LS wiring question - The 1947 - Present Chevrolet & GMC Truck Message Board Network

J48chevy · 10-31-2017, 09:09 PM

So where is everyone mounting their PCM unit and also the fuse box for the LS engine wire harness. Just got my engine kit and box and plugs are too big for me to get them thru the firewall since I have it finished and painted. Ordered a complete new engine wire harness for my 5.3 LS. I will have to put in engine bay somewhere and wanted to see what everyone has done.

clegg · 10-31-2017, 09:15 PM

I have an LS 6.0 motor in my 1955 truck. I hung the PCM, fuse block and relays on the drivers inner fender.

**mongocanfly** · 10-31-2017, 09:27 PM

are you trying to feed the plugs thru the firewall from inside the truck to the engine?..ecms just don't look right under the hood of these old trucks...my next ones going under the seat..my last one was behind the dash..but on my harness it feeds thru the firewall from inside the cab to the engjne...the harness should have a firewall grommet that dictates what size hole you'll need...the grommet will hide the hole made with a hole saw

BAM55 · 10-31-2017, 09:33 PM

If you do decide mount the pcm on the inside of the firewall, cover the area where the hole needs to be with tape then drill the hole. Use a good hole saw and you should not have any paint damage that is not cover by the grommet.

dwcsr · 10-31-2017, 09:47 PM

Quote:

Originally Posted by J48chevy

So where is everyone mounting their PCM unit and also the fuse box for the LS engine wire harness. Just got my engine kit and box and plugs are too big for me to get them thru the firewall since I have it finished and painted. Ordered a complete new engine wire harness for my 5.3 LS. I will have to put in engine bay somewhere and wanted to see what everyone has done.

This is what we did on the LS and PCM

**Dan in Pasadena** · 10-31-2017, 09:49 PM

I have an LQ4 in my ‘55.2 now. ECM s mounted under the seat. Fuse panel behind the gauge cluster between the dash reinforcements. I didn’t want the wiring to show so it runs on the floor under the Dynamat and up the toe board to under dash.

Dave0 · 10-31-2017, 10:07 PM

Quote:

Originally Posted by Dan in Pasadena

I have an LQ4 in my ‘55.2 now. ECM s mounted under the seat. Fuse panel behind the gauge cluster between the dash reinforcements. I didn’t want the wrong to show so the wrong runs on the floor under the Dynamat and up toe board to under dash.

Nice! Time to go thru your build thread

**jweb** · 11-01-2017, 10:17 AM

I extended the harness and mounted the PCM under the seat. I used an S10 PCM bracket to mount it https://www.summitracing.com/parts/n...SABEgKVm_D_BwE I ran the wire bundle down the transmission and then drilled a hole under the seat. My fuses are mounted high on the driver's side kick panel along with the OBDII and check engine light for easy access.

I don't have any pic, my truck is torn apart for paint right now.

**Dan in Pasadena** · 11-01-2017, 10:57 AM

I forgot to mention that I didn’t use the stock GM harness.

used the Holley Dominator ECU which comes with longer wires so I didn’t have to extend the stock harness.

https://www.amazon.com/gp/aw/d/B0049...UHL&ref=plSrch

dsraven · 11-01-2017, 01:03 PM

I48, post up some pics of under hood and cab so we can see what you have for room.
could you install under floor like another sealed battery box with a lid accessed from inside the truck?
under seat on a tray that slides out for access?
behind seat where fuel tank used to be? (assuming it's under the truck)
behind dash glove box, accessed from inside or through access hatch on firewall?

it depends on what you have changed etc. original park brake, steering column, dash, fuel tank etc

**Patrat** · 11-01-2017, 07:59 PM

I put mine under the dash but would never do that again. Now that everything is put together I dread ever having to get to it. Next time either under the seat or (prefer) behind the seat

J48chevy · 11-03-2017, 09:23 PM

Quote:

Originally Posted by dwcsr

This is what we did on the LS and PCM

Thanks everyone. Lots of things to think about. Can look at doing the extending but may go with this setup. So many decisions.

J48chevy · 11-03-2017, 09:26 PM

Dwcsr, did you make those shields and also did you end up making a cover or will you leave it like that? I have same vintage air set up so kind of like this idea.

J48chevy · 11-03-2017, 09:30 PM

Quote:

Originally Posted by Dan in Pasadena

I have an LQ4 in my ‘55.2 now. ECM s mounted under the seat. Fuse panel behind the gauge cluster between the dash reinforcements. I didn’t want the wiring to show so it runs on the floor under the Dynamat and up the toe board to under dash.

Very clean looking. I am just worried about cutting and extending wires on a $600 engine harness. I am definitely not an electrician.

47 Fasttoys · 11-04-2017, 07:59 AM

I mounted all my electronics behind the seat. Easy access but it is a bit of work up front.
Rob

robertsjm711 · 11-04-2017, 11:51 AM

I put both under the seat. It wasn't hard to extend, but time consuming. Just do one wire at a time and solder/ heat shrink. May consider a plug so you don't have to remove the harness from engine if you ever have to pull it.

tmoble · 11-07-2017, 12:32 PM

Quote:

Originally Posted by Dan in Pasadena

I forgot to mention that I didn’t use the stock GM harness.

used the Holley Dominator ECU which comes with longer wires so I didn’t have to extend the stock harness.

https://www.amazon.com/gp/aw/d/B0049...UHL&ref=plSrch

yikes Dan, a $2000 computer? I've got a $350 Mega-Squirt.

Crash0637 · 11-07-2017, 11:22 PM

I am using a truck harness that has been re-pinned but not lengthened. What worked for me was mounting it on the passenger inner fender, close to the battery (in the stock location).

If I had to do it over again, I would spend the extra time to lengthen the harness and hide it under the dash. Eventually, I will end up doing this.

I am with everyone else, an ECM looks out of place under the hood.

10-31-2017, 09:09 PM	#1
J48chevy Registered User Join Date: Jun 2013 Location: Michigan Posts: 304	LS wiring question So where is everyone mounting their PCM unit and also the fuse box for the LS engine wire harness. Just got my engine kit and box and plugs are too big for me to get them thru the firewall since I have it finished and painted. Ordered a complete new engine wire harness for my 5.3 LS. I will have to put in engine bay somewhere and wanted to see what everyone has done. __________________ My Build: 1948 Chevy Thriftmaster. Mustang 2 front end, LS1, 700R trans, Explorer Rear end. http://http://67-72chevytrucks.com/vboard/showthread.php?t=606670

10-31-2017, 09:15 PM	#2
clegg Registered User Join Date: Oct 2016 Location: Orlando, Fl Posts: 84	Re: LS wiring question I have an LS 6.0 motor in my 1955 truck. I hung the PCM, fuse block and relays on the drivers inner fender.

10-31-2017, 09:27 PM	#3
mongocanfly Post Whore Join Date: May 2015 Location: Alabama Posts: 14,674	Re: LS wiring question are you trying to feed the plugs thru the firewall from inside the truck to the engine?..ecms just don't look right under the hood of these old trucks...my next ones going under the seat..my last one was behind the dash..but on my harness it feeds thru the firewall from inside the cab to the engjne...the harness should have a firewall grommet that dictates what size hole you'll need...the grommet will hide the hole made with a hole saw

10-31-2017, 09:33 PM	#4
BAM55 Registered User Join Date: Jan 2016 Location: Inglewood, CA Posts: 324	Re: LS wiring question If you do decide mount the pcm on the inside of the firewall, cover the area where the hole needs to be with tape then drill the hole. Use a good hole saw and you should not have any paint damage that is not cover by the grommet. __________________ BAM 55/58 Chevy Truck Project Fixing a mess of a clip installation

10-31-2017, 09:49 PM	#6
Dan in Pasadena Senior Member Join Date: Nov 2008 Location: Glendora, CA Posts: 6,345	Re: LS wiring question I have an LQ4 in my ‘55.2 now. ECM s mounted under the seat. Fuse panel behind the gauge cluster between the dash reinforcements. I didn’t want the wiring to show so it runs on the floor under the Dynamat and up the toe board to under dash. Attached Images __________________ '55 Big Window Shortbed, Drive-It-&-Work-On-It slid down the "slippery slope" to a Frame-Off Rodstoration! LQ4/4l85e/C4 IFS/Mustang 8.8 rearend w/3.73's Dan's '55 Big Window "Build" - Well, Kinda! Last edited by Dan in Pasadena; 11-01-2017 at 12:32 AM.

11-01-2017, 10:17 AM	#8
jweb Senior Member Join Date: Apr 2010 Location: Oregon Posts: 1,215	Re: LS wiring question I extended the harness and mounted the PCM under the seat. I used an S10 PCM bracket to mount it https://www.summitracing.com/parts/n...SABEgKVm_D_BwE I ran the wire bundle down the transmission and then drilled a hole under the seat. My fuses are mounted high on the driver's side kick panel along with the OBDII and check engine light for easy access. I don't have any pic, my truck is torn apart for paint right now.

11-01-2017, 10:57 AM	#9
Dan in Pasadena Senior Member Join Date: Nov 2008 Location: Glendora, CA Posts: 6,345	Re: LS wiring question I forgot to mention that I didn’t use the stock GM harness. used the Holley Dominator ECU which comes with longer wires so I didn’t have to extend the stock harness. https://www.amazon.com/gp/aw/d/B0049...UHL&ref=plSrch __________________ '55 Big Window Shortbed, Drive-It-&-Work-On-It slid down the "slippery slope" to a Frame-Off Rodstoration! LQ4/4l85e/C4 IFS/Mustang 8.8 rearend w/3.73's Dan's '55 Big Window "Build" - Well, Kinda!

11-01-2017, 01:03 PM	#10
dsraven Registered User Join Date: Nov 2010 Location: calgary alberta Posts: 8,372	Re: LS wiring question I48, post up some pics of under hood and cab so we can see what you have for room. could you install under floor like another sealed battery box with a lid accessed from inside the truck? under seat on a tray that slides out for access? behind seat where fuel tank used to be? (assuming it's under the truck) behind dash glove box, accessed from inside or through access hatch on firewall? it depends on what you have changed etc. original park brake, steering column, dash, fuel tank etc

11-01-2017, 07:59 PM	#11
Patrat Junior Member Join Date: Apr 2015 Location: San Antonio Posts: 46	Re: LS wiring question I put mine under the dash but would never do that again. Now that everything is put together I dread ever having to get to it. Next time either under the seat or (prefer) behind the seat

11-03-2017, 09:26 PM	#13
J48chevy Registered User Join Date: Jun 2013 Location: Michigan Posts: 304	Re: LS wiring question Dwcsr, did you make those shields and also did you end up making a cover or will you leave it like that? I have same vintage air set up so kind of like this idea. __________________ My Build: 1948 Chevy Thriftmaster. Mustang 2 front end, LS1, 700R trans, Explorer Rear end. http://http://67-72chevytrucks.com/vboard/showthread.php?t=606670

11-04-2017, 07:59 AM	#15
47 Fasttoys Registered User Join Date: Sep 2013 Location: Westminster, SC Posts: 933	Re: LS wiring question I mounted all my electronics behind the seat. Easy access but it is a bit of work up front. Rob Attached Images __________________ My '47 C.O.E build

11-04-2017, 11:51 AM	#16
robertsjm711 Registered User Join Date: Jul 2017 Location: Lebanon, Oregon Posts: 241	Re: LS wiring question I put both under the seat. It wasn't hard to extend, but time consuming. Just do one wire at a time and solder/ heat shrink. May consider a plug so you don't have to remove the harness from engine if you ever have to pull it. __________________ It was said that if you cut off one hydra head, two more grow back. -Lernaean Hydra Build Thread: http://67-72chevytrucks.com/vboard/s...d.php?t=743246

11-07-2017, 11:22 PM	#18
Crash0637 Registered User Join Date: Jun 2014 Location: Orlando, Fl Posts: 36	Re: LS wiring question I am using a truck harness that has been re-pinned but not lengthened. What worked for me was mounting it on the passenger inner fender, close to the battery (in the stock location). If I had to do it over again, I would spend the extra time to lengthen the harness and hide it under the dash. Eventually, I will end up doing this. I am with everyone else, an ECM looks out of place under the hood.



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