What would cause a hard shift from first to second

You have a 4L60-E transmission in your Bravada. Since your transmission is electronic then the first place to start is by scanning the ECM/TCM to see if there are any drivability DTC's or transmission related DTC's.

Usually when diagnosing a transmission you take fluid samples. You look for things like a burned smell in the fluid, darkening caused by broken down clutch materials, metal filings, and you cut the transmission filter open to see if big metal filings are lodged in there.

Another thing is that you must use GM approved transmission fluid. For your vehicle GM recommends Dexron VI. Using any other fluid will cause either damage to clutch materials or transmission slippage.

To diagnose a hard shift we can suspect the following components: Throttle position sensor - used by the ECM to determine transmission pressures and shift characteristics, vehicle speed sensor - to determine when to shift the torque converter clutch to lockup, engine coolant temp sensor - to keep the vehicle out of torque converter lockup to warm the transmission/engine to operating temperature as fast as possible, any transmission pressure control solenoid - responsible for actually shifting the transmission, and the transmission fluid temp sensor - responsible for keeping the transmission from upshifting quickly during warm up and to initiate torque converter lockup during a transmission overheat.

If you take your vehicle to a good mechanic, expect to pay one hour of labor to properly diagnose. Repairs might be as cheap as a sensor or a fluid change to actually a transmission overhaul. Or you might luck out and have a leaky transmission cooler line. If you overfill your transmission then you get a clutch slipping event due to foaming fluid caused by the internal planetary gear set churning up the fluid.

I hope that I've given you something to think about. Good luck!

- B.