Where to put the PCM???

[bigmoe]

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.

[1971Stepside]

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.

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

[bigmoe]

Originally Posted by bigmoe

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.

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.

[Flubyu]

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

[MDPotter]

Originally Posted by bigmoe

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.

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.

[2014sierraAT]

Originally Posted by 1971Stepside

Here is what mine looks like in a 71.

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?

[1971Stepside]

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?

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.

[2014sierraAT]

Originally Posted by 1971Stepside

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.

Awesome, thanks for the information!

[dayj1]

Originally Posted by MDPotter

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.

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.

[bigmoe]

Originally Posted by dayj1

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.

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.

[bigmoe]

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

[bs46488]

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.

[nsb29]

Originally Posted by bigmoe

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

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

[Pontitruck]

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

[Pontitruck]

Originally Posted by bigmoe

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

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.

[bigmoe]

Originally Posted by bs46488

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.

I like your placement. Unfortunately the 65 has bulky inner fenders.

[Gunnin4ya]

I extended mine about 6 Foot just for the ECU section. I mounted my fuses and relays on the firewall inside the cab. The ECU is routed down the frame and under the seat now. It really cleaned up a lot in the engine bay. I read where some said not to and some said it was okay. Eaither way I was going to need a new harness. So, I bit the bullet and did it. I used the heat shrink with the solder in it. I spliced in different sections so I didn't end up with a huge "wad" of connections. I do not have any problems at all. Just took my time. It was originaly on the firewall until I got Vintage Air like you are doing. I did keep the TAC and everything up on the firewall as well.

[bigmoe]

Another thought. When rebuilding the stock harness would it be possible to reroute the main portion to point toward the rear of the engine? Then it could be run through the firewall and mount the ECM under the dash?

[Wgesnerjr]

I did not extend mine but, as I reworked my harness, I could see that being a possibilty. Briefly though about mounting my PCM under the dash where the Electronic Spark Controller was. But thought better and picked the lower area of core support on drivers side, just like the factory 2003.