3800 sc swap engine wiring harness

[SamDeegan]

New here. Have a 90 beretta gtz I'm piecing together a 3800 sc swap wondering how to do my enging harness. I would love some help from you old heads out there.

[Rettax3]

Hello, and welcome to the site! Was your GTZ a Quad-4, or a V-6?

On my '89 GTU, I piggy-backed the 3800 harness and left the original wiring intact, actually completely untouched aside from one ground wire from the fuel-pump (my Beretta from the factory ran a second ground-wire from the sending-unit, yours does not, and my 3800SC donor used a multi-speed fuel-pump control unit which I later opted to install, this required the use of that second wire so it had to be cut and spliced into.. Not all 3800s use that type of secondary fuel-pump control unit, and the car ran fine without using it too, hopefully your donor doesn't use this setup since you have a three-wire sending-unit).
Using the 3800's ECM as a standalone had some advantages, but also a few compromises. I had little option but to piggy-back, since my GTU is a digital-dash car (on the digi-dash Berettas, the ECM runs the signal outputs to the cluster directly, so removing that computer would essentially kill the gauges' function), but neither my MPG function nor my factory tach ever worked after the swap. A Dakota Digital (brand name) tach signal converter would certainly be a solution to my tach problem, but I had installed a host of aftermarket analog gauges to supplement the factory cluster, and have a good tach in that set so I just never got around to it. Look into Dakota Digital tach converters if you want to retain your original tachometer, it may be necessary.
Some questions that may help pin-down what specific issues you may face follow...
-Are you swapping in the 3800's transmission as well?
-Was your car originally a V-6 auto?
-Do you mind cutting/splicing into your original harness, or are you like me and want to be able to put your car back 99.99% to stock if the notion so strikes you in the future?
-What year/make/model donor are you using?
-Are you building a fresh harness, modifying your car's original harness (hardest option), or piggy-backing/tapping into the donor 3800 harness?
-Are you using the 3800's ECM/PCM, or something else (Mega-Squirt, custom-burned EPROM chip in your Beretta ECM, etc)?
-Do you have access to decent wiring schematics (Alldata, Mitchel, OEM Service Manual, etc) or at least half-a$$ed schematics (Haynes, Chilton, etc)?
-What is your Build time-frame? My GTU went together pretty fast -I spent more time dinking with different exhaust setups than I did on the actual engine install and five-speed swap. But it still wasn't a weekend swap either.

I am not on the Board much these days, not enough activity to justify checking in frequently, and PM notifications have been hit-and-miss for at least some of us, but if I get pinged on a question, I'll try to respond. There are a few other 3800s swapped-in here too, so someone else may chime-in as well. Good luck, be sure to share pics of your car and the work!