Autotrac with manual transmission - anyone ever made this work?

[JJorgensen52]

HiHave run into a problem with my latest project - 1999 K2500 suburban 7.4L NV4500 swap, retaining the original NP246 transfer case. Got everything put together and working (GM was kind to me so it went pretty easily!), but I can't get the transfer case controller to work properly.

I have both 2hi, 4hi, and Autotrac *appears* to work, but I can't get neutral or 4lo. I just rebuilt the transfer case and it operates smoothly and easily by hand with the encoder removed. The encoder motor is in working order. When I plug up my scan tool and try to command a range change, it keeps telling me "transmission not in neutral and vehicle speed present".

My VSS is indicating 0RPM on both front and rear driveshafts and I have tried both with and without parking brake, always with clutch depressed.

The GMT400 never came with Autotrac behind a manual, but the S-10 blazer did using the same encoder. The control modules are different calibrations (have tried swapping them and that didn't help).

I am thinking, at this point, that the problem lies in my transmission calibration file, but I'm not terribly familiar with tuning beyond the use of a Tech2.

Any ideas on this one? Different thoughts or ideas appreciated!

[EVT1]

Hi JJ, I was just wondering if you ever found the source of this problem? I am having the same exact issues, I have a tech 2 and it’s giving me the same info & results when I try to force the shift. I’ve talked to a really good ecm programmer company and they can’t come up with any solutions either, I’m just really hoping that you have made some progress and could please share some information sir.

[JJorgensen52]

Originally Posted by EVT1

Hi JJ, I was just wondering if you ever found the source of this problem? I am having the same exact issues, I have a tech 2 and it’s giving me the same info & results when I try to force the shift. I’ve talked to a really good ecm programmer company and they can’t come up with any solutions either, I’m just really hoping that you have made some progress and could please share some information sir.

So I have not come up with a solution to the problem - I am happy to report that autotrac works as it should and I now have some 22,000 trouble-fee miles on my conversion.

-That said-

I have isolated two problems that seem to be the root of my issue, one of which appears to be baked into the manual transmission flash I am currently running (2000 K2500 pickup with 7.4 and 5 speed)
1. Transmission flag always shows 2nd gear
2. VSS indicates 3mph thru the PCM, even when the vehicle is stopped

I have played with the VSS a bit and had no success fixing that problem - unfortunately the factory PCM isn't really tuneable on these trucks. I think an offset in the tune would fix the VSS issue.

On the transmission gear flag front, I have tried grounding the correct pins to indicate Neutral based on how GM wired the gear indicator in the automatic, with no luck. It appears the manual trans tune just ignores those pin inputs. If the default gear could be changed in the tune it should fix this, or ideally if it could be tied to the clutch engagement input. As far as I know the TCCM module that runs autotrac cannot be programmed.

My current theory on this is that, since my truck is a '99, I need to complete an 0411 PCM swap to allow me to access and revise those two problem areas of the tune. I just haven't had time to play with it.

[EVT1]

Good morning sir,
I’ve been calling around to a few transfer case repair shops & they have been pointing at the same thing. They all think it’s the encoder motor, I talked to one of them & explained what I that I wasn’t seeing anything with a Tech 2 & he said the Tech 2 won’t see a problem but a snap on scan tool will, I’m currently working in Baghdad at the US embassy so I can’t look at it myself but I’m going to try to get one of my buddies to plug his up to my truck and see what he finds. They also highly recommended only using ac delco as most others will crap out in a short time or possibly need ecm coding to talk to one another, I’m going to try to get some info from my friend & his scan tool. If he can’t find anything I’m going to bite the bullet & buy a new encoder motor anyways as I don’t have much choice if I want it to work, I’ll de keep you in the loop but I won’t be back home until July myself to put it on.

Laz

[EVT1]

I have isolated two problems that seem to be the root of my issue, one of which appears to be baked into the manual transmission flash I am currently running (2000 K2500 pickup with 7.4 and 5 speed)
1. Transmission flag always shows 2nd gear
2. VSS indicates 3mph thru the PCM, even when the vehicle is stopped


I was wondering how you found the vss to be indicating 3mph? In your original post you said it was showing 0mph (I think, it does say 0rpm) but I think you meant mph, I’m just wondering if something made you look somewhere else that I haven’t as I’m trying to rule out all possibilities before spending $350-$400 on an encoder motor

Laz

[JJorgensen52]

Originally Posted by EVT1

I have isolated two problems that seem to be the root of my issue, one of which appears to be baked into the manual transmission flash I am currently running (2000 K2500 pickup with 7.4 and 5 speed)
1. Transmission flag always shows 2nd gear
2. VSS indicates 3mph thru the PCM, even when the vehicle is stopped


I was wondering how you found the vss to be indicating 3mph? In your original post you said it was showing 0mph (I think, it does say 0rpm) but I think you meant mph, I’m just wondering if something made you look somewhere else that I haven’t as I’m trying to rule out all possibilities before spending $350-$400 on an encoder motor

Laz

I had access to a Tech2 scan tool at the time I was last working on this problem - in the transmission data, it allows you to read the transfer case output shaft RPM both front and rear, as well as the indicated speed output from the PCM. When the shaft RPM indicates zero on my truck, the PCM is outputting a speed of 3mph.

I don't think the problem is your encoder motor (I know mine isn't as it worked perfectly before the swap) - the TCCM (transfer case control module) has a built in logic check, which requires certain conditions to shift ranges in the case. If it doesn't see 0mph and an indication of Neutral from the PCM, it rejects the command to shift ranges. So as far as I can tell, my TCCM is doing exactly what GM programmed it to do - the PCM setup for the manual trans was just never set up to play nice with it, as they never brought the autotrac manual setup to the fullsize trucks. It was available in the S10 (tried swapping TCCMs with an S truck, that didn't work either).

[EVT1]

I have a tech 2 as well, I’ve looked at everything on it. The transfer case output shaft speed, the vss & even the abs sensors & everything says 0mph or 0rpm as well as showing park/neutral, I’m stuck. 🤦🏻