'04 Chevy 2500 slips on takeoff, but no slippage if tow/haul mode is used

[working on it]

I rarely drive my 135000 mile 2004 Chevy 2500 since I retired, usually just to tow my home-built squareback trailer (2250 lbs) on camping trips. Before retirement, it was a hauler for my drag race car (for 5 years), towed my camper (for 9 years, now), and also my daily-driver (for 5 years, then part-time after I bought an HHR panel as my daily). The bed is full of camping gear and emergency-use supplies (about 800-1000 lbs), year-round, so it sits while other cars and trucks get more usage.

 

After sitting for three months this time, I changed the oil, topped up some other fluids, and took a 500 mile trip with my trailer attached. Sometimes, if I started off too hard from a stoplight, the 4l80e felt like it was slipping, but not so much if I feathered it at the start. At first I wondered if it might've been a U-joint getting loose, but a quick test underneath proved it to be OK. Furthermore, when I pulled off into soft mud, it really slipped before it grabbed (two-wheel-drive). Though my 6.0L engine/4.10 rear combo has no trouble pulling the 2250 lb trailer, I started using tow/haul mode on the second half of the trip, whenever I was either braking in traffic in urban areas, or when taking off from a dead stop. No slippage when tow/haul is engaged.

 

I had the solenoids upgraded about 50000 miles ago, when stop & go daily driving in rush hour traffic was causing erratic behavior, and I used Lucas Transmission Fix about 25000 miles back, when I felt a slight slamming into second gear (upon part throttle). In each case, the problems went away. I didn't check my transmission fluid before the trip, I confess, but I did check the tranny pan, rear seals, differential for signs of leaks, and found none. On the trip home, I stopped just around the corner and bought a new grease gun & cartridges (for the driveline, which may need some, though I did the front end 10k miles back), and a 24 oz bottle of Lucas Transmission Fix (if the fluid level is low, I'll just top up...if the level is OK, then I'll drain/siphon 24 oz and replace with Lucas). It worked before, so why not try it again? 

Edited April 26, 2022 by working on it

[Jsdirt]

Sounds like it may be near the end of its life. Tow/Haul boosts line pressures, so there's apparently a low line pressure problem that's probably allowing the clutches to slip. Either a worn out valve body, or a stiff or broken o-ring allowing fluid leakage at some point in the hold circuit.

 

Usually this generation lasts much longer than that (Especially the 80E vs. the 60E - my 60 only made 103k before losing 3rd & 4th without warning, like a light switch, from a stop light), but it sounds like you've had a bit of a load in there between the drag car, camper, and the gear in back for some time. I would wager the o-rings inside have just aged out and become stiff as a brick, and brittle. 19 years is a long time, especially if you either live down South (or someplace warm), and/or got the transmission over 200° regularly. Usually they'll shrink and leak, which is what it sounds like to me is just happening now. I'm not a transmission expert - I farm that stuff out. It's the one thing automotive that I refuse to deal with besides body work and paint (Unless it's a rattle can), but I know a little about the GM trannys. 

 

The cheap way out would be to just use Tow/Haul at all times, and run it until she stops moving. Otherwise a rebuild, or new transmission from a builder would be the way to go. Since you don't drive it much, if it were me, I'd do the Tow/Haul option. She might be good for another 30k that way, which could be several years down the road. But once it starts slipping in Tow/Haul, you have no bailout, so if you end up a distance from home with the camper, it could end up a super expensive trip. Something to think about. 

 

At this stage it might not be a bad idea to at least change the filter. 50k is about the max you want to run one, especially towing, even with synthetic ATF. There's a possibility that could be the cause of the low line pressure.

Edited April 26, 2022 by Jsdirt

[working on it]

On 4/25/2022 at 11:07 PM, Jsdirt said:

... Tow/Haul boosts line pressures, so there's apparently a low line pressure problem that's probably allowing the clutches to slip. Either a worn out valve body, or a stiff or broken o-ring allowing fluid leakage at some point in the hold circuit

...The cheap way out would be to just use Tow/Haul at all times, and run it until she stops moving. Otherwise a rebuild, or new transmission from a builder would be the way to go. Since you don't drive it much, if it were me, I'd do the Tow/Haul option....

I used a quart of Lucas (after draining a quart of the old fluid out) and it immediately helped with the slippage, but as the fluid warms up, and viscosity decreases, it slips more, once again. So, I've been using the Tow/Haul  button when leaving a light, or creeping in traffic, and switch off after it's about to hit second gear. Works fine that way, at least for the very few miles I drive it (only 25 since April 25...about 5 miles per month, haha).

 

While checking the emissions monitors today, I decided to use my Foxwell NT510 Elite bi-directional scantool, which I bought to use mainly on my '01 X5, and also for my GM vehicles (it's got both programs), but I usually just use my dumb scantool (Innova 3100i) or Torque Pro app on the 2500HD; using the Foxwell, I tried to test the Evap system with active controls (it passed the "tests", but the Evap monitor still reads not ready (not a concern, all others will pass the truck). But, meanwhile, I discovered that the Foxwell will increase/decrease transmission line pressures as well! Perhaps I could safely increase the line pressure to help out with first gear?

 

If so, what pressure increase % would be safe, without blowing seals? or should I just leave it alone, and use Tow/Haul until rebuild time?

[Jsdirt]

Usually scan tools will just boost line pressure for testing purposes. Normally shutting the tool down, or exiting that part of the screen will end whatever inputs you just put in. I'm not familiar with that tool at all, so I'm not sure if you actually have that capability or not. Only tools I'm aware of that have a permanent programming capability are J-box programmers, or flash tuners like HP Tuners.

 

That said, if you go too crazy, at the very least you'll have uncomfortable upshifts or downshifts (Or both) at part throttle, at the worst you can break the pump or blow seals, pistons, or o-rings.

Edited September 24, 2022 by Jsdirt