92 c2500 electrical issue causes transmission issues

Hello Everyone! I have 92 c2500 2wd 5.7L 4L80E 16147060 ecm 3.73 rear gear. First off this problem is not from the inside the transmission. A known good trans was installed. if you want to know the details of that story ask.

Now with the fun stuff that has drove me crazy. This truck has weird shift points and feels like it lacks power, a known good engine is in it. The trans temp always shows 305 degrees but its not. I can just turn the key to the on position hook my scanner up and it will show that high when the truck is cold. driving down the road the speed that the computer sees is different than the speed that the speedometer shows. its a progressive difference. when the speedometer shows 10mph the computer shows 7mph, when the speedometer shows 62mph the computer shows 50mph. The trans will go into lock up when the speedo shows 62 but the computer thinks it just hit 50mph and you have to feather the gas pedal to get it to go into lock up and if you give it any throttle after it goes into lock up it goes right back out. If you give the truck heavy throttle at anytime and it goes above 3500 rpms it wont shift past 3rd gear and it stays that way until you shut the truck off for several minutes. fire it back up and you can take off going threw all gears babying the throttle. It also throws a code 28, pressure switch manifold. its not bad or the problem. If any Gear Head out there has any, tips, tricks, thoughts, or input all are welcome.

I did a data log with tunerpro the other day and was looking threw the info and noticed that the "PROM ID" doesn't match any number that is on the chip or computer. not sure if that has thing to do with that, but I noticed it. tunerpro shows the prom id as 7941 there is no number on the computer of chip that matches this.

I cant seem to figure out how to upload my data log on here any advice would help.

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Originally Posted by GearGrinder

Hello Everyone! I have 92 c2500 2wd 5.7L 4L80E 16147060 ecm 3.73 rear gear. First off this problem is not from the inside the transmission. A known good trans was installed. if you want to know the details of that story ask.

Now with the fun stuff that has drove me crazy. This truck has weird shift points and feels like it lacks power, a known good engine is in it. The trans temp always shows 305 degrees but its not. I can just turn the key to the on position hook my scanner up and it will show that high when the truck is cold. driving down the road the speed that the computer sees is different than the speed that the speedometer shows. its a progressive difference. when the speedometer shows 10mph the computer shows 7mph, when the speedometer shows 62mph the computer shows 50mph. The trans will go into lock up when the speedo shows 62 but the computer thinks it just hit 50mph and you have to feather the gas pedal to get it to go into lock up and if you give it any throttle after it goes into lock up it goes right back out. If you give the truck heavy throttle at anytime and it goes above 3500 rpms it wont shift past 3rd gear and it stays that way until you shut the truck off for several minutes. fire it back up and you can take off going threw all gears babying the throttle. It also throws a code 28, pressure switch manifold. its not bad or the problem. If any Gear Head out there has any, tips, tricks, thoughts, or input all are welcome.

I did a data log with tunerpro the other day and was looking threw the info and noticed that the "PROM ID" doesn't match any number that is on the chip or computer. not sure if that has thing to do with that, but I noticed it. tunerpro shows the prom id as 7941 there is no number on the computer of chip that matches this.

I cant seem to figure out how to upload my data log on here any advice would help.

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You have a wiring problem somehwere. Start with fixing the trans temp problem. If the PCM thinks the trans is 305* it will shift starnge because there are different tables in the PCM for that temp. That is not a normal operating temp and the PCM knows it.

This past weekend I wanted to start with swapping the ecm, it was the quickest and easiest thing to do. No luck! With the ecm swap the trans still showed 305 degrees even after sitting all night. How ever I did find that the purple wire (trans fluid temp ground) that plugs in at the ecm connector to be loose tried to easily move it around to see if it was a bad connection but no change. I would assume that if the ground isn’t making connection it would show no temp at all.
I guess the next thing is to just pull the entire wiring harness out remove all loom and inspect all wires very closely and hope I find a freyed or pinch wire making contact with another. I’ve looked at some of the wiring that’s not been covered up with the loom and all looks ok.

Unplug the trans connector and see if the temp value changes.