Of Corvette's and Kenworths

"I've tried everything on this Corvette and I can't get the AC compressor to turn on. It had a bad programmer, so I put another one in. Then it had a bad pressure switch that caused the ECM to set a code 66. I cleared the code but the compressor still won't turn on. The HVAC programmer was $780 and I think it is defective, but I don't want to get another and find out that something else is the culprit."
Max is a good technician. He graduated with a technical degree in both auto and diesel technology and is willing to learn. Some of the older technicians, just like instructors, don't like change. Some--not all--are unwilling to get out of their comfort zones and in this business, that's a no-no. An instructor owes it to the students to keep current. Students are reflections of their instructors.
The HVAC programmer is a rebuilt part. It was an R-12 system I had a suspicion what it was. I pressed the fan up & fan down buttons simultaneously until-00 appeared on the HVAC display. I was now in diagnostic mode. Once in diagnostic mode, you can use the fan up & down buttons to scroll through the PID list. To view the value at the PID location, you press the AUTO fan button. I went to PID position 00: System Faults. I found )), which indicated no codes. Then I went to PID position 09: A/C System Mode. It was 01, which meant MAX A/C. The command was there. I moved to PID 11: Program Number, which represents a heating or cooling command. The programmer was commanding 00, a command for FULL COLD. I moved to PID 34: TEMP DOOR POSITION REQUESTED. It was at -155, which means when you see a minus sign at the value, you add 100. So, -155 was 255, which was FULL COLD.
Everything looked good. I remembered the hidden programmer code:48. When I taught HVAC controls in another lifetime, I called Code 48 the "Claude Rains" Code. the young technician didn't know who Claude Rains was.
"He played the invisible man in the movie back in 1933," I said. "It was a classic movie."
"Oh," the young technician said. It was way before his time.
I plugged the Tech 1 scan tool into the data link connector. First, I went to PID 00 and used the clear codes command on the HVAC head. Then I used the Tech 1 to clear ECM codes. Then I turned the ignition key off, took the Tech 1 off the data link connector, started the Corvette, and the compressor energized.
"How did you do that?" The wide-eyed technician asked.
"With experience," I said. "The code 48 was intentionally masked from the programmer's data list, but if it set, it sent a signal over the UART bus to the PCM to disable the compressor. Code 48 was identified as a LONG TERM FREON LOSS code."
"But I disconnected the battery. Wouldn't that have cleared the hidden code?" The technician asked.
"Sometimes, but not always," I said.
That was an easy one, but only because I learned the hard way on that one, all those years ago. We learn good lessons from our mistakes. We have to--so history doesn't repeat itself.
Onward to the Kenworth in my next post.