4L80E NO OVERDRIVE

[JOHNGAAA1]

I HAVE A 1999 K2500 SUBURBAN. THE TRANSMISSION SHIFTS FINE, BUT NO OVERDRIVE. REVERSE IS ALSO OK. WONDERING IF THE TRANSMISION TEMP SENSOR IS FAULTY, DUE TO THE FACT THE TRANSMISSION COLLING FAN COMES ON IMMEDIATELY WHEN THE TRUCK IS STARTED COLD? ANY HELP WOULD BE APPRECIATED.

 

[stutaeng]

Page 43 of this document may be of some help: https://www.google.com/url?sa=t&sou...Vaw0kjoWGa02i3OULRIwx35i0&cshid=1631585292032

Good luck.

 

[Schurkey]

No OVERDRIVE, or no TORQUE CONVERTER CLUTCH engagement?

Lots of folks get those two confused.

If the torque converter clutch won't engage reliably, look for
1. Faulty wiring to the trans including poor connections at the electrical plugs
2. Failed TCC solenoid
3. ENGINE MISFIRE. The computer disables the TCC if it senses misfire. I "fixed" the torque converter clutch on my '97 K2500 by replacing a broken spark plug in #8.

If there's no overdrive, but the TCC engages, look for
1. Faulty wiring to the trans including poor connections at the electrical plugs
2. Failed 4th gear solenoid

A scan tool can tell you if the engine is misfiring; and if the TCC or 4th gear (Overdrive) is being commanded "on".

 

[stutaeng]

There's away to check the 1-2 and 2-3 solenoids. All that has to be done is removing that gray connector and checking there. You should be able to see which pins you need to check by Googling it...

They should be 20 ohms each. The other 2 solenoids (PWM and TCC) are 10 and 5 ohms...I can't remember which one is what at the moment.

That source I linked has a table that shows what component should be on or what clutch pack should be on or holding on each gear... it's really handy for figuring this stuff out.

Ditto what @Schurkey pointed out on other things to check. I suppose if everything else is okay you could drop the pan to see if you any excessive burnt smell and clutch material, indicating a burnt overrun (4th) clutch.