Mariokart

Iv never posted on a forum before. But I have a 99 silverado z71 with the old body style. My father has owned the truck for over 18 years and replaced the motor around 2017 and had the transmission rebuilt in 2018 he parked the truck in 2020 because he finally bought a new truck. So I bought the truck from him and now it will shift perfect from first gear through 3rd gear but then the check engine light turns on and it will not go back to first or second gear or overdrive. I can manually put it in second gear from the Column. And I can turn the truck off for about 5 seconds and start it up again and it will start in 1st gear until I reach 3rd gear again it will not down shift or go in over drive. Reverse works fine. I hade a code for p0758 so I changed the 2-3 solenoid and I changed map sensor and throttle positioning sensor. I unplugged the transmission harness and plugged it back in and I made sure transfluid is at the perfect place on the stick. All my dash lights work fine and rpm gage works speedometer is about 5 mph off though but it has been like that for years. So I don't think it is the ignition switch and I don't have many tools or time or money to keep throwing parts at it. All fuses are good. And I can't find anyone with these exact symptoms to even have a starting point. The transmission must be receiving power to shift all the way to 3rd but goes into "limp mode" when it gets there. Could it be the transmission harness? Or ignition switch with different symptoms from others.

Mariokart

I also have been grabbing every wire and harness I can get my hands on and moving them around to see if there may be a damaged wire but iv had no luck

silveradoboomer

My 1994 Silverado 5.7 would not shift into overdrive. New spark plug wires solved the problem.

Mariokart

Iv changed the plugs wires and distributor and mass air flow sensor and throttle positioning sensor. I am leaning more towards the PCM now. I can hook a scanner to my truck and delete the p0758 code while I'm driving and everything will work and go into over drive but as soon as it down shifts back to 3rd gear it throws the p0758 code again and I'm stuck in 3rd gear again. But I can shut the truck off and it will start in 1st gear again but will get stuck in 3rd after hitting 3rd gear. Iv had 7 of these trucks in this body style and this is the first one that's given me this type of problem.

silveradoboomer

Common Mistakes When Diagnosing

When diagnosing a solenoid related DTC, it’s important to follow the manufacturer’s solenoid testing procedure to get accurate results. The wiring may also to blame, as could clogged internal fluid passages.

What Repairs Will Fix P0758?

• Changing the transmission fluid and filter
• Replace faulty solenoid
• Repair or replace a faulty transmission pump
• Repair or replace defective transmission valve body
• Perform transmission flush to clean passages
• Cleaning corrosion from connectors
• Repairing or replacing wiring
• Flash or replace PCM/TCM
• Install a rebuilt or remanufactured transmission

Related Codes

P0755 – Shift Solenoid ‘B’

P0756 – Shift Solenoid ‘B’ Performance or Stuck Off

P0757 – Shift Solenoid ‘B’ Stuck On

P0759 – Shift Solenoid ‘B’ Intermittent

Mariokart

if i hooked a scanner to the truck and deleted the code while I was in 3rd gear and then it shifted correctly for a few seconds but soon as it went to 3rd gear it would get stuck there again until I delete the code again. The truck shifts perfect when the cel is off. So I believe everything is working like it should mechanically. It's really irritating to have to keep a scanner hooked to the truck and delete the code every time it shifts into 3rd or down shifts into 3rd

Mariokart

Thought I should also add that I just noticed that when the truck goes into "limp mode" and I pull the trans stick to check fluid level the fluid reads low. But I shut the truck off to and crank it up a few seconds later and delete the cel the trans fluid reads perfect. Would this be something to worry about or is it just something that happens in "limp mode"?

Mariokart

Changed ignition switch and still the same problem.

Mariokart

I will run a new power wire to the transmission connector (pink wire) and I will run another wire to the VCM (yellow/Black). And if the problem is still there I will have no choice but to focus on the transmission internal harness or the VCM. But I am leaning more towards the VCM because of the sequence of how the solenoids turn on and off
Solenoid 1-2 2-3
1st on on
2nd. off on
3rd off off
4th on off
so this explains why the transmission work good in first and second gear. And then throws a code for 3rd. But if I delete the code while driving it will shift into 4th but after about 5 seconds the CEL comes on and then it will down shift back to 3rd. Sense the trans harness is supposed to have a constant power the VCM must be the culprit or a bad wire to the VCM. Because it throws a code when it needs to activate the 2-3 solenoid to the on position. If the CEL is off then it will shift gears. So maybe it's getting what it needs but there is some type of problem with the wire or the VCM is just not recognizing the 2-3 solenoid because it is bad? The truck had 380000 miles before the motore was changed in 2017 an the trans was changed in 2018. So the VCM has been in the truck for 24 years and probably has well over 400000 miles on it by now. I will keep replying in this tread for anyone else who has this problem.

Mariokart

I ran a new wire (yellow/black) for the solenoid control from the vcm, it did not work. Still has the same problem. I will be changing the VCM soon the solenoid is obviously getting power for the truck to shift between first and second gear and the solenoid control wire from the vcm is working properly for the solenoid to change to the off position and go into 3rd gear. If I turn off the CEL with a scanner the truck will shift into 4th gear and it will down shift back to second gear and first gear. But the CEL will turn on after about 5 to 10 seconds in 3rd and 4th gear and the truck will go into "limp mode" a dew seconds after the CEL comes on. So I believe that the VCM may have an internal problem. I will try to get a new one soon from autozone they are only 100 dollars and they come pre flashed to my trucks VIN.