1995 Chevrolet 6.5 t.d. 4x4 with a 4l80e tranny in it. I can not get this thing to upshift at all. On our way to church it suddenly dropped down to what feels like second gear. it cruises at 30 m.p.h. at about 3,000 r.p.m. When I let off the pedal there is no engine brake, just kinda rolls along.. I replaced the 2-3 shift soenoid, filter, and trans fluid but no change. Checked the wires, grounds, disconnected battery over night still no shift. I had the truck scanned today and the parts store guy said I have DTC 78,81,82,83.. Disconnected the battery again, drove it home, stuck a paper clip in her and got the same code as the parts store guy. He suggested I replace my ignition switch as they have been known to be faulty. Before I start dumping money into this thin has anyone here dealt with this before or can someone give me some sort of direction on what to do and where to start.....Thanks in advance for any help

 

That is "limp mode". Locks the trans in 2nd to protect it and locks out the TCC.

78 is a waste gate fault. Not likely the problem, but the vacuum system waste gate becomes a bit unreliable as these vehicles get older. Does it smoke a lot of black when you accelerate? If so, you're probably got a vacuum pump or wastegate solenoid problem which is unrelated to the transmission codes.

The others are the 1-2, 2-3 shift solenoids, and a TCC code.

Unless your transmission has calved and blown junk through the internals, it's unlikely you would get all those faults at once. Possible, but unlikely.

At the risk of sounding like a broken record, I'm going to say your PMD may is on it's way out. If it's failing, it can cause a "stutter" in the powertrain that sends the transmission into limp mode. I've had that same thing happen when towing; PMD started to give up the ghost and it locked into limp mode with the 35 foot trailer on the back, in a 1 lane construction zone with no shoulders. Boy oh boy, were the people behind me p!ssed after 40 miles of 30 mph...

The only thing that doesn't quite match with a PMD problem is that limp mode should go away when you clear the codes. It will come back, but it shouldn't come back until you drive it a little.

The other guess I would make is transmission fluid in the big electrical connector on the side of the transmission. It weeps in over time due to a old seals. Known to happen on these particular electronic transmissions as they get older.

But keep in mind, without actually "seeing" the truck myself these are all "guesses".....

 

Check all of your fuses with the key on . In the dash and under the hood. This will help determine if you may have an issue with the ignition switch or elsewhere. Is your speedometer working?

 

Speedo works great, my plan was to get it into the garage after Church and double check all the grounds but the wifes rig went down this morning....easy fix just time consuming..stupid egr valve on a safari van is gunked up and sticking.... I do have a test light in the truck and I will quickly check for power at the fuse real quick. My plan now is to get it back in the garage after work Monday......the wife comes first... Another thing, I work about a mile from my residence and its all back country roads with very little traffic, do you all think its ok to limp to work at 25-30 m.p.h or am I putting unnecessary strain on the trans????
A Big Thank You To All That Have Responded So Far, Its Very Much Appreciated.............

 

The trick to not hurt the trans is very light throttle acceleration. If you push it at all you can fry the clutches very quickly. But I would say try to avoid any more use than absolutely neccesary. Good luck and check all the fuses might seem pointless but for the few seconds it takes it is time well spent.

 

O.K., checked all the fuses in the side of the dash board and under the hood. There is power to all of them with the key in the run position.

 

I will check wiring schematics tomorrow see if I can help give direction.

 

Well, you can drive it in limp mode.

Line pressure defaults to max with solenoid a and b reported as bad and it won't shift up.

can build up excessive heat if driven too long this way.

Make sure you check the ground that's at the back passenger corner of the intake manifold. This one is the PCM ground and has been known to cause all kinds of heartaches.

My preference would be to not drive it like that.....but if you're stuck, you're stuck. A 1 mile drive isn't likely to hurt it.

I'd probably just bite down and take a taxi or bum a lift from a bud for the moment if it were me....

Heck, it's only a mile.

Use the Black Cadillacs for a day....

 

checked wiring schematic all solenoids do run off of the same circut powered by fuse #20 rated for 10 amps .I would suggest checking for power at the trans connector on the rear left hand side of the trans (drivers side )it will be a round plug about 1 1/2"-1 3/4" diameter there is a retaining screw in the center of it .The wire to check is the pink on in position E this wire should have battery voltage with the key on if it is not easy to determine what wire it is as long as the connector is un plugged only one pink wire should have 12 v with the key on . If you do not have power at the pink wire with the key on you have a problem in the vehicle harness between trans and the fuse panel assuming the fuse is good .If you have power at the pink wire you can check for the circut in the internal trans harness by checking for voltage at the following wires with the trans harness plugged into the transmission ,dark blue position s ,yellow and black position b ,light green position a , voltage will be close to battery voltage if internal harness is ok . if no voltage at the wires assume internal trans harness needs to be replaced . Good luck hope this helps .

 

i had the same issues and it ended up being the speed sensor on the transfer case it gets corooded and pushes itself out all i did was pulled it out clean all the corrosion off and put it back in it works like a charm now