khtruong0615

The A/C is not working at all, compressor and blower not running, in either manual or auto mode. No faults in ACC module but shows pressure sensor at 38 bar which is higher than limit 35 bar. Multifunction sensor also shows -40 on dew point. In wiring diagram shows both sensors connected to Front Sam and Pressure sensor should has 5 volts and multifunction sensor should have 12 volts but I got 0 volt on both. Checked continuity from plugs at Front Sam and at sensors and they are ok. Is there something else prevent the Front Sam to output the voltage to the sensors or I have a bad Sam? Front SAM has no fault either.

DaveW68

Sounds like your system could have a leak. The compressor will not turn on if the system is out of refrigerant.

snovvman

It is curious that the ACC module reads the sensor at 38 bar while you measurement is 0 volt.

I would first put a pressure gauge on the low side to verify the sensor reading. As a shot in the dark, have you tried resetting the modules (disconnecting battery)?

You can also disconnect the sensors to see if the SAM throws an error (as you would expect). This may give you a clue to something?

khtruong0615

I did some searching and yes if you loose power to the sensor then it will show high pressure. I have no gauge to test but I have try to reset and even update the modules. I disconnected the sensor and still showing 38 bar.
I think my main problem is I have no voltage out to sensors from Front SAM but don't know if something disable them or my SAM is bad. I'm going to try to tap the multifunction sensor to the battery 12 volt to see if I have some reading. Do you know any 5 volt wires in the Front SAM or near by for the pressure sensor?

snovvman

I do not. Sorry--my knowledge of the M-B systems is very limited. I'm going off of general automotive and electronic knowledge.

It's unusual for the lead powering the sensor to be 5 volts. How many wires are going to these sensors? Power, ground, and return (sense)? Another silly question: Have you checked the fuses?

khtruong0615

The multifuncstion sensor uses 12V, pressure sensor uses 5V and they all came off front SAM with no fuses in them. They may have an auto-shut down in case of a short or something else, other modules disable them. And yes 3 wire system, Pos, Neg and Signal.