Hey all,
Just joined the forum. Looks like alot of good info here.
Name's Mike, I'm in IL.

I'm having some problems with my 4L80e not wanting to shift into 4th, or lock-up when it's cold.
It's in a '92 (mechanical) C3500 dually with 213,000 miles on it. I bought it used with 142,000 on it, and don't know any history on the transmission.

Typically, when it's warm outside, just jump in and go...
shifts and locks up fine.
BUT......now that it's dropped below 40 degrees here and staying cold, I can't get a 4th gear shift from a cold start until I've driven far enough to get the engine up to normal operating temp, then I have to pull over, shut the key off, restart, and it will shift fine the rest of the day.
I've owned the truck for about 4 years, and had this trouble last winter also. really got to be a PITA doing the restart thing every morning.
I did 2 back to back fluid/filter changes (flushes) to get the fluid cleaned up right after I bought it.
It was a little dark, and had the usual sediment in the pan.
It's been staying pretty clean so far.
I don't know alot about these transmissions other than they have way too many electronics for my tastes. (budget!)
I've made sure the coolant is always topped off, and wiggled the coolant sensor plug a time or two, but nothing short of a warm restart seems to get me into 4th gear when its cold out.
When I first got it, the shifts would bark the tires and snap your neck if you pushed it too hard.
I replaced the TPS and it's shifted like OEM since then.

I do quite a bit of 1500 mile plus towing around 8-10,000 LBS. with it, and have to depend on it not leaving me stranded.
I finally figured I'd try a new temp sensor, thinking that maybe the time it takes to shut down/restart may be allowing just enough "heat soak" to push the tired sensor into closing.
Sounded good anyways!
Now I see from some other posts that it could be speed sensor related (code tripping)...........or leaking or weak solonoids in the valve body.

Any suggestions on what might fix the problem?
Any possible transmission damage driving it this way?
Best source for updated solonoids?
Speed sensors?
Anybody got a NV4500 setup they want to get rid of?LOL!

I just need to get another season out of this trans, then the 6.5TD is going into a better truck.

Thanks in advance,
Mike

OK, surely someone knows enough about the 4L80E to point me in the right direction.
Here's what I've come up with so far................

I know that the engine operating temp has to come up to somewhere between 130-170 degrees (depending on who you ask) before the 'puter will allow 4th gear or OD.
Basically, the truck has to be warmed up enough to not labor it in 4th.

I replaced the temp sensor, but it doesn't seem to have helped any.
The only code the ALDL will give up is 28.
(transmission range pressure switch fault)??

Next, I checked both of the speed sensors and related wiring. Wiring looked fine, but both IN/OUT sensors had fine metal "hairs" collected on the magnets.
Fines, like grinding dust.
Cleaned them off and re-installed them.

While I was under there, I decided to take a peek in the pan.
The fluid had a slight burnt smell, but not fried.
It was medium dark, but it always has been a little darker than average. (last 70,000 mi.)
The pan had the normal black streaks and no debris at all.
The pan magnet had the normal acculumation of fines, about what it did the first time I changed the fluid/filter 70,000 mi. ago.

Would a momentary difference in IN/OUT sensor readings set a code 28? (metal "fines" causing a bad reading)
Evidently code 28 reverts the 4L80E to "limp mode 1" where it is limited to direct drive (3rd), no lock-up.
Warm restart resets the 'puter............but why doesn't it repeat the no 4th,OD after it's fully warmed up?

What EXACTLY is the "transmission range pressure switch"?
Where do I find it, and can it be repaired?

Thanks,
Mike

The transmission range pressure switch is the flat silver pressure manifold you see when the pan is off,it has a flat connector plugged in one end and (if I remember correctly) 4 bolts sandwhiching it against the valve body. It seals to the vb with seals similar to 0-rings. Its possible to have a bad switch in it or a blown out rubber seal but what I have saw more often is a bad connection on main plug to pcm or bad wire in the harness. There were ALOT of harness problems with the 93 and older 4l80e's,the majority of them were upgraded by the factory though. Its easy to tell if it was ever upgraded,the old style problematic harness had a screw threw the center where it went on trans to hold it on,the new style harness has clips that lock on similar to the 4l60e connector. If it has the old harness you will want to upgrade it,you replace the internal trans harness and replace or splice in a new style external harness,not easy BUT helps eliminate nagging electrical issues. The connection and screw I'm refering to are on drivers side right next to the front driveshaft,the screw is either a torq or phillips and goes right thru the middle of the connector,Smitty

Looking at your mileage and dark streaks in the pan; I believe you need major work. Smitty is correct in his diagnosis. Code 28 is pressure switch manifold malfunction or, leak at trans connector at case, or stuck valve in valve body none of which should react to temperature change.

Thanks guys, for the feedback.
I studied things over last night and decided to go ahead and pull the range pressure switch down and just check to see if there was any evidence of a blown seal.
I see nothing to indicate any loss of sealing at the switches, no fluid tracks, no burrs, no "blow throughs".

How sensitive are the pressure switches?
I'd like to check all the contacts to rule out any bad switches. (check for continuity)
Price of a replacement TRPS?
If this one checks out good, will they usually re-seal OK?
Torque specs?

The internal harness/PCM harness is the first design with the torx screw through the PCM connector.
(Truck was built 12-92.)
The internal wiring looks OK, and checks good for continuity, BUT... I found the PCM side wasn't tight with the torx screw. Just barely finger tight.

Also, found the connector has been leaking trans fluid past the factory red urethane seal on the inside.
Plug was half full of tranny fluid.
I don't know if that's relavant, as the rest of the internal harness runs in fluid anyway.
Does the second design harness seal better?

Also, from what I can see through the window by the driver's side of the VB, the (foreward?) clutch pack looks to still have good color and an acceptable amount of friction material left.

I hate to imagine it, but if the harness and switches check out OK, I'm thinking there might be an internal pressure loss somewhere that hasn't closed the right switch when the PCM calls for 4th gear. Likely?

Still, when it works, and I get 4th and LU, it works fine.
However, during my last test drive, when I kicked down from OD to 3rd to pass, (high load) it didn't want to shift back to OD until I did the restart thing.
The PCM may be detecting slip related to 3rd gear somewhere and going to default.
Which brings me back to the IN/OUT speed sensors possibly sending incorrect ratio readings in 3rd............
putting it back in default.

Gotta ask myself, "Do we REALLY NEED this much sophisticated high tech JUST to roll a truck down the highway?" (grin!)
At that point, I'm outta my element, as I don't have the correct tools or technical knowledge to empty the case.
Well, I could probably empty it.......just the putting it back together correctly would be the problem! LOL!

Again, thanks a bunch for the feedback.

Mike