hello all,
so I am not getting any juice to the compressor.... here's what I have done so far. this is a conversion to 143A and so have flushed the system and replaced the compressor, the switch control unit (but it was a used one!! ) the temp switch for the aux fan, the low pressure switch, and the expansion valve under the dash. As some of you might know I am still trying to find the A/C compressor relay which is somewhere other than the fuse block on the drivers wheel well (must be something specific to the '85 300TD). I thought maybe it could also be the ETR valve so I pulled the glove box and bypassed the ETR valve circuitry and still nuthin, anyway so I was thinking as I do some research on where that relay is, it could also be something wrong witht the used switch control unit I got. everything works fine with the entire system except the compressor wont turn on. while I was bypassing the ETR switch I did some checks for power there and there is no power at the ETR switch, there is also no power at the pressure switch (neither side) so i am now questioning the switch control unit. so with the background said here's my question........ does anyone know how to test the pushbutton switch unit to see if it is actually sending out the power to the compressor?? thanks all for sticking with me on this. specifically I would like to know which terminal on the control unit I should be testing...

__________________
Thanks! Scott
sdpettitt@yahoo.com

oh, forgot to mention, the compressor works fine and is hooked up correctly. with the engine off I con take a lead directly from fuse 8 to the compressor and I can hear it click on loud and strong, so that is not the problem. the problem is before the compressor, before the pressure switch, before the ETR valve....

__________________
Thanks! Scott
sdpettitt@yahoo.com

hey does anyone know if the A/C compressor relay is before or after the pressure switch? that might shed some light on whether it is the relay or something else. I am thinking......... or trying to.......

__________________
Thanks! Scott
sdpettitt@yahoo.com

The compressor relay coil is powered through the evaporator temp switch. Power to the compressor comes directly from a fuse.

Sixto
95 S420
87 300SDL

thanks for the info,
so where does the power to the evap temp sensor come from? I am not getting any power to that switch. so starting at the end this is what I am picturing. power to the compressor comes directly from fuse 8. control of the compressor comes from the pressure control switch on the receiver which in turn comes from the ETR (evaporator temp) switch under the dash but where does it come from before that??
thanks
Scott

__________________
Thanks! Scott
sdpettitt@yahoo.com

In defrost, total ventilation and normal modes the PBU sends +12V to the ETR which opens below 40*F.

Sixto
95 S420
87 300SDL

" control of the compressor comes from the pressure control switch on the receiver which in turn comes from the ETR (evaporator temp) switch under the dash but where does it come from before that??

From the " ON " button on the AC controls ?