no air conditioning - DME issues

[cobb535]

I just got a 94 540 and I need a little help sorting out the air cond. The circuit to activate the compressor is good through the pressure switches (voltage going in and out), the DME is getting the signal I think because the motor rpm increases about 200 when I switch on the AC. I have traced the circuit for the compressor relay where the DME where it is not grounding to complete and energize the relay. My question is are there any other inputs to the DME that would tell it to not engage the compressor relay (the DME doing what it is supposed to) or do I possibly have a glitched up DME. Other thoughts would be welcomed.
I do not have any internal circuit logic or info as Bently stops short of this information.
RESOLVED
I was reverse trouble shooting. The DME needed the climate control to complete a circuit, not the other way around. My problem wasa bad evaporator temperature sensor. Infinite resistence instead of around 3,000 ohms at 70 degrees F told the climate control it was too cold and do not run the compressor. Good temperature sensor from the parts car - 10 minute job - fixed the system. AC works great now!
Thanks for all the help and suggestions.

[632 Regal]

if you are low on freon it wont kick on, also the low/high pressure switch could be the culprit. You should be able to jump power to the switch and get the compressor to kick on and see if its working.

[cobb535]

I have plenty of refrigerant in the system. The pressure switch is closed and passing voltage. The compressor relay is good but the circuit controlling it requires the DME to make a connection to grond to complete it. My DME is not doing that. The DME is getting a signal from the climate control computer. I was looking for other inputs to the DME to check that would tell the DME to not energize (ground) the compressor relay.

[Ferret]

I have plenty of refrigerant in the system. The pressure switch is closed and passing voltage. The compressor relay is good but the circuit controlling it requires the DME to make a connection to grond to complete it. My DME is not doing that. The DME is getting a signal from the climate control computer. I was looking for other inputs to the DME to check that would tell the DME to not energize (ground) the compressor relay.

This might sound daft, but you've not got a fuse out somewhere have you?

What happens if you short the correct line to ground?

[Bellicose Right Winger]

Some E34's disengage compressor at WOT. Check your throttle position switch.

Paul Shovestul



....... I was looking for other inputs to the DME to check that would tell the DME to not energize (ground) the compressor relay.

[Alpinewhite]

I had a similar problem. After I switch on AC it would blow cold for a moment and then it will turn itself off. Also heater was always blowing hot irrespective of the setting in the knob. I had to change climate control unit; a black box behind micro filter.

Arun

[cobb535]

Sounds like where I am heading. the compressor relay kicks on the ac when I short the coil wire going from it to the DME to ground.
Thanks for the help.
Anyone have any other ideas?