Vexing problem with transmission

[Brian Scott]

First, let me apologize if this is in the wrong subforum. If it is, can a mod please assist me in moving it?

 

So... I drive a 2007 Silverado 1500 Classic. It's an LT1, with an L59 engine and 4L60e transmission with about 230k miles on both. Neither have had an overhaul.

 

Some months ago, I noticed I was suddenly accelerating sluggishly, and thought my transmission was slipping, so I limped it to my destination and parked it. After I got done, I was going to limp the truck home, but it drove normally. So I drove the truck to an auto parts store to see about checking the fluid levels and all were fine. On the way home, it was still driving normally, so I stopped at the grocery by my house to pick up ingredients for dinner, and when I started my truck, it again drove like the transmission was slipping. So I parked the truck when I got home and left it.

 

A few days later, I took the truck (not driving well) to a local transmission shop, and left it so they could evaluate it later that day. On the way, I noticed that it never changed gears. Turns out the truck was staying in 3rd gear. Anyway, they drove it, and could find no obvious problems, because it was working fine for them. They asked if the gauge cluster looked odd when it happened, because they felt like it might be the ignition switch.

 

Some details: The truck goes through intermittent times where it will work fine for several days, and where it will never work right for several days. Most days, it's a bit of a crap shoot. But one thing I noticed is that the truck never goes from working well to not working well during the course of a drive. If it starts well, it stays well, even if on for hours. If it starts badly, same story.

 

It feels, to me, like there's some reason the TCM isn't booting, or it's booting with a fault that forces the transmission into some kind of a limp mode. Has anyone ever encountered this problem before?

[swathdiver]

The wiring harness is probably contaminated with fluid causing shorts.  If yours were mine I would put a scan tool on it and monitor all the transmission functions and try to isolate the problem.

[Brian Scott]

15 hours ago, swathdiver said:

The wiring harness is probably contaminated with fluid causing shorts.  If yours were mine I would put a scan tool on it and monitor all the transmission functions and try to isolate the problem.

I know that the transmission controller has power at least to the point of the fuse block. I'm not sure where further on down the stream I could measure to see if there's power getting somewhere.

 

Is there a particular scan tool which is good at monitoring transmission functions?

[swathdiver]

3 hours ago, Brian Scott said:

I know that the transmission controller has power at least to the point of the fuse block. I'm not sure where further on down the stream I could measure to see if there's power getting somewhere.

 

Is there a particular scan tool which is good at monitoring transmission functions?

There is a wiring harness inside the transmission.  When transmission fluid starts getting inside the wiring, it causes all kinds of shorts and weird stuff.

 

These cars were made to be worked on with the Tech-2 but any bi-directional scan tool can be used, such as the Snap-On Solus.