woodybunch

Hi everyone I recently acquired a 1994 GMC K1500 that they said had a bad transmission because it only has reverse and first gear(or second there's just no shifting). I checked all the fuses and did a quick look at all the wiring inside and outside the transmission. Then proceeded with a new filter and fluid.
I then took it to my brothers and scanned it for codes and wow it pulled five codes. They are 59,66,67,73, and 82. I cleared them all and they instantly came back. I noticed that on his computer it showed all the solenoids on but the 1-2. It also shows the transmission is in second gear even if you shift it. The indicator on the dash does work correctly also.
Now the person told me that the transmission never acted up until it all the sudden just wouldn't shift out of the forward gear. I think judging by all these code definitions that it sounds like the ECM. Does anyone else have any ideas?

Allan In NE

Check the body ground behind the right valve cover, the grounds at the thermostat housing and also at the battery. Pull the transmission plug at the right side of the tranny behind the heat shield and make sure the cavity isn't full of transmission fluid.


The chances of it having a bad ECM are just about non-existent.


The default for this transmission is second gear; the ECM is keeping it from shifting up until the fault is cured.


If all this is okay, replace shift solenoid "A" only after you have checked the valve and spring behind it. I think the valve is sticking "on" however.


Allan

woodybunch

The plug on the right of the transmission? The harness plug? It looked like the only wires going into it is through a plastic tube visible when you drop the pan. I guess there could be a plug in that tube but I have never looked that close.
Also when a valve sticks will it throw that many codes? Thank you for the fast response on this too!

Allan In NE

On the OUTSIDE of the transmission, top, passenger side, behind the heat shield. Turn the lock to the left a quarter turn and the harness will lift off. Get a mirror and look down into the cavity. Those earlier electrics had issues with fluid in that cavity.

You've only got two transmission codes. No big deal......something is telling the ECM not to shift. If that 1-2 shift valve is stuck in the "on" position, it is probably a broken shift valve spring. This would hold 'er in first.

"Usually", when the ECM throws multiple codes, it means that there is a poor ground somewhere.......

Allan

woodybunch

Thank you Allan!! OK The plug on the transmission was partially disconnected. I plugged it in,cleared the codes and went for a test drive. Still just second gear but now it will manually downshift into first. I rescanned it and it kept throwing a 73 code TFP switch. Still maybe the shift valve spring? I also noticed when I put it into Drive or reverse it engages hard. I will pull the pan again but I would like to rule everything electrical out first.Do these switches usually go out or would I have a better chance of it being wiring?

Allan In NE

Is this truck a diesel?

Allan

woodybunch

No it is a 5.7 gas.

Allan In NE

Okay,


Ya gotta drop the pan again. Check the internal wiring, if it's okay, replace the Pressure Control solenoid. That's the guy that's went south and is causing the ecm to hold the transmission from shifting higher than 2nd gear.


Allan

woodybunch

Well I dropped the pan and found the TFP motor unplugged. I plugged it in and checked the shift valves. Everything looked good. So I buttoned it up and test drove it. Still just one forward gear. I think it is first gear. Just when I drop it into low 1 it drags the motor with it so it seemed like it was in second but the rpm's are the same. I am going to check codes today and see if clearing them will help. Or see if any new ones show up. The guy I got this from really tore it apart.

woodybunch

Well no luck. Ok first I have a snap-on graphing scanner that shows everything that is going on with the transmission during operation. I cleared the codes and drove it a round and it never left first gear. The scanner showed everything right like what gear selection I was in. It also showed that It was in different gears while driving as the speed increased (like 4,3,2,1) but it actually never left first. It also showed all the solenoids on and off at the right time. It never generated any codes while driving either. I think it might be toast? What do you guy's think?