If anyone is up for a challenge, I require assistance.

The truck is a 1994 GMC K1500. Already rebuilt trans.

I have first and second gear, manual shift only. With the scanner hooked up it shows second all the time. If you unplug the main harness at the transmission you get 1st, 2nd, and 3rd, manual shift only. like this the scanner shows 3rd.

I have not checked power at the transmission yet.

I am going to clean and/or replace pcm and engine to body/frame grounds tonight.

I have visually inspected the harness from the transmission to the PCM and there are no breaks or frays. Also have checked continuity from the PCM connectors to the Trans plug and everything checked out good.

Checked TPS operation with my scanner, looks nice and smooth.

Have not road tested with the scanner to check the VSS, may do that tonight.

Just rebuilt this transmission. Was having exactly the same problem before it was rebuilt. Nothing looked bad on the inside, I did not do anything to the valve body nor did i do any upgrades, repacked with new clutches and torque converter, no change what-so-ever.

changed both shift solenoids and the TCC lockup solenoid (because i broke it) with no change.

I have 2 extra j/y PCMs and have tried both of those with no change.

If you put it in drive it will pull ever so slightly, but will not move significantly.

Any suggestions would be greatly appreciated. I'm running out of ideas.

 

Hello,
This trans may be in failsafe mode, do you have a good scanner(snap-on) to scan this to see if any codes are present.
What was this truck doing that you rebuilt the trans on it.
Gary(I have a trans shop in new york)

 

Yeah I have the snap on one. No codes when it's all hooked up. Vss is good. Cleaned all the grounds. Ran a new body to frame ground. The scanner shows the prndl switch is working but the current gear says 1st for everything except 2nd, which will show 2nd. Even park neutral and reverse show 1st

But I drove my truck to a gas station and it drove fine, came back out and only had 1st and 2nd in manual shift only. And had to drive it 20 miles to work. And then back. I just figured it had 208000 miles on it and it probably needs done anyway. The 3rd and 4th clutches were smoked but everything looked good overall. Reinstalled and nothing changed.

 

Hello,
Ok, so if i understand this correctly, sometimes your car will not move in drive, you will have to pull the shifter down to manual one and car will move. Is that correct? Gary

 

Yeah it doesn't move at all in drive or 3rd. If you unhook the main harness to the transmission or pull the fuse you get 3rd too. The transmission only goes into gear if you manually put it in each gear. It will not shift automatically at all.

 

Hello,
When you rebuilt this trans, did you change the low sprag located in the center support? More than likely that is your problem. When you pull the shifter into low, the low reverse clutch will come on, thats why it will pull. In manual 2nd, i believe your taking off on the band.
Gary

 

No I did not. All i did was a master rebuild kit and shift solenoids/tcc lockup solenoid.

should my snap on scanner show my current gear as 1st even if im in P N R D 3? and why would i get 3rd when i unplug the trans if that's the case? i can drive the truck all day long with the trans unplugged just manual shifting 1st 2nd 3rd. It just will not shift on its own.

 

Hello,
The scanner may show that as a gear command,probably because thats the way it programmed. Its important that the switch itself reads correctly, which it was(park shows park,rev shows rev, etc.)
Now, The trans shows 3rd when the harness is unplugged because at that point you are cutting off all eletricity to the transmission and in 3rd gear both shift solenoids are off(no eletricity)so the trans defaults to 3rd, also called failsafe mode. If the computer detects any problem in the trans it will turn off all eletricity to the trans.
Absoluly DO NOT drive this car with this harness unplugged, you will burn the 3-4 clutch and that is the weak point to this trans. The best thing to do is drive it like a standard shift, pull it down to manual one and upshift.
This trans will have to come back out to change the low sprag, while its out i would change the input sprag as well because they where updated from 26 to 29 elements. Gary
I hope i explained so you understand.

 

yeah i understand. just was hoping it was something that the trans didn't have to come back out. I drove it one day like a standard shift back in march or april and it has been sitting ever since.

Thanks

 

so this is what you're talking about when you say the low sprag?

TH700 R4 4L60E 4L65E 4L70E Low Roller Sprag Borgwarner 87 Up Wide Design | eBay

and this for the input sprag?

TH700 R4 4L60E 4L65E 4L70E Sprag Forward Clutch Borgwarner 29 Element | eBay

any special tools required to change? I haven't changed these before and am not too awful familiar with inner workings of transmissions, i know enough to rebuild but not so much on diagnosing.