i have an 03HD chevy with 8.1/Allison. DIC shows range shift inhibited. no reverse, 3rd gear only. 14 trouble codes are displayed on my scan tool for the tcm. So I have had trouble with it before but it cleared up by itself. Dont know why but it did. To help me diagnose any future problems if it did it again I installed a voltmeter and tapped it in the ignition power circuits and ground wires at the tcm gray connector. Ground and power are monitored there in both circuits. I also monitor transmission fluid pressure at the tap with a gauge. None of these gauges show anything amiss when I get the range shift inhibited fault. So I pulled the connectors off the tcm and checked to make sure all of the pins in the connectors were in the correct position. They were. Then I went to the plug on the RR of the trans and pulled it off to check for oil. No oil. Then I pulled the connectors off the nsbu switch and found a pin on the switch pushed in. Replace the nsbu switch, cleared codes and tried it again. No luck. Still have these 14 codes. Then got wiring diagram and saw that there is a 12v reference on pin 31 of the red connector at the tcm. Checked that also with the engine running. I got 13v. So I dont think there are any shorts to ground or bad grounds. Im at a loss. One other thing, those codes will set with just the key on without the engine running. I would appreciate any help yall could give me.
Range shift inhibited
- Thread starter clevor
- Start date
How about listing those codes?
For sake of argument, have you tried clearing them and then running the truck to see which codes come back. This is a common procedure we do with our own equipment to make sure that the codes you are seeing are current and not something from years ago that were never cleared.
For sake of argument, have you tried clearing them and then running the truck to see which codes come back. This is a common procedure we do with our own equipment to make sure that the codes you are seeing are current and not something from years ago that were never cleared.
I have cleared these codes several times and they come back. doesn't matter if the engine is on or engine off with key on. I have tried two different scan tools for code clearing and again it doesn't make any difference. As far as the codes go, I went out this morning, to check the codes again to list them. With key on I had 4 codes, I cleared those codes and came back with 40 codes. Started it up, cleared those codes and it came back with 41 codes. Compared those codes with what I wrote down and some of those has disappeared and new ones have come up. You see why I have reached out to this site for help. Seems TCM has a mental illness. I can list those codes if you want but I don't think that will help. I have been involved with GM products since 1981 in a shop setting and was their go to guy for anything computer related. I have seen as many as 3 to 5 codes stored in an engine management computer but never this many in any ecu. I want to be certain that it is the tcm before I go and plop down 500 to 700 for one.
It does sound like a failed TCM. Suggest that you unplug anything transmission and transfer case related. I did not see where you said is 2wd or 4wd. I would also double check the grounds at the actual ground connection spot(s). Leave unplugged overnight. If the codes all come back the next day, source a used TCM before you buy reman or new.
2wd. tcm is unplugged. I monitor the grounds and the ignition voltage at the tcm. This truck has given me this problem before so I installed a voltage monitor(gage) and monitor both 12 ignition supply at the tcm connector along with their grounds. I don't see any issue there. I also checked the 12v reference at pin 31 on the red connector with engine running and got 12 volts there. I also check the batt supply, pin 3 on clear connector. 12volts there also.
Some of my suggestions are based off of experience with these GMT800's and some is from years in the shops. You have ran thru a range of back up info gathering help. It is the rarest of rare things for a module assembly to fail by itself. Usually it is module environment and/or contamination or animal chewing or even rub through.
So, this morning I went out and connected the TCM. Turned on key with engine off and got some new codes and some went away. The new ones are: U1041, U1064, U1300, U1301, U1016. These are all CAN buss related. When I unhook the clear connector on the TCM they go away. This doesn't surprise me as the comm wiring is on this connector. Also, I get a U1000 now that tells me a module is missing from the CAN buss. Is there a scan tool that can verify that the TCM is pulling the CAN buss down. Seems like I'm narrowing it down, but I would like to be certain.
I am honestly not advanced enough to answer that question. Hopefully someone here is or knows someone who is.
99Sierra2500
Well-Known Member
It's a shot in the dark, though here:
They talk of trouble with the neutral safety switch. I know a little about auto electrical, my philosphy is always trace the problem before getting any parts.
A/T - Slow/No Gear Engagement/Engine No Start — 2003 Chevrolet C 2500 Truck 2WD V8-8.1L VIN G Service Manual | Operation CHARM
Detailed repair manual for the 2003 Chevrolet C 2500 Truck 2WD V8-8.1L VIN G.
charm.li
They talk of trouble with the neutral safety switch. I know a little about auto electrical, my philosphy is always trace the problem before getting any parts.
After seeing all of these communication errors, I researched this and eventually found that I have a class 2 serial data buss that needs to be checked for proper operation. As I said previously, disconnection the clear connector from the tcm clears all of these codes. I now only have a U1000 that says some component is missing from the class 2 serial data buss. Which is the tcm. So, I have some diagnostic equipment coming tomorrow that I hope will help me further confirm the tcm is bad or point me in a different direction.
I replaced the nsbu switch b/c of a pin pushed back in the switch itself. This switch has two connectors on it. One for the tcm and the other for reverse lights, start inhibit and so on. The start inhibit was the problem socket. But I replaced it anyway as there is a ground for the tcm connector in that socket. Still didn't make a difference. I'm about frustrated to the point of walking away or spend gobs of money for a shop to look at it. The only shop I trust gets $100/hr and a minimum of 4hrs diagnostic time. My money comes hard, so I don't want to spend it if I have too. In another life I was a GM certified technician in the 80's so its not like i don't know what I am doing but there are way too many computers on these cars today.
Cadillacmak
Well-Known Member
Wow, I have been following this in hopes you find something so I could learn, look frustrating.
99Sierra2500
Well-Known Member
Sorry it didn't help, you are probably right it is the can bus or something. Modern trucks do have too much electronics, old ones seemed to work fine. I think a lot of people want them like cars.
Latest update on this saga. Purchase a dlc breakout box and a pocket o-scope. Connected everything up and began testing. It appears there is nothing wrong with class 2 serial data line. o-scope showed a normal wave form. So, I took setup over to my 01 suburban and connected that to the dlc and got same waveform. While I was there, I used my scan tool to scan for codes on this vehicle. This is where it gets strange. it showed 41 codes in my suburban ecm and there is nothing wrong with it. No check engine light. Could this be a bad scan tool. I wonder. Used and older scan tool on suburban and it didn't show any codes. I'm about ready to throw in the towel. This is not my daily driver so I may just walk away for a while and think about this.
Are they P codes, B codes or C codes or U codes?
p codes, I now suspect my scan tool reporting codes that are false. Does anyone here know of a scan tool that will let you look at the transmission inputs and outputs like looking at live data of the pcm?
For those of you that have hP tuners, should vcm scanner show the different gear positions according to the position of the shift lever. I have a 03 8.1/Allison trans giving trouble. It has a dead IP prndl display but it has been that way for a while before trans stopped working. I nowhave No reverse and one forward gear. I get park - park, reverse-D4, neu-park, D5-D4, D4-D4, D3 -D4, D2-D4 and so on. My 01 suburban w/8.1-4L80E shift position correctly coincides with what VCM pndrl shows. any help would be appreciated.
Cadillacmak
Well-Known Member
I would have to play with a newer truck to see.
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