1993 Blazer 350 tbi.
Son came in from school and said trans would not shift. I drove it and it had Rev, 2, and 3. Limp home mode. I checked fluid and it was good. Then i checked codes. found a error code for the knock sensor. Found the wire had come loose. Reconected the wire. I disconected the battery overnight and reconected in the morning. No codes, Then drove around. the trans shifted great for about 10 miles then back in limp mode. Got home checked code and none were shown. Disconnected battery again for a while and tryed again to see if it would work. Again after about 8 miles it went into limp mode. I checked for loose wires and didnt find any. Any ideas would be apreciated.

 

Try wiggling the ignition switch when it comes in and out. The computer commands "limp Mode" by just turning off the circuit that powers the solenoids and they default to 3rd, although sometimes because of like pressure variance you can get 2nd or manually. So, Anything that causes loss of power to the solenoids (bad connections, blown trans fuse(probably not your case if it comes in and out) , or a bad ignition switch (fairly common in many later GMs) or a component in the circuit shorting to ground and killing voltage will do it. If wiggling the key doesn't give you a hint start at the "pink"? power wire at the trans when it isnt working and trace back till you find where the power is interrupted. You can check the voltage that "powers on" the whole system right at the PCM if you can find a pin-out diagram, and even setup a voltmeter so you can observe it while you drive if needed to help catch the ignition switch in the act. Make sure to clean any corrosion as it can often be the sole culprit. I'm betting ignition switch. Todd