'01 GMC 2500 HD hard shifts
 

Been getting hard shifts from my truck lately. Intermittent issue, but may be getting more frequent. If it was in one of my Pontiacs with a TH400, I would say it was the vacuum modulator, but doubt seriously that this truck has one. Replaced by electronics? Tow/haul kicking in randomly maybe?

Fluid level fine, w/ bubbles on the stick when you check it running. Got about 120k on it.

What have I got going on?

If you have the 4L80e trans, then check for a knock sensor code. I forget why or how it's tied into the trans wiring but we had that once and it drove us nuts on a dually and only applies to the 4L80 trucks. Had firm applies into drive and reverse. Just general high line pressure

Thanks. That makes sense with this vehicle. Had a knock sensor replaced about a year ago. Still have to run premium in it to avoid knocks, but majority of issue resolved. When gas formulas change seasonally I still get the knock even with 93 octane. Also more prone to do the hard shifts when A/C is on.

It's like I got a street strip kit in it!