I have a 1999 Ram 150 5.2L 4x4. The truck runs perfect without the A/C. As soon as the A/C is on it immediately throws a check engine light ( Code P123 ). I actually changed the ECM to a reman unit. Drove the truck for 20 miles with no issues. Got back home and turned on the A/C and it threw the CEI on again. I pulled it into the garage and turned the A/C temp control and the truck shut down. It would not restart and was reading NO BUS in the odometer. I removed the reman ECM and put back in the OEM unit. Truck fires right up, But I am still having the same original issue. I know the code p123 is high voltage at the throttle position sensor. Basically what is happening is I turn on the AC and the scan tool reads 100% up from 13% at the TPS. The company is sendin me another ECM, but I guess that really isn't the issue. Any ideas?

 

i'm not an expert, but i'd guess that inside the wiring harness, one of the 12 volt AC control wires is shorted to the TPS wire. the TPS control is normally only 5 volts, so when it shorts across, your getting 12volts on the tps, which trips the high voltage limit check.

look for any obvious damage or unusual look to your wiring harness. also check for loose or bent pins in your pcm connector. if nothing obvious, trace your tps connector backwards and separate it out of the harness.