4l80e doesnt shift up after downshifting to pass

[ahansel1229]

I got a 4l80e in my 1992 c2500 and whenever the truck downshifts out of overdrive to pass it will not go back in and if I let off the throttle it will drop to idle speed will going 55-60 mph. I then have to rev the truck up and catch up to the speed of the transmission before it starts to speed up anymore. I'm not sure if it's related but the truck also has a super low idle in gear it drops to about 450 or so and when hot 250 and stalls sometimes.

 

[Schurkey]

How old is the fluid and filter? Fluid level correct?

SOMETHING or (more likely) several "somethings" in that truck are unhappy. Verify fuel pressure, connect a scan tool to find out what the computer is trying to tell you about the idle speed and the trans shifting.

Wouldn't hurt to inspect/replace plugs, wires, cap, rotor, fuel filter, etc.

 

[tayto]

probably low AFL oil signal. i would remove valve body and get it vacuum tested. most likely the AFL valve bore will need to be reamed and valve replaced. Transgo makes a kit, check around with local transmission shops they can probably do for you.

TransGo - 48-ACT-TL - Actuator Feed Limit Valve Boring Kit

At 100,000 miles, you can bet the actuator bore will be worn. Worn bores cause low pressure hot, clutch, band and converter slippage, wrong gear starts,

transgo.com

 

[ahansel1229]

Plugs, wires, cap and rotor fuel filter o2 sensor map sensor tps egr and controller iac the coolent temp sensor for the computer vacume lines have all been replaced. How much does a test like that run usually?

 

[Westieterrier]

Hi Ahansel, I have a similar problem w. my '99 2wd C2500 under the heavy load of my truck camper when passing on steep hills. The truck shifts into some sort of safe mode. It only did this w. the TC in the bed and on steep hills. Pulling over to stop/start again always reset it back to normal (big pain in the a$$, though).

I changed the tranny oil and filter to no avail. I posted here and was told to bring it to a shop that had a good scanner, which I did. Turns out engine codes (P0130s and P0150s) showed bad O2 sensors and the Mass Air Flow Sensor (MAF). There were no 700s codes to indicate a problem with my 4L80E, only the emission stuff codes above. My grizzled old tech said he was sure that was the problem (didn't even charge me!). Man, everything is interconnected with these trucks! But I digress . . . .

I have replaced the O2 sensors (mine has 4, arrggh!) and am waiting for the GM MAF to arrive. It's already running better and I can't seem to replicate the problem with the bed empty despite REALLY trying. Once I replace the MAF and re-load my 4,000 lb. TC, I will update my post.

Keep us posted on how yours turns out too. Thanks! P

 

[ahansel1229]

Thanks for the info I'll take a look at my maf and o2s again

 

[tayto]

1992 doesn't have a MAF sensor and codes are more limited for OBD1 vehicles (1995 and pervious). while the trans does have a limp home mode it is only activated when power is lost to the transmission.

 

[ahansel1229]

So my transmission could be loosing power when it downshifts in overdrive?

 

[tayto]

I doubt it.. in limphome mode the transmission defaults to 2nd gear and full line pressure.

 

[Sabinoerc]

Would agree with above that more than one thing going on. I have not clue on transmission, I’ve noticed tayto is one of the go to on forum for that topic.
That said, I don’t think a bad tranny would cause idle problems but messed up fuel pressure, tps, engine running can cause shifting problems. I had bad fuel pressure once which was causing low idle, studder, etc and that was also cause of hard and late shifts. Fixed the fuel pressure, engine ran well and shifting problems went away. I don’t know for sure why but think fuel pressure was so low I had to put on more throttle than normal for the speed and it put pcm in cell where pcm thought I was trying to accelerate and was controlling trans as if I were.

I think I’d get engine to run correctly first as easier (at least for me) than getting into transmission. After engine runs well, it may be easier to figure out trans problems.