but it should be doable. You're ahead of the game by having both cars. My comments are below in red.

Never too late to get in on one of our help requests! Thanks for taking the time.

This whole project is quickly becoming a nightmare. You folks offering advice is very much appreciated.

Here's what we've found so far as real problems -or perceived problems, that are piling up to the point -we are about ready to turn it into a stock car!

94 TA donor car (running)
99 V6 Firebird (project car)

Feel free to correct any points I make -please!

Our original intent was to use the computer from the 94, the engine harness firewall forward and change gauge cluster if not compatible with 99.
Leave the dash in - the 97+ dash is an upgrade compared to the 93-96 dash. The specific gauge cluster in the 99 might not work, though, and you might need to get a 97 gauge cluster... I'm not sure if there are gauge cluster harness differences between 97 and 99, but 97 was an LT1 year with the upgraded dash.


The ABS system has different plugs and computer system, appears that we'll have to swap it with the complete engine side harness (including the one going through the firewall), to keep it functional.
Personally, that's where I'd say function is outweighed by convenience. Is the ABS that critical to have? Most people remove them, anyway.

The AC also appears to be completely different at least from the evaporator out. Hoping we can use all lines and system from 94 in 99 using the original 99 evap.
The A/C condenser in front of the radiator should be the same I believe, but the canister and hoses are going to be different.

GM factory alarm and key chip and computer issues. The computer I think would have to be reprogrammed to use the key from the 99 with the 94 computer. Or we'd have to swap the ignition switch and possibly the column as well.
As long as you keep the column, ignition switch, and BCM from the 99 you should be OK I believe. In 98+ I think the VATS system was integrated into the BCM, so as long as the key matches what the BCM is looking for, you should be gold. We put in the LT1 and did not run into any security issues because the key still matched our VATS system under teh dash - regardless of the different PCM.


At the end of the day yesterday, we'd pretty much decided we would have to pull the complete harness from one end to the other from the 94 and swap it out in the 99 to keep everything working. Pretty depressing thought since it would mean disassembling both cars.

The other option since I'm an old hot rodder was to replace all the gauges with custom units, rework the ignition switch to go old school on the the ignition switch eliminating the GM alarm and computer tie in. Using the computer only to run the fuel injection (likely a nightmare all by itself), ABS and AC -not really what we had in mind at the start of the project.

Any help and advice is more than welcome! At the rate we are going with this -I may have to give someone a Florida vacation!

Chevy

[Phantom409]

If you are going 94 LT-1 into a 99 you are going to have issues because they had totally different ignition systems to start with.

The Opti runs using an optical sensor inside the Opti to fire the coil vs a crank trigger system on the LS cars.

You are going to need the harness and gauges from the 94 to run the LT-1 considering the 94 is OBD 1.5 and the LS1 is OBD 2+

You might be better off going with a FAST or Accell stand alone system and running that to control the fuel injection but it's not cheep

This project is going to take some time and research!
Never heard of anyone going LSX to LTX though

[meissen]

I don't think that's an issue though because he has the engine wiring harness with the PCM connected to it. If he tried using the 99 harness on the LT1, then yeah... but otherwise the engine component harness is only giving the engine harness power for the injectors... the rest (in regards to the ignition system) is all self contained in the engine harness.

If you are going 94 LT-1 into a 99 you are going to have issues because they had totally different ignition systems to start with.

The Opti runs using an optical sensor inside the Opti to fire the coil vs a crank trigger system on the LS cars.

You are going to need the harness and gauges from the 94 to run the LT-1 considering the 94 is OBD 1.5 and the LS1 is OBD 2+

You might be better off going with a FAST or Accell stand alone system and running that to control the fuel injection but it's not cheep

This project is going to take some time and research!
Never heard of anyone going LSX to LTX though

[Phantom409]

Here again B your going OBD 2 to OBD 1 so the computer is going to work different and the 99 car will have different connectors on it.

The ABS system has different plugs and computer system, appears that we'll have to swap it with the complete engine side harness (including the one going through the firewall), to keep it functional.

The AC also appears to be completely different at least from the evaporator out.

You can delete the ABS unless you plan on winter driving.

You would just have to add a paportioning block in for the rear breaks.

I think you are going to have to get into the dash though
If you are running the LT-1 in the 99 you are going to need that harness, there is no way around that in my opinion.

I do know that there is a retainer ring around the area where the wires go into the fire wall and it has a 7 or 8 mm screw that holds it I believe.
I will go look at the 93 that I have a part in my garage.

[meissen]

Here again B your going OBD 2 to OBD 1 so the computer is going to work different and the 99 car will have different connectors on it.

Right - and the OBD 1 computer is only using the engine harness for the optispark. The OBD 1 engine harness gets power to the injectors from the right hand shocktower plugs and those might be different but again if you're using a 94 computer and a 94 engine harness on a 94 LT1, the optispark wiring should all be self-contained within that 94 engine harness.

[Phantom409]

Right - and the OBD 1 computer is only using the engine harness for the optispark. The OBD 1 engine harness gets power to the injectors from the right hand shocktower plugs and those might be different but again if you're using a 94 computer and a 94 engine harness on a 94 LT1, the optispark wiring should all be self-contained within that 94 engine harness.

Correct!

But it sounds like he is trying to use the 99 Harness and that isn't going to work on a OBD 1 computer.

[meissen]

Correct!

But it sounds like he is trying to use the 99 Harness and that isn't going to work on a OBD 1 computer.

Ahhhhh alright - now we're on the same page. I had interpretted the below quote that they're using the computer, harness, and everything from the firewall forward from the 94.

Our original intent was to use the computer from the 94, the engine harness firewall forward and change gauge cluster if not compatible with 99.