I need some LS swap wiring help and yes I’ve done some searching here before posting. Most of the posts regarding swap wiring are how to modify an LSX wiring harness or how to mate an LSX harness with a thirdgen harness. My situation is not either of these conditions.
My car is a 1984 Trans Am and I am using an FiTech Ultimate LS EFI system for my LQ4 engine. This is a standalone system with sensors and ECU that will control the engine. I am now trying to figure out which wires on the stock C100 connector need to remain. Referencing the C100 diagram below, the smaller side of the connector will still be needed for headlights, turn signals and horn. It looks like the other side of the C100 has wires that I will not need anymore so I'd like to remove them if possible.





The first thing I am trying to figure out is how to connect the engine oil pressure and temp sensors to my gauges. I have two sets of oil and temp sensors installed - one set are stock LS sensors that connect to the FiTech kit and the other set are thirdgen type sensors for my Intellitronix digital dash. I assume the dark green temperature wire in the thirdgen harness connects to the thirdgen type temp sender (picture below) on the engine and that’s it. Is that correct?





My thirdgen oil pressure sender has three wires. I’m not sure how to connect this to run the dash gauge. Do all three wires need to be connected somehow? It looks like one of the wires might connect to the factory cowl induction setup which I won’t be running anymore. Should I just get a sender with a single connection?





I know I’ll need to keep wires for wipers, starter/power and ignition. Below are a few pictures of connectors that I did not label and am not sure where they originally went. Anyone know what these are and if I need to keep them? All of this wiring was from a '84 TA with a carb V8. This plug is connected to the oil sender wiring. Anyone know what that was supposed to go to?







I think these might have been connected to the carb - is that correct?






Not sure what this is for. Pretty odd shaped connector with a light blue wire.






I think these might have been connected to the distributor - is that correct?




Any help would be greatly appreciated. Thanks.

 

I have Holley HP EFI. Similar situation. I have hyperlinks to my methods and wire diagrams in post #1 of my build thread, including all my connections to C100 for an '89 Firebird. Pinouts might be a little different, but the concepts are the same.

https://www.thirdgen.org/forums/ltx-...-qwktrips.html

I wouldn't entirely remove any wires from the C100 harness. Just clip any unused wires with about a foot length and leave it in your harness. Those extra wires are handy when you need a firewall pass-thru some time in the future.

You don't need multiple coolant temp sensors. Just toss the two you have and buy a sensor for a '98 Camaro LS1. It has 3 pins. One goes to the temp gage, and the other two go to the engine ECM. Done.

There are several different "stock" LS oil pressure sensors that work in different ways. I don't know which you have. The F-body LS1 oil pressure sensor has a single pin in it (the other two pin slots are blanked out). That sensor can be used to operate the oil pressure gage in your dash. The LS3/LS7 sensor can't operate your oil pressure gage, but it can be understood by an engine ECM if the right voltage/pressure scale is used. I've done it both ways. Right now I'm using the LS7 sensor so I can datalog oil pressure, but that requires another pressure sensor to be installed for the dash gage.

I know for 1989 that the V6 and V8 oil pressure gages use a different resistance/pressure scale. The V6 gage will read high when coupled with a LS1 oil pressure sensor. The V8 gage will read a bit low but much closer to reality. If you use a stock 3rd gen sender, then make sure you use the right one for your gage.

 

There are ALMOST NO wires you will be able to eliminate from C100, especially if you retain the stock gauge wiring. And as said, might not hurt to leave them there anyway but just tie them back into the harness, for future-proofing.

You could use that stud & nut temp gauge sending unit, or a stock slide-terminal style. Problem will be more mechanical than electrical... the threads in the LSx blocks are different from the ones in our cars and therefore making that adaptation is more critical than "which sensor". Same thing applies to oil pressure. You might have to get pretty creative with brass T's and taps & dies.

2 of the wires in your stock OPSU are related to the fuel pump. It only takes 1 wire to work a gauge. Qwk is correct, there are 2 different OPSUs, the 6-cyl is for a 0-80 psi gauge and the V8 is for a 0-60. Get the OPSU for a 84 L69 or something and you'll be golden. Remember: match the sensor to the gauge, electrically; you don't have LSx gauges, so LSx sensors may or may not match it, but if the gauges you have are designed for SBC sensors, then those are the ones to stick with. Not sure what he's saying about using a LS1 coolant temp sensor for the oil pressure gauge though.

Those connectors in both photos are not related to the carb. Not sure what they are but I'm sure they're not for the carb.

The blue wire with the grey connector body is for the electric choke. That's 1 wire you CAN get rid of out of C100. I don't see it listed in the pinouts you posted though; you might have to just trace it.

Pink is ignition power to the dist. White is tach, goes to the (-) side of the ign coil. Obviously won't work w/ LSx motors.

 

It was a typo. Good catch.

 

Thought it might be something like that. I was pretty sure you didn't mean it exactly as it was.

 

I haven't worked with a FiTech system, but it has to be similar to the Holley as far as connections.
You just need to look at the terminals in the C100 list #4-#7 are the engine side harness. The early cars use almost all the wires still since there weren't many other wires for fuel injection, later cars have more wires that can be removed.
A4 - power for inside
A7 - ignition power, can be used to signal the FiTech the car is in run
B4 - Starter solenoid
D6 - Tach, will probably need to connect to the FiTech
E5 - Oil pressure for gauge
E6 - Fuel control,
F4 - ignition coil. This wire is unfused. Can be used for a different ignition source
F5 - coolant temp for the gauge
F8 - Generator/Alternator. You will probably need to connect this to the alternator, but you might want to look at QwkTrip post about turning on the alternator instead of using this
G4 - Fuel control,
G5 - This is also power for the inside of the car
I forget which wire does what on the early car without looking at the schematic for those circuits. You should only need one of the two, but I don't know how the FiTech is going to turn on the fuel pump.
If the FiTech uses stock LS sending units for the temp, then definitely get the 98 temp sensor, unless the intelletronix needs a special sender. It is a 3 wire, less parts to replace later. Also the OEM is better because it is longer. The aftermarket senders are always too short once you screw them into an adapter. Like the one you pictured, the sensor won't actually be in the coolant flow and will be slow to react and may be influenced by the ambient temperature in the engine compartment.

Honestly it is so easy to just remove the unused terminals from the C100. You pull the comb out and then just push the tang of the terminal with a small screw driver than they slide right out. It's not worth even shortening them.

 

Thanks for the replies so far everyone. Below is some more information on my install in case it sheds any additional light on the situation and some replies to what has been posted.

I removed the C100 connector from my firewall and infilled the hole when I had the bodywork done to try and make the engine bay cleaner. In hindsight that might have been a mistake. I was trying to get wiring out of the engine bay or away from heat sources since I will be running a turbo setup. I intended to run any needed engine related wiring through a new hole behind the engine. Stock wiring to the front of the car (for lights, horns, etc...) would be ran along the top of the fender like DSE did in their install here.

I moved the battery to the trunk and installed a Ford solenoid with a 1/0 cable to the starter. I will not be running power through the starter to anything else as it will only be hot to start the car. A separate wire will be run from the alternator to a distribution block somewhere in the engine bay and then back to the battery for charging.

QwkTrip
You're build thread is very organized and detailed. I've referenced it a few times already and it has been very helpful - thanks. I looked at the '98 coolant sensor you recommended. I'm not sure I want to use that though since it looks like that would require modifying the FiTech harness coolant plug which will void the warranty. The FiTech system came with a coolant sender which I installed in the front of the DS head. This required me to install a separate coolant sender for the gauges.




Sofakingdom
The stud and nut type coolant sender in the picture above can be installed in the LS block with an Autometer LS adapter kit: https://www.autometer.com/ls-install-kit.html
This sender is required for the Intellitronix dash. Maybe you were eluding to the same thing Scooter said where aftermarket adapters move the sender out of the coolant flow and may not reflect actual temperature.
I installed one of the adapters on the rear of the LS block so I can install a factory style oil pressure sender. Luckily the intake manifold in the FiTech kit is a high rise type so the factory thirdgen oil pressure sender can be installed vertically without extra fittings / adapters. The stock sender I have is the 3-prong type like in the picture above. Could I just run one wire from the sender to the gauge and not worry about the extra wires on the sender plug?

Scooter
I believe the C100 diagram I posted may be for a Camaro. I couldn't find one specific to the Trans Am so I posted on TGO for help and that image was provided. It is basically the same, but my C100 has a dark brown wire with white stripe in the G7 location. It goes to a relay on the firewall:



D6 - The FiTech kit has a tach wire from the ECU so no need for this wire.
E5 - I'm thinking I only need this wire on the sender to get the gauge to work and could remove the extra wires from the stock plug. Not positive though.
E6 & G4 - What do these wires do? The FiTech kit has a wire that provides 12V to the fuel pump. I have a Squash Performance dual fuel pump setup with a plastic LS1 tank.
F8 - Not sure if I need something connected to the alternator from the stock thirdgen wiring. Haven't even thought about that yet. Could be another obstacle I'll have to deal with later.
You have a good point about the aftermarket coolant sender. The sender will be directly above my turbo exhaust pipe so ambient temperature could skew readings. Only problem is the Intellitronix setup requires a specific sender like pictured.

 

That G7 brown/Wht wire has to do with the cowl induction flap.


D6 is the lead to the tach inside the car, you would connect that to the output from the FiTech
E6&G4, if your FiTech has a driver that will support two fuel pumps you can just leave those out, remove them. I seriously doubt it can drive two pumps unless it has a seriously heavy duty relay and some large wire, at least 12 gauge. From all I see the FiTech system is cheaper than the rest for a reason.
As for the alternator, if you don't hook up the field wire it won't charge. QwkTrip had a solution, hopefully he chimes in, I just read it the other day, but I don't remember it or where it was

I bought the Equuis sender adapters for my New Vintage gauges, which looks similar to your sender. I cut the Equuis adapter head down, then re-tapped the hole, 1/8" NPT deeper so the aftermarket sender is at the same depth as the factory sender.

 

I'll be using the FiTech 12V to a relay for the primary fuel pump. The dual pump setup has two relays - one each. The second pump is triggered by a hobbs switch. Each pump has its own heavy gauge wire to the battery and circuit breakers. Install for this part was pretty easy since I have my battery in the back.
Good idea about tapping the adapter deeper so the sender can go in further. I'll try that.

 

This works for the Delco Remy alternator from the LS1 Camaro. I don't know anything about the Valeo alternator from the Corvette.

post #884,
https://www.thirdgen.org/forums/ltx-...ml#post6172816

If you're ever looking for something that I tried then odds are there's a link to it in post #1 or #2 of my build thread. I couldn't find crap without that either! LOL!

 

Good deal, thanks!

It's not just my story, it's also the user manual to my car. It can be darn handy when I forget what I did -- especially when it comes to wiring!

 

That’s smart. I should’ve done a better job documenting things I’ve done in my build thread. I find myself working on the car a couple hours here and there when I have free time so I often forget to write things down and take pictures.
Wiring sucks. It’s definitely the worst part of this whole swap.

 

I'd like to encourage you to do the documentation now. If you think wiring sucks now, just wait until you have to troubleshoot an error after it's all built! Or you end up with an accidental electric short and, *POOF*, part of the harness goes up in smoke. You're doing an extensive rewire and handling a whooooole lot of wires... it's pretty easy to make a mistake without a blueprint!

I always sit down and make wiring diagrams before I do any wiring job on the car. It's part of the error proofing process. You'll notice in my build thread that all the diagrams are posted before I do the work.

 

^^^ This ^^^

Draw what you're going to build, build what you draw. If you build it different, update the drawings as you go. NEVER "assume" you'll just build something and document it later; it NEVER works right that way.

This applies to LOTS of things besides just wiring or just cars. It's an all-around good sound engineering practice and mental discipline.