Transmission slipping

[kristi77]

I have a 2003 Chevy 2500 HD that has 180,000 miles. It started with a clank in the front end only when you let off the gas all the way, does not do it when changing gears. A couple of days ago the transmission started slipping only when running slower, below 60 mph. Also the transmission temp does not get above 150 and most of the time. There is no leakage of fluid at all. I changed fluid, internal filter, pan gasket, and external filter about a year ago. I just changed external filter and fluid again today to see if that made a difference and it did not help at all. Does anyone know what this could be?

[DmaxMaverick]

Welcome aboard!

What is happening that leads you to believe your tranny is slipping?

[kristi77]

It jerks really hard and loud knock, the rpm goes up then it's like it will finally catch and go into gear. I doesn't do it when running above 60 mph and in overdrive. It has a lot of highway miles, my husband works about 45 minutes from home and drives it everyday. Haven't really done a lot of heavy pulling with it, only a few times.

[DmaxMaverick]

That's not "slipping", in the traditional sense. You have an electrical, electronic, or power issue. Is the SES lamp on? The first thing to do is change the fuel filter.

[kristi77]

No lights on at all. We change the fuel filter on a regular basis, every other oil change and do run seafoam through a full tank a fuel every time we change it.

[DmaxMaverick]

Any abnormal transmission performance, including any noticeable slipping, will trigger the SES lamp. It is more sensitive to problems than any other system on the truck (GM could learn from Allison Division). Something is causing low power, and is not causing the PCM to complain. What happens is, the power input to the trans is less than what is expected, according to environmental conditions, gear range selection and throttle position, which causes the erratic shift pattern. This is almost always a low power condition that isn't triggering an engine, fuel or emission system fault. The air filter, if plugged, can cause this, although it will usually also cause MAF errors. If the air and fuel filters haven't been changed since this began, change them. All of this assuming there is NO power enhancing mod's involved. If this is the case, it/they must be removed for a proper diagnosis. Also, don't rule out fuel contamination (like a bunch of gas in your Diesel fuel tank).

If it becomes necessary, have this diagnosed/repaired at a medium duty truck or Allison certified shop. Any "car" dealer will cost you about double, take a LOT longer, and not always actually fix the problem (they will end up sending it to the medium duty/Allison shop). Funny as it sounds, you are almost better off taking an Allison problem to a Ford or Dodge dealer. They'll send it out to a medium duty or Allison shop as a first option, not the last. Dealerships are too arrogant with their own brand.

[kristi77]

Thanks, I took it and had shop and they hooked it up and stated there were codes for transfer case and #8 glow plug. He also said that the universal joint needed to be replaced. Could this be my problem or should I get a second opinion. He said there is a sensor on the side of the transfer case that is bad. He also said the first code for the glow plug came up 26,000 miles ago. Which one is causing it to feel like the transmission is slipping?

[DmaxMaverick]

The glow plug should do nothing but make it complain (turn on the SES and store a code). No performance or drive-ability issues. None.

It's hard to say about the rest. I'd have to know the code(s). Some of them, perhaps a transfer case code, could prevent the torque converter from locking, which can seem like slippage.

The U-joint won't cause shifting issues or tranny slip, but should be repaired if necessary.

Fix what you know needs fixing, then try to repeat the problem. Like I said, if the tranny is, in fact, slipping, there will certainly be a P07xx code stored. If there is no code, then the tranny is fine. If the transfer case code is preventing a torque converter lockup, then that may be the fix. Fix what you can, and we'll take another look at it, if it continues.