I have been having this issue with my truck for a little bit now. I used to think it was a transmission problem, but am not sure anymore...since the transmission shop keeps telling me it isn't (I had a rebuild in September and was PLAGUED by problem after problem and went back to the shop like 40 times, but those issues have already been discussed in another thread, for the most part).

The problem I have is that every once in awhile, I will let off the brake at a red light or stop sign, and the truck won't move (usually will roll slowly). I will press on the gas, RPMs will raise, and the truck will BARELY move. It's hard to explain, but sort of feels as if I had the clutch halfway engaged in a 5-speed...like it isn't "catching" if you know what I mean. Once the truck gets up to 20-30 mph or so it will start driving normally.

This is a completely random occurrence...as far as I know, at least. I can't replicate it nor do anything specifically to make it happen. It just does. And when it does, it USUALLY only happens once per drive, and then won't happen again for awhile (only one time did this happen twice while on the same drive).

I have told the transmission shop about this numerous times, but they always tell me "we can't seem to make it happen, and without being able to make it happen we can't diagnose and fix it." They told me they believe it is an electrical problem because if it were mechanical it would happen regularly. They have replaced all the solenoids (twice, each), the valve body, re-did the torque converter once AFTER the rebuild (rebuilt torque converter as well) was done, etc. This is what leads me to believe it is not actually a tranny problem. Please help!

 

New transmission pump? It sits behind the torque conver. The problem you are describing sounds like a fluid problem. Is it full and was the correct Automatic Transmission Fluid used?

 

As far as I know all the transmission parts are new. I am out of town at the moment, but can ask the shop to confirm when I get back.

The transmission fluid is full, and I can't confirm or deny whether or not the "correct" fluid was used...but I would assume that the transmission shop knew which type to put in when they did the rebuild. Even if I ask them though, I am positive that they're not going to tell me "no, we didn't use the correct fluid."

 

I agree with KingsShadow, it sounds like the torque converter is occasionally running low on fluid, in which case it won't transmit any torque. As to why, I'd be looking at the pump or at whatever valves regulate and keep the correct amount of fluid in the converter. Since it "barely moves", indicating some residual torque, that makes me suspect the torque converter. If it acted like it was completely in neutral, I would be more likely to suspect a shift solenoid/valve failing to engage the low-gear clutch pack.

 

Being a mechanic for many years and still learning a lot of the newer stuff, I have had brand new parts fail too. When it screams and not moving, is it first "cold" start or after it has warmed up?