automatic tranny Problem???

[frogcity]

I've got a 95 GMC 1/2 ton with 6.5 TD 4wd with auto trans. 90K miles. I thought I was having a motor issues, but now I am not sure. Under a load it was like skipping or stuttering. I replaced the pmd/fsd with an aftermarket one, changed the fuel filter, lift pump, relay, and ops. I am still averaging 17 mpg last I checked and there is no unusual exhaust smoke. But the problem is getting worse. I have tripped the codes and find no faults. Tonight was pretty bad. I tried different gears all with the same resulting stuttering. Before it started around 2500 rpm, but tonight it was starting around 2000. Was really disgusted with it tonight, so on a long downhill, I shifted into neutral and held the motor at 3500 for about 5 min. with not a skip or stutter. Put it back in gear, and the problem was back, so now I am thinking tranny problem and not motor. I have also noticed that I can be on the highway doing 70 mph, let off the gas, the motor drops to idle and the truck basically freewheels. Any suggestions???

[frogcity]

Went out this morning to take another look at it. I now have code 39 on the computer.

****update****
I disconnected the harness from the tranny and cleaned it out. I left it disconnected and took it for a test drive. Does not shift like this, runs 3 grand at 50 mph, but no skipping or stuttering. Plugged it back in, now shifts through the gears, but the stuttering has returned. So I guess that means it is a trans. problem. So now what do I do? Do you think that if I changed the fluids and filter that will clear it up or do I have other issues?

*******
just realised that I posted it as a 95, mistake, it is a 94.

[moondoggie]

Good Day!

Does it feel just like a gasser with an ignition miss? That's most frequently a failure of the LP system. IMHO it's time to quit replacing parts & start troubleshooting actual function.

Get a helper (a kid can help), put a hose on the fuel filter bleed port on top, crack the fuel filter nut to open the bleed port, & stick the hose in a can. Block the wheels or set the parking brake, put the tranny lever in any position EXCEPT park or neutral, & have your helper turn the key to the crank position - the LP will be powered, but the engine won't crank. You're blessed - you're truck's a 95, & from what I've read over the years, this is the only year where this works.

Fuel should squirt out the hose at a pretty good rate. If it does, you've verified the function of everything in the LP system except the OPS & assoc. circuitry. If it doesn't, start troubleshooting. The LP system includes the sock filter in the tank, the LP, the relay that energizes the LP during cranking, etc. - you get the picture.

Once you've verified and/or fixed the LP system, let us know & we (you, me, DP members) will take the next step(s).

Good Luck & Blessings!

[frogcity]

LP system was my first thought originally. I did the lp diagnostics and did find it to be faulty. I replaced lp, relay, and ops. I did the pmd/fsd at the same time just to stay one up on things and because of the oem reputation for this. I have since checked the lp system and have found no issues. in fact I have checked it several time since replacement, because it does seem so much like that is the prob.

I am now leaning toward the tranny because of what I previously posted. But I am in unfamiliar territory here. For the last 30 yrs I have owned and serviced nothing but sticks. I actually prefer to drive and tow with a stick, but this truck is what I got now, so I need to learn a little bit about autos. esp. since that appears to be the issue.

[MTTwister]

frogy - That part seems to be the normal operation, unfortunately. I think there might be some ways to get some transmission braking on the '95, but not sure.

How does the tranny fluid look and smell? Good luck!

[moondoggie]

Good Day!

"...on the highway doing 70 mph, let off the gas, the motor drops to idle and the truck basically freewheels." Completely normal.

Code 39: "TCC stuck OFF" I'll start by saying I know absolutely nothing about trannies. That, however, has never stopped me from having an opinion. That being said...

I wonder if what you're feeling is a TCC that's being commanded by the PCM to lock, but it's worn (or something) & it can't. The PCM can (I think) determine whether the TCC actually locks or not by comparing input & output shaft rpm's. If this is so, then the relative rpm's being wrong sets the code. I don't know much about the PCM's algorithm - it would seem odd that the PCM would determine the TCC wasn't locking, sets a code, but continues to try locking the TCC, causing what you're feeling.

After the code was set, did you notice firmer 1-2, 2-3, or 3-OD shifts? The way my Gear Vendors Aux. OD is hooked up (non-standard), I have to be careful to never have it engaged in any gear except OD. For some reason, the PCM's algorithm doesn't check for slippage in OD, so it's unaware that the input/output relative rpm is wrong due to my GV being engaged. When I've occasionally forgotten to kick out the GV & driven in say 3rd gear, the shifts get real firm after that - the PCM has commanded higher line pressure in the tranny to try to fix the slippage.

Good Luck & Blessings!

(signature in previous post)

[frogcity]

I think I'm getting closer.

Talked to a tranny place yesterday. Didn't think it was right code, but if computer says 39, replace this relay. only 25 bucks, everything else is big bucks. I've been meaning to change tranny fluid anyway since I got the truck 3000 miles ago. Fluid looked good, no bad smells and pan and tranny was free of sludge. Changed filter, fluid, and relay. Much better, but not 100% yet. But definatly better. Really got to work her hard before she falls on her face, but now she falls off a little differently, feels like a fuel starvation problem. Cleared computer codes and retested, still get a code 39, but not going to worry about it at this point, since she runs and shifts better and the fact that it feels like fuel starvation now.

This morning I tested the LP sytem after she was well hot and running hard for half hour or so. I usually tested when the motor and surroundings were cool. Definately seem something is off. She pumps, but not as strong as I would like to see. Decided to drop the tank. Internal filter is clear and it appears that there is a valve in it that opens if the screen does get clogged. But what I did see was a partially collapsed intake line. The rubber line from the steel lines to the tank is old and dry rotted and very soft. The crease in the rubber and the memory of it makes it appear that it has been there for a while. Might also explain why it never gave me a problem when it was cold, rubber was stiffer in cooler temps. Going to replace that and hope for the best. Will post updates.

[frogcity]

Fuel line fix has removed the engine issue. Pumps fuel better than ever and runs great. Ran it yesterday for 100-150 miles with not a beat missed.

Still not happy with the tran. Cleared the codes, checked to make sure the codes were cleared and ran the truck for 1/2 hour or so. Came back and rechecked the codes and found the code 39 tripped again. 1 to 2 shift is a firm noticable shift. After that you cant feel it, but the rpms do drop as it goes up the gears. And it does appear that I am running about 2-300 rpms higher at highway speed. Last week it was about 1800 rpm at 60 mph, now about 2100.