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1993 4l80

B

BigMikey

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Greetings, this is my first post.
1993 Chevy Silverado 3500 Dually 454 4l80.
Bought truck with rebuilt 454 with 100 miles.
Seller says needs rebuilt trans...
Trans shifts firm when I first leave and shifts thru all gears. After a mile or so it down shifts to 3rd and will not shift back to 4th. Also, there is no engine brake unless I move the shifter lever out of overdrive selector to just drive. Trans fluid is red, replaced both speed sensors, the map, and cleaned the wiring plug of excess Trans fluid... I did notice a green wire in the plug was pinched by the cover but didn't take plug apart because I'm bad with those.
Any ideas ?
 
Welcome to the forum @BigMikey there are a few things you can check which allow your trans to function without tearing into it. do you have access to a GM tech 2 scanner ether by a local shop or friend nearby? you can use the paperclip truck and count the flashes on the SES light but I'm not positive it will reveal trans codes, I may be wrong though. you need to verify your coolant temp sender for the PCM is functioning along with the TPS is working and properly adjusted.

if the trans is shifting and there is no slippage I wouldn't deem the trans needing a rebuild right off the bat. there is also a temp sensor inside the trans but I have not seen one of these go bad on it's own. it mainly comes down to what codes the PCM is showing along with verifying all grounds are good and clean.

being the engine has been out, there is a good chance there might be a wiring issue or a ground accidentally left disconnected from the block.
 
BigMikey said:
After a mile or so it down shifts to 3rd and will not shift back to 4th. Also, there is no engine brake unless I move the shifter lever out of overdrive selector to just drive.
Click to expand...

Engine braking with this transmission: FORGET ABOUT IT!!!
Yes, it only brakes in manual 3rd with the weakest afterthought small clutch pack there is. I have burned out several although that clutch pack being fried doesn't affect going forward. Because these transmissions snap off the input shaft the TCC unlocks when you lift off the throttle.

Brake switches are a common failure point affecting TCC lockup and draining the battery. It's one of the few parts it don't hurt to just toss at it.

You will not get a Service Engine Soon light when transmission codes are stored. Pull codes. Likely your transmission is slipping. Your description indicates it's going into limp mode so it will have a code stored. Common slipping causes are the TC clutch or valve body wear to apply the TC clutch. IMO the TC has a very weak clutch to start with to the point GM won't lock it in below 45 MPH. (Other reasons like emissions and engine load. Point is it will slip below 45 MPH with any throttle when forced to lockup by a switch.)
 
ak diesel driver said:
So it's normal to not have engine braking in OD. Are you positive it's downshifted to 3rd and not the torque convertor unlocking?
Click to expand...
I count the gear changes, starting in 1st, then 2nd, 3rd then 4th. Then the down shift to 3rd and the rpm goes up to what 3rd was.So, I'm assuming it is a downshift to 3rd. But, also I can shift the lever down to just regular drive and get engine brake.
 
dbrannon79 said:
Welcome to the forum @BigMikey there are a few things you can check which allow your trans to function without tearing into it. do you have access to a GM tech 2 scanner ether by a local shop or friend nearby? you can use the paperclip truck and count the flashes on the SES light but I'm not positive it will reveal trans codes, I may be wrong though. you need to verify your coolant temp sender for the PCM is functioning along with the TPS is working and properly adjusted.

if the trans is shifting and there is no slippage I wouldn't deem the trans needing a rebuild right off the bat. there is also a temp sensor inside the trans but I have not seen one of these go bad on it's own. it mainly comes down to what codes the PCM is showing along with verifying all grounds are good and clean.

being the engine has been out, there is a good chance there might be a wiring issue or a ground accidentally left disconnected from the block.
Click to expand...
I replaced both speed sensors and the TSP with AC Delco units.
I'll check for grounds and Temp Sensor next.
Also, there is a Green Wire in the Trans wire harness at the Trans plug in the was severely pinched in the plastic. Any idea what this wire controls ?
 
You can verify of the TCC lockup is working by getting up to a cruising speed (55 mph) and without letting off the throttle pedal (keeping a constant cruise speed very slightly accelerated) use your other foot to lightly tap the brake pedal. you will feel a small upshift and about 200 rpm increase momentarily then taking your foot off the brake pedal it will lock back up lowering your RPM. you should be able to do this with the shifter in OD and in D.

testing your throttle position sensor for proper adjustment is a crucial part of the trans shifting correctly. we would need to lookup the exact readings for the adjustment, but if you use a volt meter on the TPS (key on-engine off) the meter should read 0.40ish without pressing the go pedal and 4.80ish volts at WOT. moving the pedal from nothing to WOT slowly you should only see a gradual increase in voltage between those numbers with absolutely no spikes or dropouts.

again don't quote my voltage range 0.40-4.80 that is just a general guide. we will need to find the actual numbers to adjust it properly. others here might have that info available.
 
verifying shifting between gears sometimes can be a bear feeling for shift point. some trans have a very soft shift. try dropping the shifter down into 1st and manually shifting to 2nd, then D (3rd) then to OD (4th) each time accelerating slightly over where it would normally shift so you can feel each shift. remember when you get to D (3rd) if the TCC is working you should feel two shifts as it will lock up the TCC in both 3rd and 4th.
 
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