I JUST BUGHT A 91 1500 XTRACB 4X4 PICKUP. IT RUNS FINE BUT, WHEN I GET ON IT FROM A DEAD STOP IT BOGS OUT. SO I HAD A FRIEND BRING OVER HIS SNAP ON SCANNER AND MY ECM WILL NOT COMMUNICATE WITH THE SCANNER. MY FRIEND TELLS ME THE ECM IS BAD BUT I CANT UNDERSTAND WHY THE TRUCK DRIVES SO GOOD WITH A BAD ECM AND HE CANT TELL ME. DOES ANY ONE HAVE ANY INPUT ON THIS? ALSO IF I NEED A NEW ECM DO I NEED THE OE 7747 OR THE 8747 THE STICKER IS MISSING. THANK YOU FOR ANY HELP YOU CAN OFFER

 

you are either running in limp home mode or have a bad ecm. You should try to jump the aldl with the key on and see if you can get communication that way. The prom in the ecm has a broadcast code that will identify it and let you know what ecm that prom goes into.
http://www.gearhead-efi.com/BCCFind/

 

Key on engine off you should see a blink of CE/SES and solid on. are you seeing that?

 

connect to pin A8 on the ecm with a voltmeter and see what voltage you get. That is the serial data line. It could be that the ALDL connector (diagnostic port) may not be getting a signal or it could have been wired wrong. If still no voltage ,try another ecm. My car ran great but I could never connect with a laptop/scanner. Turned out to be the ecm. Low voltage on the serial data line.