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No Overdrive or TCC lockup

A

avsgunnar

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Location
Denver, CO
Noticed a couple of weeks ago, high rpms. can feel gears 1 through 4 and also manually shift through them. Just no overdrive/TCC lockup. Only codes were 84 and 99 (APP sensor which I replaced). Have since swapped out brake switch, VSS sensor, and the coolant temp sensor which has given me problems through the years and has restored overdrive before by swapping harnesses (I think they may melt up there.) Previously though it would go into limp mode and throw codes until I either jiggled harness, unplugged it and plugged it back in, or replaced. I carried extras with me.

Currently, no codes, no limp mode, just high rpms in excess of 2500 to 3000 at 65 mph. Everything usually ran around 1500 rpms. Only started to notice because having to drive on highway now to get to a job site.

Speaking of coolant temp sensor, if I unplug non-voltage side no codes are thrown. If I unplug voltage side, I do get code. Although anything is possible, is it probable that the non voltage wire is not sending info to ECM causing no lockup? I am not looking forward to wire chasing through that rats nest of wires if someone else knows better info than me on this.

This is 4L80e trans(automatic). As signature states, replaced trans 30k ago, too soon for solenoids to go? Work truck so trying to avoid mechanic and being without truck. Would like to try and troubleshoot before surrending it to them as trans is under warranty.

Any ideas or guidance?
 
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Welcome to the truck stop. I’m no help when it comes to transmissions but there are a few that can help you out. @THEFERMANATOR should be able to get you going.
 
Welcome to the forum Gunner.
if the PCM does not see a certain temp, the TCC will not lock up.
If You have a repair manual, in it there is a listing for the various ohms reading of the temp. sensor. I think You can probably do a google search and find it too. You might try running through the test.
Not a terrible expensive part, so You could just replace it and see what how it performs.
 
The sensor is fine/new. Skipped resistance testing on sensor since I have a few of them and just swapped them out. Apparently I should have 5V at pin 1 at harness (or voltage wire) which I do have. I had continuity at pin 2 (non-voltage wire) yesterday but just rechecked this morning and seem to have lost it (showing infinity).
I was thinking that maybe it is jostling around while driving. It is just hard to trace the wiring since the truck is so old old and dirty they all look the same color and disappear under IP. Im leaning now toward that return wire. If I knew where it was exiting harness I could just run new wire to that point and then start checking for breaks/continuity. Problem is my wire colors dont match any schematics I have on this truck. Previous owner did a lot of his own wiring and tapping also.
 
My first thought is you have actually lost 4th gear: You are feeling 1-2-3 and TCC lockup. Move the selector from 3 to 4 and the TCC unlocks and locks back up after the assumed shift works. There is no way you went from 1500 to 2500-3000 RPM on TCC not locking up. It's ~500 RPM max unlocked to locked and you can't stall it any higher without a downshift. IMO you have more RPM due to higher freeway speed and depending on tires and rear end ratio 70 MPH around 2500 is normal.

At this point without the proper tools you are stuck. You need a scanner to see what the ECM is seeing and gears commanded including TCC lock or not. The computer should code if the trans doesn't shift as commanded. Ratio error for not shifting, and slipping code unlocks the TCC going into limp mode till the next key off/on. So something is telling the computer not to shift or lock the TCC. You have mantioned some of these, but, without a scanner you really don't know IF the ECM thinks the brakes are on or not. Bad switch, new or not, wires to switch, ECM died, bad ground(s), low ECM voltage.

Fine/new will drive you NUTS! New parts do NOT mean GOOD parts!!! Bad out of the box, shorted to ground, wrong values, wrong part... You need to probe at the ECM pins to see if the wire run to the sensor is good.

Again I suggest you check and clean grounds then get ahold of a scanner. If you can't get a scanner let the mechanics who have the tools fix it. It will be cheaper than throwing parts at it. Unless they try and throw parts at it...

There is a real possibility the ECM locks the TCC, sees something slipping and then (limp mode) unlocks it till the next key cycle.
 
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avsgunnar said:
The sensor is fine/new. Skipped resistance testing on sensor since I have a few of them and just swapped them out. Apparently I should have 5V at pin 1 at harness (or voltage wire) which I do have. I had continuity at pin 2 (non-voltage wire) yesterday but just rechecked this morning and seem to have lost it (showing infinity).
I was thinking that maybe it is jostling around while driving. It is just hard to trace the wiring since the truck is so old old and dirty they all look the same color and disappear under IP. Im leaning now toward that return wire. If I knew where it was exiting harness I could just run new wire to that point and then start checking for breaks/continuity. Problem is my wire colors dont match any schematics I have on this truck. Previous owner did a lot of his own wiring and tapping also.
Click to expand...

I was having lots of problems with My truck until i cleaned avery connector block that has anything to do with the engine controls.
It is now even showing boost when driving on the road at 70mph, before it was showing 0.
 
Thanks for the responses. Mentioned RPMs because when working correctly goes a little above 2200 to 2500 then always seemed to drop and rest near 2000 in OD higher speeds and 1500 to 1800 when 55mph or so. Just have had the truck so long you can "feel" when it is not acting right. Acceleration is strong but with tach sitting near 3000 at 75 you can hear it kind of "stuck" not wanting to shift again (TCC) and just burning through fuel. If in D, (4th) and I manually shift down to 3rd RPMS surge higher then drop when manually shift back back into D. Am i incorrect in assuming that all 4 gears are there?

I just removed air intake and fuel filter and have gotten back to the harnesses where all the various sensors from front of engine connect before branching again to ECM and through firewall dash. Have a little corrosion on some and looks like critter was building a nest until some oil from a valve cover leak overtook him or he got trapped there. There was a ground disconnected and wedged in the firewall insulation. Large ring connector on it. Going to start some back probing and hope something turns up.
The lack of continuity on the coolant temp sensor is bugging me since that part has specifically caused me problems through the years with no starts, limp modes etc. I have always had to mess with that connection to get truck running. May have nothing to do with it and just a coincidence, but hard to take these blinders off.
 
Like @WarWagon said. Make sure to clean all the grounds. Especially since you found the one at the firewall unhooked. You would be amazed at how dirty grounds and unhooked grounds can effect these trucks. Dirty grounds cause all kinds of different symptoms.
 
You need a scanner, no ifs ands or buts about it. From your description of rpm's and such, it sounds like you're getting 1st, 2nd, 3rd, and 3rd with lockup. Your rpm's sound like your not getting to 4th at all. Try this 1st, pull your gear shifter ALL the way down to 1st manually, then click it up to 2nd, drive, then overdrive, and see if your gear display is following actual. The gear display is run off a switch mounted to the shift shaft, and I've seen them read wrong, and say it's in a different gear than you actually are in. You may be actually putting it in 3rd when it says overdrive. The ecm reads the pressure manifold switch internal to the trans for gear position, so they can be different.
 
Does the disconnected ground wire go into a harness or is the other end connected to the fire wall ?
If other end is hooked to firewall, hook it to the back of the intake manifold, it needs to be hooked to the engine if it goes into the harness too.
 
Starting to put back together. The disconnected ground is a larger guage black wire taped to a smaller ground wire that is connected to intake. The ring for the larger guage looks like it should sit on one of the intake studs also (same length wire as smaller ground). These grounds are coming from a different harness connecter, but right next to the one that my Coolant temp sensor ties into. The Coolant temp sensor feeder harness at firewall when disconnected from front sensor harnesses showed continuity and voltage so I just kept probing wire from front (after replugging in front harnesses) until continuity alerted. Cut wire there and spliced. Now coolant temp sensor at front showing both voltage and continuity.
Am checking all those connections at firewall, unplugging and replugging and cleaning grounds as I come across them. Will test drive in a bit after I get everything back together and move on to the other suggestions you guys have provided and post back results.
 
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yessir, back up and running and TCC lockup working again. Nice to feel that distinctive "drop" in OD. Decided to do an oil and filter change while I had it the garage. That coolant temp sensor wiring was yellow for voltage and black for continuity for reference purposes. The connecting harness was right behind fuel filter with 2 yellow, 2 black, along with orange, green, and brown. 8 pin connector, with mine having 7 wires.

Thanks to everyone who offered advice and suggestions. I really appreciate it and am relieved it turned out to be a squirrely wire in the engine valley. Long drive tomorrow so we will see how it holds up. Thanks again guys.
 
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