Automotive Air Conditioning Information Forum

Had a chance to work on the Montero. Pulled the evap housing out, and turned the set screw 2 turns CCW.

I'm making headway, but problem continues. LP side at idle will hover 27-28 psi. Previous was high teens. I researched and found the low pressure switch numbers in the service manual. LP cutoff is 28 psi +/- 2psi. There seems to be a dwell time of about 6-8 seconds to disengage the compressor once the switch opens the circuit, or signals the BCM to disengage the compressor. This corresponds with the previous clutch on time once below 28 psi as it pulled the low side down into the teens.

Said another way, previously clutch would engage, and low pressure needle would dive down pretty fast, but give me 6-8 seconds of time below 28 psi.
Now, clutch engages dives slower, stops at 27-28 for 2-8 seconds then disengages.

I think I will pull the evap again and open it up another turn.

'LP side at idle will hover 27-28 psi.'
What is wrong with that?

Don't know how much looser you can go. Be sure tension remains on the spring else it is going to come apart while operating...

PSI!!!!!!!
What are the vent temperatures? Lower PSI means nothing if you're already freezing the core.

tbirdtbird wrote: ↑Mon Mar 27, 2023 7:58 am 'LP side at idle will hover 27-28 psi.'
What is wrong with that?

Don't know how much looser you can go. Be sure tension remains on the spring else it is going to come apart while operating...

The Low pressure switch cuts off at 28 psi. Clutch on off cycle is 10 seconds. Vent temps fluctuate from 49-55. 800 rpm.
Increase RPM to 2000 and the cycle time is 5 seconds. Vent temps 58-63. I’m sure the evap will get nice and cold if the clutch will stay engaged for a meaningful amount of time.

I have a TXV on my bench I disassembled and it it took 5 turns (from factory preset) to loose contact with spring.

Something just isn't making sense to me. Vent temperatures of 58-63°F suggest that the coldest the evaporator is getting is about 53°F and means that it's running at about 50 PSI. Yet, according to your Manifold Gauge Set, the compressor is cutting off at 28 PSI with the LS pressure continuing to drop. The 28 PSI reading on the low side corresponds to an evaporator temperature of about 33°F, which obviously can't be the case here, not with sustained 58-63°F vent temperatures. The cause of such a scenario might indicate an obstruction somewhere on the high side of the system, a bad LPCO switch, possible reheating of the cooled air, or a combination of these.

Does this vehicle have automatic A/C with a digital read-out on the dash instead of analog controls?

Think I may have an idea.

There is no LP cutoff switch. What I thought to be the LP cutoff switch is a refrigerant temp sensor in the compressor. The dual pressure switch (mounted on R/D only sees high side pressure and the specs for that are ON 28 +/-2 psi / OFF 428 psi (converted from Kpa).

I believe the thermistor (mounted on exit center of evap) is cycling the compressor off prematurely.

There is no constant Low side pressures. The needle is always dynamic. Compressor comes on at around 45 psi, and pressure begins to decrease, with a decreasing RATE as the pressure decreases. Great, the cycle is working, then the compressor cuts off around 27-28. But it's not the dual press switch. I jumped that switch and something else is cutting comp off.

Im sure if the comp stayed on it would get ice cold. Im going to open the evap access door and pull the thermistor wire out of the clip and move it out of the evap flow and test.

Manual temp controller, not auto.

What can cut compressor off.
Coolant temp above 225*f
Refrigerant temp sensor (new, came with compressor)
Dual press high side switch >28 / <428
Evap thermistor

Making headway.

I opened the evap housing access hatch and removed the thermistor wires. There was ample wire in the harness to pull it down and out. Put my temp probe for my Fluke in the evap core. Temp probe in the center vent and gauges hooked up.

10 min run time
Ambient 75*f
Engine 1500 rpm, recirc on, fan on max.
AC clutch engages, and not short cycling (thermistor not in evap airflow)
Helper blower fan on condenser.

LP = 22
HP = 195
Evap Core 40
Vent = 42

Placing one of the thermistors back in the flow resulted in no change (may be a duct probe) Can't find any info on it.
Placing the other thermistor in the flow, resulted in short cycling after about a minute.

A new thermistor is in order, and will replace that.

Service manual says this for "performance testing"
25*c close to Phoenix temps today.
LP 13.5 psi - 28.3 psi. (Converted from Kpa)
HP 135 psi - 165 psi.
Vent temps 39 - 43

The fan clutch is weak, and have a new one in order.

Burning question is why the need for the second thermistor. Those things do not last long at all and are known to cause trouble in about every device they are in.
Keep up the good work

That's the million dollar question. The service manual (online) shows a thermistor with yellow/yellow Red wire as the evap sensor. This was the one installed in the factory entry into the evap case, which at present doesn't do anything.

On top of the evap case is the AC Controller. Which is plugged into a main harness. Coming out of the AC controller is another thermistor with yellow/yellow jacketed wires, which the wiring diagram doesn't show. (Maybe a SB on an updated part controller).

Multiple iterations of the AC controller were used/superseded for these models. The Mitsubishi dealer installed the AC controller and just opened the evap access hatch and shoved the additional thermistor wire in there, and clipped the cover back on, pinching the wire.

I was unsure of which one was effective, so in my evap install, I routed the additional thermistor alongside the one that was already there.

In my trouble shooting, I discovered the additional thermistor (hooked to AC controller) is doing the signaling.

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