2000 Mitsubishi Montero Sport

[JohnHere]

I didn't know it had a thermistor, also known as a "frost switch." From what you said earlier, I was going on the supposition that it had a LPCO. Moreover, having two thermistors installed doesn't make any sense to me, either, because the only reasons for its existence are to prevent evaporator core freeze-up and blocked airflow. It's designed to monitor the temperature of the evaporator core, turning the compressor on and off as needed.

I would get rid of the extra thermistor, hook-up and position just one thermistor (OEM preferably) in the proper location, and I believe your problems will be solved.

[Mark86]

Yes. I had a list of the pressure specs and had incorrectly assumed the in compressor TEMP sensor was a LPCO switch. The specs say LP and HP cut off but only looking on another page did it show those numbers were for a binary switch, installed in the R/D. Which never sees low pressure.

The LP operating range seems low to me, has anyone seen a low side (vehicle) operate at 13-28 psi?

[Mark86]

I'd be tempted to just leave the troublesome one out of the picture and see what happens.

Thermistors are used in resi AC to help the controller that runs the variable speed fan motors (which is a hoax). These thermistors are constantly burning up, costing homeowners major bucks

That's what I did to trouble shoot. With both out it was well on its way to freezing up. Then I would introduce one at a time. Only one had any impact on cycling the compressor.

[JohnHere]

The LP operating range seems low to me, has anyone seen a low side (vehicle) operate at 13-28 psi?

At 13 PSI, the evaporator in a malfunctioning clutch-cycling system would be operating at about 10°F, icing it up very quickly and blocking airflow. At approximately 28 PSI, you'd be looking at an evaporator temperature of about 34°F, normal for a correctly operating clutch-cycling system.

[Mark86]

Success.

After doing more homework I found that the replacement ACC (air conditioning controller) which was a field replacement for a rash of failed controllers. Finding the literature for that states the original thermistor to be left in place, and to drill a small hole in the access panel and locate the new thermistor which comes with the replacement ACC. This was a Mitsubishi SB for the field repair.

I removed the thermistor which was in a factory harness with plug, as the original ACC had been replaced, making this thermistor irrelevant.

The superseded ACC which came with a thermistor is the one to use, and the SB had very clear instructions on where and how to locate the thermistor. My guess is this in the field fix could be accomplished without having to remove the evap housing, or discharging the system.

On to the results.

The system is blowing nice and cold. Clutch cycles off at an evap core temp of 35* (measured via Fluke probe) and a vent temp of 38*. I installed a new OEM fan clutch and that helped with the idle pressures and temps.

Thanks for all the help and direction. I learned a lot about this type of system and broadened my overall AC knowledge.