i have a 1990 buick reatta coupe when i plug in my snap on scanner the dash and heater go crazy and goes into error

when the car runs it runs great

I had a car do weird things once when one of the prongs in the scanner plug was either broken off or bent i can't remember. check the plugs on the scanner and the wires going into the dlc.

Is it the right kind of scanner? I think the OBDII was 1996 and later. Not sure what a 1990 car would require.

on my crt i would press the off and warm button simutaneously and it would go into diagnotic mode and show me the codes plus the sensor operation. maybe your crank sensor is going out.

The OBD ll systems did not come out in all cars until 1997. The older On Board Diagnostic computers were not "user friendly" (Gov. quote). They were not regulated so 5 different scanner tool companies cold make the tool and all have slightly different results. Most of GM'S Diog. computers were made by Toyota and some were made by them, which makes it harder to Diog. Close but not quite is was made the Gov. to mandate the OBD'S to all perform the same. Hence the OBD ll systems. For proper Diog. the pre 97's had to be taken to the dealer to find out what computer you had and then Diog. it.

Your crank sensor does sound like it is shorting out. External crank sensors sometimes need a good power washing.