Where to put the PCM???
 

Hi guys,

Where is everybody putting their PCMs on the 64-66 trucks? I have a 65 GMC 5.3 swap going on and I'd like to hide all the wiring but am also adding Vintage Air so space is limited. Please post up your pics and suggestions, thanks!

Joe

Re: Where to put the PCM???
 

Put mine under the seat

Re: Where to put the PCM???
 

And the fuses and relays? Do you have carpet over all the wiring? Any pics?
Thanks

Re: Where to put the PCM???
 

I'd go under the seat if possible with everything..

Re: Where to put the PCM???
 

Orignally had mine mounted on the inner fender well with fuses etc, but when I rebuilt my truck and ditched the inners, we relocated all the ecm stuff to the passenger upper footwell area. May not be an option since you'll have a/c ducting.

Re: Where to put the PCM???
 

For those that are locating them under the seat on a 64-66 what harness is long enough, aftermarket? Rewired OEM? I would appreciate any pics you have.
Thanks

Re: Where to put the PCM???
 

I don’t have pictures but wiring onder carpet made my own harness

Re: Where to put the PCM???
 

I've used PSI on all my swaps...their harnesses have 4-5 ft of wire from the back of the head...I contacted several of the "store bought" harness makers and none would make a longer harness.....with the LS swap being so popular ,you would think someone would offer a long harness...if you build your own you can make it however you want....I know PSI is coming out with a DIY kit...
Of all the engines I've bought I have yet to get the factory harness that wasn't damaged in some way....

Re: Where to put the PCM???
 

5ft mite be long enough

Re: Where to put the PCM???
 

Ask for the Van harness instead of the truck harness. It adds length to the main trunk of the harness.

Re: Where to put the PCM???
 

Call BP Automotive. Their harness is made to put the PCM under the seat. The one for my 71 is more than long enough. The harness is a work of art as well. Labled and easy to install.

Re: Where to put the PCM???
 

Thanks 71 step...Wish I knew that before....BP is one I didnt contact...
and thanks brew...I'll have to remember that next time about a van harness

Re: Where to put the PCM???
 

I've got computer below the radiator and the relays are on the inner fender tucked up against the core support. Makes for a nice and seamless install that doesn't draw a lot of attention to the things that aren't supposed to be there.

https://farm5.staticflickr.com/4164/...3a84c5ea_c.jpg

https://farm5.staticflickr.com/4193/...82bed74a_c.jpg

https://farm1.staticflickr.com/836/3...de2d4b4f_c.jpg

Re: Where to put the PCM???
 

Mounted mine right behind the cluster, right above the parking brake assembly. Fits snug but it fits.

Re: Where to put the PCM???
 

Who sells a converted van harness??

Re: Where to put the PCM???
 

Must see pics!

Re: Where to put the PCM???
 

Won't there be a lot of water hitting that if driving in a rain storm? I definitely like the clean look though.

Re: Where to put the PCM???
 

Called them, they provide 5ft of wire from the back of the passenger side of the intake. My PSI harness for the manual trans I'm taking out is only 4ft so the extra foot may allow it to be mounted under the seat.
They will also extend it 3' for $100

Re: Where to put the PCM???
 

Here is what mine looks like in a 71.

Re: Where to put the PCM???
 

I am mounting mine under the seat. I am using a PSI harness. I can attest that I wish the PSI was about a foot longer. It will work, but the extra wire would have been nice.

My mount is from Motobilt https://www.motobilt.com/product/ls_engine_ecm_mount

Re: Where to put the PCM???
 

Potentially, yes but remember that these computers use sealed connectors and they also have a water intrusion rating. I don't drive in the rain on purpose but when I have been caught in it, it hasn't been an issue. They get doused with water all the time in 4x4s so it really isn't any different.

Re: Where to put the PCM???
 

If your providing a donor harness for rework, anyone will do it the same as a regular truck harness. If your having a harness made, just ask, some will make you the harness instead of just selling a premade truck swap harness for everything.



Back when my ECM and harness were on the inner fender, I drove without a hood to the CarCraft Nationals and got caught in a freaking torrent. The intake tube was less happy, the harness and computer didn't care.

Re: Where to put the PCM???
 

Thanks guys! I ordered my new harness from BP today. I measured the PSI harness and it is only 48" and now on eBay. I will post more when I get the project more complete.

Re: Where to put the PCM???
 

can you give me a measurement to compare my harness to so i can see if my harness will be long enough? Mine is from speartech but theres just not really a good way to tell without drilling the hole first i think. Love the utube vids btw.

Re: Where to put the PCM???
 

Bolted it to the rad support on the grille side, tucked away nice out of sight. My harness was a 2008 van harness so was extra long but was easy to tuck all the slack wire in under grille/rad support

Lengthen a stock harness?
 

Is it an option to lengthen a stock harness? I assume it would involve splicing to make the PCM end of the harness longer. I like the idea of mounting the PCM somewhere out of the way like under the console/seat or behind the glove box but I assume the stock harness is too short. I've seen them mounted to the inner fender but it's not a very clean look.

Re: Where to put the PCM???
 

BP harnesses are 5’ long from the back of the intake

Re: Lengthen a stock harness?
 

For anyone considering lengthening a stock harness I read another thread where someone said the purple data wire needs to be one continuous wire from the PCM to the OBD connector. Butt splicing or soldering or changing the wire gauge cannot be done.

Re: Where to put the PCM???
 

I also put mine under the driver seat. Plenty of room for the PCM and fuse box.

Re: Lengthen a stock harness?
 

Has anyone extended this wire and had issues or success? I've extended mine and my OBD port came with a pigtail so I will end up with (2) soldered connections, but I won't have the engine in the truck for a few months to test it.

Re: Where to put the PCM???
 

I have the same harnsss, trying to decide where to bring mine into the cab. Did you put any extra protection on the harness for heat from exhaust/trans?

Re: Where to put the PCM???
 

I did not. I have the high hump and ran the main trunk of the harness at the top of the hump. It clears the trans by several inches so I don't think it will be impacted by the heat of the transmission. The wires to the starter and the O2's already have heat protection loom on them, so I think you should be good. If you are running the low hump you can pick up a 3" diameter by 3' long heat shield sleeve at O'Reilly's for $15.00 Might be cheap insurance.

Re: Where to put the PCM???
 

Awesome, thanks for the information!

Re: Lengthen a stock harness?
 

It is fine to extend the serial data wire (or any other wire in the harness) as long as it spliced properly. I have done it many times with no problems.

Re: Lengthen a stock harness?
 

Good to know. I’m reworking my stock truck harness and was hoping to avoid fender mounting it in my 65. I’ll solder and double heat shrink.

Re: Where to put the PCM???
 

What kind of grommet is good to use to go through the tunnel to mount the pcm under the seat or console?

Re: Where to put the PCM???
 

I extended my harness so that I could hide he wiring and tuck the ecu up front in the drivers fender.

I made a bracket to mount the ECU, fuse pack, relays, and the CEL

I have no issues with extending the harness.

Not the best pic but you can see it all tucked in this fender pocket.

https://i.imgur.com/g7U0hTQh.jpg

Re: Where to put the PCM???
 

I went to my local junkyard and found one off a late model with a large harness don’t remember what it came from

Re: Where to put the PCM???
 

The BP harness is long enough to go under the seat on my 65 GMC.

Re: Where to put the PCM???
 

Moe, not sure where guys are buying the grommets but I suspect you could call BP, PSI or others that sell the harness and buy one.