I had the trans modulator and shift solenoids replaced with upgrades about 12K ago on my 2000 MC. Recently, the trans began to shift hard into 1st and 2nd. Thinking the solenoids were defective, I took the car to the original transmission repair shop. The tech conducted a scan that revealed a load sensor and thermostat issue causing the problem. The defective load sensor happened to be the mass air flow sensor. Evidently, the MAS sends info to the computer that regulates shift timing, and the thermostat was not allowing the engine to maintain a stable temp (I was told to change out the thermostat every 2 years - the higher temps degrade the metal). The shop wanted $650 to change out the 2 items. I said no thank you and paid them $58 for the diagnosis. I purchased a new 92 degree Motorad thermostat and seal for $17, and a new LLT warranty MAS for $133. The problem is solved and the MC runs like new.

 

Wow, sounds like an easy fix to what could feel like a scary problem. Good info for members. What codes did you have? Maybe put them in here as keywords for members who search for the issues in the future.

 

MCSS had 123K on the speedo: Fortunately, the tech did share the hidden codes as the engine check light did not come on when I noticed the shifting problem: PO102 - MAS, 128 - coolant temp/therm low, & P1810 - trans. press manf. switch fault (this had been changed 12K miles ago at the first repair - so may have set a code because it was not working correctly due to the failure of the other items). The tech did state that the MAS was not giving the proper voltage reading (it was low). What I did notice before I took the MC to the shop is that when I checked under the car for any leaks, etc. with the engine running, the engine stopped dead. I tried to start it to no avail since the battery was dead. I put the charger on the battery and that did the trick to restart. But, little did I know that the alternator was also on the fritz, so I had it checked at AZ to find that the rippling test was all over the map - the ripple numbers were not stable, even though the alternator was pumping out 14.5 volts. So, there is a lot to be aware on these electronically controlled cars. If one thing stops functioning properly, it can effect a chain of trouble events. And as I found out, determining what is wrong may require taking it to a shop that has sophisticated equipment to find the hidden codes. For me the $58 charge was well worth the diagnostic. In the long run I saved myself frustration and $450.