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4L80E Tranny - no over drive, no slipping, no codes everything else drives fine

G

Grumoly

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Hello all
New here and nearly new to the 6.5 diesel - I love it.

I have a 1997 6.5 diesel crew 4x4 1 tonne
Starts and runs great drives straight.
A few months ago I was camping with family - drove 150 miles and tehn another 40 miles on really rough road (lots of shaking) pulling a 2500lb trailer. drove slow and no probs.
Camped for a few days and then on way home realized no overdrive. TC seems to be engaging but wont go into overdrive.
It is same when hot or cold. no slip. no hard shifts.
Had it into a shop and he drove it with the computer hooked up and said the computer is sending signal to tranny to go to OD but eh tranny isnt hearing it. He mentioned a problem with these that there is a short sometimes in a dignal wire (orange?) sometimes?
He hasn't had time to work on it so I took it to another shop who is supposed to be pretty good and he drove it and said definitely a hard parts problem and needs a rebuild...

I have been driving it since for a few months and no change for better or worse. tows fine, doesnt overheat - tranny fluid looks fine.

Any thoughts?
 
Thanks for the suggestion. I have checked and there are no codes showing. I have checked and the 2 mechanics I had it to confirmed.
I appreciate the reply.
Have a great evening
 
Yup. It took a new two wire coolant sensor to fix the no od on my K3500.
It is the two wire sensor on the thermostat crossover right top and center of the engine. Cant miss it.
Have the new sensor in hand, preferably a Delco brand, unscrew the new one and right quick get the new one in the hole. Only a very small amount of anti freeze lost, not even enough to worry about having to do a topup.
 
To be clear: No Codes mean the ECM isn't commanding OD, Period. It would throw a ratio error or a slipping error code if it commanded a gear and didn't get a shift. I have seen one ECM not store codes, but, that's a rare failure.

I doubt it's a trans failure.

Does the Cruise Control work? And by that I mean is the brake switch bad keeping the brake lights on? Tapping the brakes will unlock the TCC. This is a quick and dirty way to see if it's in 4th with the TC locked: tap the brakes and watch the tach jump some RPMs. No RPM jump means the TC is unlocked. Many mistake the TC locking as a shift. The engine running badly will also unlock the TCC as a engine stall saver and for less emissions. Manually run through the gears from 1st to 4th below 45 MPH counting the shifts as you select the next gear. Over 45MPH the TCC can lock up.
 
Another voice in the chorus to check the ECT sending unit at the crossover (not the one in the head). Best way to diagnose is to hook a scan tool that can read the PCM in real-time and load the ECT PID. If the engine is clearly at operating temperature, but the ECM sees cold coolant, this from a failed sending unit or possibly a grounded wire.

When the computer sees cold coolant, it starts to do lots of tricks to warm the engine. This includes not allowing a shift into OD.

If the motor now sounds a bit more machine-gun like than usual, this is another clue that the ECM is seeing cold coolant temps and advancing the timing to compensate.

Welcome to TTS!
 
WarWagon said:
To be clear: No Codes mean the ECM isn't commanding OD, Period. It would throw a ratio error or a slipping error code if it commanded a gear and didn't get a shift. I have seen one ECM not store codes, but, that's a rare failure.

I doubt it's a trans failure.

Does the Cruise Control work? And by that I mean is the brake switch bad keeping the brake lights on? Tapping the brakes will unlock the TCC. This is a quick and dirty way to see if it's in 4th with the TC locked: tap the brakes and watch the tach jump some RPMs. No RPM jump means the TC is unlocked. Many mistake the TC locking as a shift. The engine running badly will also unlock the TCC as a engine stall saver and for less emissions. Manually run through the gears from 1st to 4th below 45 MPH counting the shifts as you select the next gear. Over 45MPH the TCC can lock up.
Click to expand...

Thanks for all the help

Yes the Cruise control works.
Brake switch not keeping lights on
Engine Runs Smooth
Maually running thru gears gives 1-3. move to overdrive and no change.
 
Thanks again for the reply
how do I do this? I have a basic scan tool that I can plug in and it usually runs at 90 ish celcius while running. Not sure how acurate it is or which sensor it is picking up from.
 
90C seems about right for operating temp, especially if it stays in this range while driving.

For a double check, might try going back to the shop which hooked-up a computer and specifically ask to get readings of the coolant temp.

@THEFERMANATOR, any thoughts on this one?



Toward other diagnostic tools, one low cost potential item is the Torque app on a mobile device and a dongle that attaches to the OBD-II port. The free version of Torque will let you know whether it is able to read the PCM at all. If Torque is able to see the PCM, then it is just ~$5 more for the Torque Pro app to get better features. One thing of note is that I do not recall whether anybody got Torque to work with this early of a GM pickup.

If Torque does not work, then the next set of options are things like AutoEngenuity and the Edge Insight monitor. These are on the somewhat expensive side, although they are worth it from a diagnostic ability. For example, AutoEngenuity has the ability to disable a single injector which is useful when just one injector goes bad and you are trying to figure out which one.

There are other tools which can read the PCM in real-time and am sure that others will chime-in with a list :)
 
Last edited:
Hi again
I took advise adn replaced the etc. Made no difference. but at least eliminates one thing.

While out for a test drive I paid more attention the the shifting and while manually goin gthru gears there is def 3 gears but cant feel lock up in any. Temp was a steady 89 degrees celcius. load up to 40% I think( not sure what that means on the scanner)
Dont feel any slip but when I take foot off gas revs dropp down maybe 1000 rpm. ie 3rd gear at about 50 mph is 2300 rpm, foot off and drops to 1500 ish
 
Can you read transmission data? You need to confirm good feedback from both the engine and transmission temperature sensors. A failed trans fluid temp sensor can prevent 4th gear. Also check APP, VSS, RPM. Then drive it and watch for the commanded gear and gear ratio feedback. If its not commanding 4th or TCC then something is telling the PCM not to. If it is commanding both, then you more than likely have a mechanical issue with the transmission. I would confirm good wiring and solenoids before I'd pull it though. If the two shops you took it to are any good thats what they did and they already told you the diagnosis.

Codes are always the first place you start. But don't rule out something because its not setting a code.
 
Shop one
Put it on a computer and drove while attached. no codes. computer was signalling to go into OD but nothing happened. "I seem to remember these old trannies use to have an electrical short problem in the harness. an orange wire?". Too busy to look at it.

Shop 2
No Codes. Took for a test drive and no overdrive. figures it needs a rebuild. cant be solenoids or wouldnt just loose overdrive. something about a sprag clutch siezed on maybe. - all from a 10 minute drive.
 
There is only 2 shift solenoids, if you have 1st, 2nd, & 3rd gears, then it cannot be a shift solenoid or a wire to it. Without seeing the datastream of what it is doing, it is tough to say what is wrong. You need to monitor the shift solenoids, gear requested, calculated gear ratio, and pressure switch positions. You need to find out if it's requesting 4th or not, if it is requesting it, is the pressure switch showing the shift valve change. If it is showing it, but not going into 4th, you have an internal issue. It could be a blown out seal in the overdrive section, or even the 4th gear assembly bolt sheared off(it's not common, but can happen), but without seeing the datastream, it's all a guess.
 
Ok, I'm new to the forum. I came across this thread looking for insight to the 4L80E in my 1999 K3500 454 4x4 with 103,000 miles losing overdrive only after it gets to operating temperature. I've changed the ETC, all the solenoids, TPS, fresh fluid and filter. When you first start it up in the morning it shifts through all 4 gears fine but once it gets up to temp it goes into limp mode and loses overdrive. I'm not 100% sure the converter goes into lock up when it's cold or warm. Any insight would be greatly appreciated. received_799163290755622.jpeg
 
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