85TAracer

iTrader: (5)

Hey guys,
I have owned a 1985 Trans Am for 7 years now and I have decided to swap in a 1995 LT1/T56 from a Z28. I have done lots of searches to help me with the swap but I have a lot of questions still... mostly I think I overthink and confuse myself so I will ask a few questions at a time and those answers should help me figure out the other questions on my own.

1.) LT1 PCM location - I see people put them all over the engine bay. I was thinking about putting it in the AC box where the evaporator used to be.. this should keep it was from engine heat.. I will still have a blower hooked up for defrost.. it looks like I can still get some flow around the PCM to make it work.... opinions on this?

2.) MAJOR QUESTION here - Ok, my car was originally a 305 carb automatic.
I need to know what harnesses I am splicing into. I have read about what wires to use, etc. I am pretty much ok with that. BUT where am I hooking into? I have an engine bay harness, instrument cluster/dash harness, and a TBI harness from a car I parted out. And I have the complete uncut LT1 harness. From the LT1 harness it looks like I have a good length to bring the connectors into the car where the stock PCM came in. Do I splice these into the dash harness via the TBI harness? ( I believe these are the guage wires)

3.) Fuel pump and electric fans - My car originally had a mechanical fan, I don't have any relays or such.. I guess I can just splice into the harnesses and wire them in with some help as to which wires go where...

4.) Fuel pump - Since the car was originally carbed I have no pump in the tank... I have the tank from the 95 Camaro with a good fuel pump. Can I put that tank in the car and try to wire it into my harness so it will run like it did originally? (meaning key on engine off pump cycles, engine start pump runs cont.)

Ok no more questions till I get these figured out and wired up.

I appreciate the responses.

Thanks,

Tim

Pocket

iTrader: (24)

1) The LT1 PCM is weather proof. It can take nearly any environment you can throw at it. You dont have to shield it inside the HVAC box

2) Decide what dash setup you want to use. Id suggest using the TBI dash harness for the electric speedo and full fuseblock. The carb harness will require a speedo cable driver box ($300) and a secondary fuseblock/relay center to be added. Once you pick one, cut down the engine harness to just the gauges as laid out in the prep thread. The remaining wires will be your splices to your gauges/fuseblock

3) The LT1 has no provision or possibility for a mechanical fan. You will have to swap in some form of electric fan. Thankfully, OEM single fan setups are more than capable of cooling the new engine and fully compatible with the newer PCM. Other alternatives would be 3rd gen dual fans or even later 4th gen fan setups. The 4th gen setups require minor fabwork to fit, but function and control are the same

You will have to add a fuel pump relay with the 85 carb harness. This includes running the long wire all the way back to the fuel cell
The TBI harness will come with a suitable fuel relay and necessary wiring back to the tank

4) You can use an external fuel pump if you like, but the cheapest route would be to drop the tank and use an EFI sending unit. V6, TPI, TBI whatever, its all compatible. You will have to update the fuel lines to EFI standards with the new sender, but youd have to swap them to do the swap anyways. The 95 tank will fit right in there. Some trimming around the neck is needed for a perfect fit. Fuel lines will be about 2 feet short, but again youd need to update them to do the swap regardless

Use the PCM control for the pump. It will run it when its needed. Its quite simple

The more you know, the easier the swap will be

85TAracer

iTrader: (5)

Thanks for the info. I am reassembling the car from the bare unibody up, so running whatever needs to be run isnt a problem. I am using all new steel fuel lines and plan to merge in the plastic lt1 lines somewhere under the drivers seat. I do plan to use the 95 tank now that I know it will fit. (they looked the same)

I cut down the TBI engine harness, and then looked at my original harness and noticed that some wires are different or ran in different harnesses (such as the wiper/washer) So i think if I tried to modify my instrument cluster harness (stock) I will end up with problems at the C100 pins. I guess I will look for a dash harness from the same TBI setup/year.

I was going to stuff the PCM in the HVAC more for astetics than protection...

I was planning to use electric fans all along, and using a TBI harness will be the best way to handle that. I have a single stock fan, and dual LT1 fans... I can cross that bridge later.

I will source and organize my harness situation and report back..

Thanks for the response, from what I am reading you are the guru.

Tim

85TAracer

iTrader: (5)

Update: For various reasons, including very cheap price, I just bought a complete 1989 GTA with a 350 TPI. Besides having SLP runners everything appears stock, and the wiring is intact. I plan to part the car out, and am thinking of using the dash and engine harness for my LT1 swap unless someone comes up with a complete uncut TBI harness(dash and engine) that has foglights. (or someone talks me out of cutting this harness)

Pocket

iTrader: (24)

Find an 89 V6/TBI engine harness and sell the TPI harness with the engine

The dash harness isnt excessively modified in the swap and the engine harness is stripped to basics. Everything V6/TBI/TPI about the harness is removed

The buffer box needs to be bypassed to get the speedo working with the new PCM and the clutch switch needs to be added for auto to T56 swaps

85TAracer

iTrader: (5)

10-4 on selling the harness. I am swapping the LT1 into an 1985 TA, which is why I needed a dash harness as well. I have a TBI harness that I cut down, but upon close inspection the C100 is different than the 85 Carb harness.. ie the wiper motor and squirts run through the lighting harness in the TBI harness, but originally ran in the engine harness of my 85... so the pins won't line up.

So, Thats why I figured on getting the whole harness from a car..

Is 89 V6 the easiest to convert or were you just referring to the year of the car Im parting out and keeping it the same?

Pocket

iTrader: (24)

Year and original trans type needs to match up

Year sets:
90-92
89
86-88 EFI
85-87 carb
85 EFI
Pre-85

85TAracer

iTrader: (5)

Alright, so basically my options are to find an 85 efi engine harness, or find a V6 TBI car to part out and use the whole engine and dash harness. Finding an 85 TBI harness may be harder than it seems... plus my car was auto to begin with so I would have to do extra modifications for the pedals, reverse, etc.
So the 87 firebird V6 5 speed for sale on craigslist for $1000 obo might be worth looking into if I can part out the rest of it to break even on whatever I pay for it. (not $1000)

Pocket

iTrader: (24)

Find your engine/dash harness combo in a JY. If they want more than $50 for the pair, pick a new JY

V6 didnt come in TBI and TBI was 88-92

85TAracer

iTrader: (5)

Update: a long time in coming. I have a complete wiring harness from a 87 firebird V6 5 spd. (engine harness, headlight harness, dash harness, body harness to fuel pump/taillights) I prepped the engine harness, and am in the process of building the LT1 harness. I stripped it down per your instructions found here, I will not be running A/C or any emissions. Can I leave out the wiring for the EGR solenoid and the canister vent valve? Also, am I correct in thinking I need the HO2's for it to run properly? Where would I mount my HO2's? I have long tube hedman headers. add bungs into the collector or add bungs to the beginning of the exhuast pipe? (I can extend the wires if ness. because they are already cut from the PCM, so I want to set the length so I can splice them back in)

Oil pressure sensor - I have a lead from the 3rd gen harness for my guage, can I just splice the LT1 connector into it or does the PCM need a reading from it too?

Tach signal wire - Can I just "T" into the signal wire near the coils/module or do I "T" into the wire from the PCM?

These are enough questions for now. Once I get further I'm sure I'll have more.

Pocket

iTrader: (24)

Use the 87 dash harness in the 85 dash to save alot of headaches

EGR and EVAP can be left out if you're not building an emissions friendly swap

Front HO2s are required for proper PCM function. Some have gotten away without them, but why bother. Lengthen as necessary. Try to keep the O2s reasonably close to the collectors if not in them. Moving them a few inches down into the Y-pipe or beginning of duals is fine, bu8t try to keep the distance from the collector flange pretty close. you dont want one in a collector and the other two feet down the Y-pipe. Readings will be all sorts of screwey

I recommend swaps use the LTx/LSx oil senders instead of retrofitting their originals onto the new engine. The main reason is simplicity. The new sender is compatible with your stock gauge, so simply connect the new connector wire to the proper pin in your C100. Most of the later 3rd gens use the same sender connector as well, only with 3 wires incorporating the fuel pump relay bypass switch. I cut these out and run only the single wire to the gauge

The PCM tach output is quite a bit more accurate and stable than the signal from the coil. Most LT1s dont even have a coil tach wire, the connector generally has an empty pin. Ive seen a few B-bodys with them though. Run the PCM output to the tach wire in your C100

85TAracer

iTrader: (5)

Thanks for the info and quick response. This project is going better than expected as far as actually doing the work, and knowing what goes where. Once you study and research enough things you start to memorize what everything is. -- makes it simple and easy.

Another question about the LT1 harness: (I am building it on the engine 30 mins at a time after work)

Injector harnesses: I cut out all the commons as instructed, now I am joining the pink power wires from the injectors.. I have spliced all 4 together and am running one wire out of the splice into my harness towards where the power will come from. I have done this on both sides. Do I run these to whatever I make as my fuse box and use 2 seperate fuses just for powering injectors? If so what do I use as a fuse? Also, I will save this question for when I am actually making the fuse box/ connecting powers, but what power source would I use? one linked into the PCM somehow or just switched power from battery source?

Thanks a lot for the help, this would take a lot longer with more errors if it wasn't for your help!

Tim

Pocket

iTrader: (24)

Splice all 4 injectors on each bank together. Left bank will go to the INJ 1 fuse and right bank INJ 2

See if this helps place the fuses


For an external fuseblock from stock you will want an IGN relay. Use either the big pink coil wire from the C100 or some emissions fuse wire from the C100 to engage the relay

The PCM requires both 12v constant and 12v IGN power wires, both will have to be ran correctly for proper long term function

85TAracer

iTrader: (5)

I have the "stock" fuse box for under the dash, but for the engine bay I have to put something together. I have the 3rd gen bracket with 2 relays on it and associated wiring to go to power, fan and fuel pump, I assume to run the dual fans I got from a tpi car I need to add an extra relay... but that can be looked into later.

I have the LT1 engine bay fuse box cut from its harness, with enough wire sticking out I could splice it into my system.. I do not have a manual to reference LT1 wiring from, if someone has the fuse bock diagram I could probably just use the factory box?

I would like to try and finish the wiring this weekend while I have the time, so I can move onto the next phase.

Has anyone used the "stock" LT1 fuse box under the hood? where did you mount it? Near the battery seems to be the most logical spot to me... maybe make a bracket off the core suport next to the battery?

85TAracer

iTrader: (5)

Hey guys,

I still need help anwsering the questions in my post above, and these additional questions:

Wires left coming out of the C100 -

Big purple wire - goes to terminal on starter

2 big red wires - I assume these are both 12v power supply, but where do they tap into the battery power? Just wire directly to the battery?

Big pink with black tracer - this is the ignition wire? where does it go? (used to go to coil on dist.) I don't think I have a Tach wire coming out of my C100, could it be routed via the ECM harness? (stock)

small brown wire - I believe this wire gets connected to the brown wire on my fan relay?

I have a few questions about the fuel pump relay but I can leave it til later..

Thanks,

Tim

Pocket

iTrader: (24)

Wire colors dont help a whole bunch, pin assignments are better. Check out the additional info thread in the LS1 sticky, all C100s are posted

Best guesses

Big ppl is starter solenoid

Reds are main battery power to the IGN switch, fuseblock and lights. Connect these to 12v constant whether its on the starter lug or a remote power distribution block

Big pink is old coil IGN, this is your 12v switched source if adding a new fuseblock, otherwise it runs the HO2s

BRN, either alt/fan fuse, or oil gauge

See if this helps with the fuel

Firebat

iTrader: (7)

You may want to look into junkyards/savageyards and swap the entire fuel lines and sending unit from a fuel-injected thirdgen.

85TAracer

iTrader: (5)

Pocket,

These wires are about what I expected. I do plan to make a new underhood fuse box. The box will be for injector fuses, HO2S, etc.. So if I use the pink/blk wire (old coil) do I need to up the fuse its on?

I am running a power dist. block.

On a side note about 12v power whether switched or direct, do I still need to run the fusable links that were in the stock thirdgen harness? or just make sure all circuits are fused?

For the fuel pump relay - diagram makes sense. Question, what is the red wire for? Also I know the gray wire goes to the fuel pump but I do not have a gray wire coming from my C100. Did it go through the stock ECM, through the dash and then connect into the body harness? I am thinking of running the wire that way because my relays are by the battery (as seen below)





I am running all new lines. There are currently no lines in the car as you might deduce from the photos. The car is on its ways back together. I have a gas tank with pump/sender from the 89 GTA I parted out. I also have the tank with pump/sender from the 95 Z28 I parted out for the LT1 and T56.