Hey group,
Mods, please move this post where you deem appropriate.
I'm a professional auto mechanic who generally doesn't work on medium duty but I often deal with very advanced wiring and drivability issues at my day job and found myself with a long term good client who needed assistance on his Truck, note this is not actually a school bus.
I'm posting in this forum because I used a lot of information from various posts in this forum to create a repair plan for this truck and as another poster said, this forum really has great info on some of the weird crap for modifying medium duty stuff. In addition to getting gear 6 unlocked on this truck I also was able to get an ECM CAT rerated for 350HP from another brand truck to work in this Chevy. If there is interest in what was needed to overcome and consequences you'll live with with the HP rerate I could build another post.
Information in this thread was helpful:
https://www.skoolie.net/forums/f10/a...ial-21661.html
The truck in question: 2005 GM C7500 has a rear axle ratio of 6.17:1, my client purchased it sight unseen because it had around 25k miles on it, he was upgrading from a K3500 chassis with 600k miles on it that he daily pulls a 30k pound trailer with. He target the model year due to the overall lack of emissions equipment, he upgraded chassis size in hopes to add 8k payload to the truck, in addition to the trailer payload. The problem is once he had the truck he really could not sustain highway speeds above 55mph at a safe RPM.
A secondary issue of low pulling power was satisfactorily resolved but top speed was 62MPH at 2650RPM+ (approx). After doing the math I figured a 0.65 overdrive would get us down to about just under 2400RPM at 65MPH. Not a great cruise RPM for economy but probably a safe sustained RPM for this steel pistoned CAT.
I did attempt to contact GM through their uppfitter website to see if I could get a letter to allow Allison to reprogram the factory TCM with software that would enable gear 6 but to this date I still haven't heard back from them, I suspect I will not. The Allison dealer I spoke with was unwilling to enable gear 6 without GM permission.
I did not have any luck getting the information of the technician in the above thread in order to have the transmission control module (TCM) calibration file updated.
I started looking on eBay for a used TCM, I found one (vehicle make unknown) with a build date of 10-11-2004 and labeled for the transmission I have: 3000RDS.
Reviewing wire diagrams and the original TCM I concluded it would work with a J1939 throttle, the donor TCM was rated for 2400RPM, 100RPM less than my original TCM, I concluded it would shift 100RPM earlier.
I picked up the TCM for $300USD on eBay after making an offer for $100 less than it was listed for.
Identifying the 6 speed TCM was by simply looking at the PRI Mode, the numbers 1-6S1 designate it as 6 gear enabled. The SEC number is only a secondary mode the TCM can operate in but for most of us the primary mode is all we care about.
The TCM was plug and play, the CAT ECM didn't skip a beat when the new TCM was installed, immediately the #6 comes on the shift indicator and gear 6 shifts on at highway speed at an appropriate time. This swap did net the RPM range I was expecting, now at 65MPH the truck is taching just under 2400RP (approx).
The RPM range could be a factor on engines that spin higher RPM, so I suggest stay pretty close to it. To prevent unforeseen issues I suggest get a TCM with similar build date to yours. I don't have any reason to believe the donor vehicle manufacturer matters much so long as the model, RPM and throttle numbers match.
All in all it was a very easy upgrade with zero downside.
I attached images of the replacement TCM and the original for comparison. Maybe some could use the CIN # to get a local Allison dealer to program a TCM for them.
I hope this helps someone.
Linear Mode
