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6.5 cruise problems

Hal

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I just did a 6.5 swap into a 1977 FWD GMC motor home in a unique way. The GMC motor homes used a Toronado fwd trans axle arrangement so the swap was a little different. The 6.5 and 4l80e came from a 2000 H1 Hummer and I kept all the electronic engine controls. It runs great but I can't seem to get the cruise to work. I have the wiring diagram and have checked everything from the ECM through the stalk mounted switch, the brake light switch as well as the TC switch. Everything checks out electrically and it shows no trouble codes. Of course I only have an OTC scanner so there may be something that won't turn the light on that's not showing up. I mounted the PMD out at the grill with an extension I got @ Amazon so I'm not certain that couldn't be an issue. I hate to just start throwing money at it replacing parts but I'm at a loss right now. I'm guessing the PMD, extension harness or the ECM. Any thoughts?
Thanks, Hal
 

THEFERMANATOR

FRANKENBURBAN
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When you did the brake light switch, did you wire it up to have the normally open AND normally closed brake switch inputs. The normally open brake switch input powers the brake lights, and must be hooked to the ECM through the same wiring that goes to the brake lights as the ECM must see the circuit grounded when the brakes are not pressed. If it doesn't see that grounded circuit, it will disable the cruise thinking your brake lights are out. Also there is a 2nd ECM brake light switch that is normally closed. It sends 12V+ to the ECM when the brakes are not pressed, and goes open when the brake lights are on. If there is any problem in either circuit, it will disable the cruise.
 

Hal

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I wired the brake light switches per the wiring diagram and as you suggested and trouble shot it through the H1 manual with no luck. Everything checked out per their chart. I need to check the voltage on the brake light cruise switch and make sure there isn't some stray voltage there. I hadn't realized the PCM looks for the ground and something as small as .2 volts can cause a problem. I was under the false impression that the voltage applied when the brake lights are energized was the signal to the PCM to kill the cruise and not that it (PCM) was looking for the ground signal from the brake lights. The wiring in a near 50 year old vehicle could easily have some stray voltages in it's brake light circuit. I just had a spare PCM flashed also I can try that if all else fails. I think I'll have a close look at the Dorman PMD extension harness too.
Thanks for the reply, Hal
 

Hal

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Well, I've been fighting this for way too long and still haven't resolved the problem. I swapped out the PMD, removed the extension wire, tried a different DRAC, had another PCM flashed and still nothing. I even mounted in a set of switches temporarily for the cruise stalk without success. The trans shifts fine and I can read the speed off the ADL with my OTC reader. I've traced out the wiring numerous time. No trouble codes show up. I understand cruise won't be enabled if it's in 4wd low range with a signal from the range switch indicating that condition. I have nothing going to the PCM for this but it looks like it needs a 12 volt signal to tell it that the vehicle is in 4wd low . It looks like abs ties in somehow but I've been advised by one of the 'chippers' that it shouldn't influence the cruise. I'm at a loss, any thoughts?
Thanks,Hal
 

Hal

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The H1 has a stand alone ABS ECU with no ties to the PCM that I can see. I built the harness using the H1 manual and have no trouble codes or other running issues. Of course the 1977 GMC MH had no ABS. On part of my harness for the VSS's I didn't use twisted pair cable but with the trans shifting well and being able to read road speed at the ADL port with my OTC reader you'd think the VSS signal was fine. I'm really at a loss here.

Hal
 

marvgarr

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#9
I have a couple of those GMC motorhomes one with a 403 the other with a turbo 455. I love them. Now for your problem you need to see if you have proper vss signal to the ecm. Make sure the speed is proper. Next you should be able to check brake pedal position through the scan tool. I am not sure on the OTC scanner if it is possible. This is about it not much else will cause no cruise. Good call on the ABS I am not sure if the 2000 Hummers have an ABS signal to the ECM. I will checked right now and see how it is wired.
 

Hal

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I have a couple of those GMC motorhomes one with a 403 the other with a turbo 455. I love them. Now for your problem you need to see if you have proper vss signal to the ecm. Make sure the speed is proper. Next you should be able to check brake pedal position through the scan tool. I am not sure on the OTC scanner if it is possible. This is about it not much else will cause no cruise. Good call on the ABS I am not sure if the 2000 Hummers have an ABS signal to the ECM. I will checked right now and see how it is wired.
Well, I finally found the problem. When I wired the DRAC I used a pair of connectors I sourced from a wrecking yard. There were 3 'extra' wires not needed for my application so I just taped them together and left them in the new harness. It looks like two of them were in contact, probably the 120,000+ ABS signal and the one of the other unused signal wires. It must have killed the cruise signal to the PCM although it didn't seem to effect the trans signal. Something so simple but very difficult to locate without the proper diagnostic tools. Of course the H1 manual I used had three different wiring diagrams for the DRAC, which really confused things. Finding ANY information on the H1 is extremely difficult. I certainly appreciate the help.
MarvGarr, you'd probably get a kick out of my GMC diesel swap. I used a transfer case from a Revcon behind the 4l80e, went forward to a 2012 GMC 9.25" front differential out to one ton spindles/hubs. Just shy of 14 mpg on the first trip solo and 12.97 mpg on the second pulling a 2010 Jetta wagon TDI. Fun stuff. Hal
 

marvgarr

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#13
I have seen your swap. Definitely well engineered. I want to try and make a 6.5 go in there. It is no problem hooking it up to the th425. The problem is oil filter and oil cooler lines being on that side. I need to get a junk tranny and see what would need to be done to get fit.
 

Hal

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Congrats on fixing your problem. I wasn't sure if that new of a 6.5 used a DRAC.[/
I'm embarressed to admit how much time I spent on fixing it. The H1 is a bit strange in using the stand alone DRAC for sure, but it does make them easier to convert as the PCM needs little or no work. No ABS or VATS to deal with and the DRAC can be manipulated with a few jumpers changed if needed. But most everyone just goes the mechanical route and gets rid of most if not all the electronics. I like for motor to tell me whats wrong with it and having the motor and trans talk with each other really makes for a sweet combo. I don't think the electronic cruise can be beat (if working :) ). It 'should' help with the economy too.
The 6.5 has been done a number of times in the GMC. There's a guy up near the Bay area that is working on a kit to install one. He's got a couple of them running and a pair of them being done in Florida at the Co-op right now. He's the GMC trans guru and also sells some other GMC specific upgrades. I think he does a remote mount on the filter.
Hal
 

marvgarr

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#16
I had seen Manny advertising a kit for the future. I would really like to see what they do with the filter and if there is enough room for a remote filter adapter.
 

Hal

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I had seen Manny advertising a kit for the future. I would really like to see what they do with the filter and if there is enough room for a remote filter adapter.
I've seen two of his installations, they were both parked beside me at the Pueblo, CO rally in September. There was another 6.5 there by Bill Hubler that he completed in 2003. Four of us in the same row, crazy. Manny and I had a friendly competition doing the swaps. Mine is a whole lot more complicated than his though and not in the realm of most of the GMC crowd-afraid I got a little carried away. A lot more time and money than most would invest.
Hal
 
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