Hey everyone, I'm new here and I've done some looking around but can't find anything helpful as far as the problem I'm having. I have a 2012 Silverado 5.3, 6L80. since I've had it every now and then when I'm accelerating (could be hard acceleration or mild) sometimes the thing shifts so hard its like I've been in a head on. if the roads are even a little wet or icy the rear wheel will lock for just a second and then everything is fine. if i try to get it to happen again to get a better idea of what it is most times it doesn't happen again. its driving me insane. I'm no talking about a light clunck or stutter, I'm mean at 110 KM/H the thing wants to stop hard for about a second and then its like nothing ever happened. and insight is greatly appreciated

 

1) Have you checked the transmission fluid level and condition?

2) Sounds like the ABS or one of the stability systems may be malfunctioning.

I’d recommend checking for codes in the ABS module. This can be done with SOME off the shelf code readers, an OBD dongle and phone app, or any service shop.

 

Fluid is good, and I don't think it's ABS because it happens during a shift. I'll check tho I have a decent code reader

 

Suggest start with looking at the wheel that seems to be locking up.
If it has drum brakes on the rear, and a return spring is broken it might produce this.

Other than that, might need to look at the differential.

 

That’s my misunderstanding then - I thought the wheel lockup was a separate issue from the shifting.

Hard shifts are usually caused by worn mechanical components or faulty sensor readings. Since the hard shift is difficult to reproduce, it’s unlikely a mechanical problem is to blame, however the more often the hard shift occurs, the more likely it is that it will cause damage to the transmission.

Sensors that affect shifts can include the crankshaft, transmission input & output shafts, and transmission fluid temperature and pressure.

If there’s no codes to follow - or all you get is a generic P0700 - there’s a lot of paths diagnosis can take.

The first step is to look for obvious causes - since the fluid level and condition in the transmission is good, visual inspection of the wiring harness and connectors is next. Look for areas where the harness has clearly been damaged as well as areas where it could be chafing on the frame, engine or transmission. Check connectors for moisture and oil intrusion as well as corrosion. Clean and repair as necessary.

This includes verifying all fuses for the transmission and drive line systems are good - not just visually inspecting the elements but also checking for loose contact.

GM has issued several TSB’s regarding shift problems with these transmissions with causes ranging from air getting trapped in the clutch packs to software updates. Your best chance to determine the cause (especially with no codes) is to pay attention to specific conditions when the problem occurs. Even if it seems random there’s usually a few common factors that will help narrow it down. Here’s at least a few things to record:

1) Which shift is occurring. If this is a highway problem, it’s likely a shift involving gears 4, 5 and 6, but is it an up shift or down shift?

2) What is the vehicle speed?

3) If your info center will read it, what is the transmission fluid temp?

4) Is the engine up to operating temperature?

5) How long have you been driving it that day and was it primarily highway, city, or both? Did you stop somewhere and shut the truck off (or let it idle) for a few minutes? Did it occur during the first or last trip of the day and how much time passed between trips?

You don’t need exact answers but the more specific the better - for example, “8 hours passed between trips” is good vs. “it sat all day while I was at work” isn’t helpful because nobody here knows how long you were at work.