1994 6.5 4L80E slow to engage

[markrinker]

One of my plow trucks has started to engage slowly into forward gear. The 'slip' is intermittant and only happens when applying throttle from a complete stop - and then only about 1 time in 5. The more your foot is in it, the more noticeable.

Any ideas? Should I investigate and rebuild now, or just let it be and wait for bigger issues? Could it just be a torque converter?

Don't know if parts are all original, but if they are, its 123K miles with lots of heavy use.

[JohnC]

Check for codes and report back...

[john8662]

I know this is straight forward, but did you check the fluid? My company truck at work started to do that (K3500 5.7L gasser w/ 4L80E). Told the maintenance guy about it, said they checked it and it was fine, since its GOV, they don't really like us to mess with it. Well next time I had to put fuel in it I left it running, pulled the dipstick for the tranny, and it was dang low, barely touched the stick. But not running, looked full. To check these trannies the engine has to be running and transmission in park. I topped the tranny off with fluid, never a problem again. I know it wasn't my rig, but I do drive it, so I take care of it.

[markrinker]

No codes set, full of new fluid and new filter. Same symptoms.

[Diesel Dan]

Check the trans line pressure.
My '93 did similar and my '00 is showing some signs. The pressure modulating solenoid/valve wears out and can't control pressure.

On my '93 the clip that retains the solenoid actually broke and let the pressure push the solenoid back and bypass lots of fluid.

Pressure on the '00 starts out at 250psi and then drops back to 100-125. It's setting a code for trans component slippage, just recently set. Tommorrow I'm gonna pick up a new solenoid.

[damork]

Mark Rinker,
Before jumping to any costly conclusions, take the output speed sensor out of the tailshaft of the tranny or the transfer case and clean the tip.

I was down the path chasing this problem, went to 2 transmission shops and was almost convinced something was wrong inside the trans.

I found this solution after hours on the web in a Hummer area. Apparently many people have had this problem and didn't know where to go.

This sensor builts up fine metal particles on the end of it and is not able to provide a clean signal for the ECM to process, so you will get erratic starts from time to time.

Hope this helps...

[Marty Lau]

Damork;
Hey thats a good plan for all of us with truck with plenty miles as a maintance item. Thanks Man I'm going to do mine when I change the oil, can't hurt.

I don't care what anybody says I have gotten more from this TDP & it's Forum than all the other Diesel sites put together. Just MNSHO

[RonD]

this may not help, but i once had problems shifting from second to third gear.Once i shut down and start again it was ok. turned out to be a battery on its way out. replaced the battery, no more problems like that.

[92gmc]

I've never been inside a 4L80E but I've had 4L60E's do similar things, in the valve body separator plate there are several (four??) small filter screens that pop into the separator plate, they get plugged and will make the tranny sluggish, since fluid can't move freely within the valve body and cause shifts. It's easy enough to check without pulling the tranny too.

[Diesel Dan]

Had my components backwards. The pulse width modulated valve is for the TCC solenoid. The force motor controls line pressure. After looking thru my service manual for my '91 sub, that I kinda forgot about redface.gif , turns out my pressures are ok. Gonna use the snap-on scanner and a my pressure gauge to compare solenoid amps to line pressure. Pulled the TCC solenoid and force motor and found screens clean, pan was clean and fluid wasn't bad. Looks like my converter may be slipping :( .
Little more time and diagnosis.....

[markrinker]

How can a converter 'slip'? Bands inside the trans can 'slip' due to excessive wear or misadjustment, but converters either work (hold pressure) or they don't, right?

[Diesel Dan]

A lock-up converter has a clutch disc much like a manual trans. The front of the converter is smooth, that is the 'steel' portion of the clutch, then there is a friction disc that is applied with hydrallic pressure. When it locks up there should be no slippage, just like a manual trans. Unlocked it has stall ability like any other converter.

I just talked to trans-go about one of their shift kits. The kit won't address my issue, he said the '00-01s had cracked clutch pistion issues causing slippage. Just like mine. So looks like a converter is in store.