Explain my AC system

[WobblyElvis]

I cannot understand how the ac system in my Dodge Van works. From looking under the hood and from what the shop manual says, the only pressure sensing device it has is a pressure transducer in the high side line. This transducer reports to the powertrain control computer and the PCM will not engage the compressor clutch if the pressure is to low or to high. Makes sense to me. In very cold temps the pressure will drop and you cannot engage the clutch in winter. Should the high side pressure rise to almost 500 psi the PCM disengages the clutch for obvious reasons. Again, this pressure transducer is in the high side line.

When checking the refrigerant level the other day with a manifold gauge set, the high side would rise to about 250psi and the low would drop down to 20psi and the clutch would disengage. This sounds about normal but HOW DOES THE SYSTEM KNOW THE LOW SIDE PRESSURE IS LOW?
I cannot find a low pressure switch and the manual does not show one either. Don't make no sense to me.

 

[EA82]

There could be a temperature sensor, low temperature = low pressure, same goes for high temperature.

 

[zrxkawboy]

What year/model is your van?

 

[WobblyElvis]

99 Caravan

 

[PWMDMD]

There could be a temperature sensor, low temperature = low pressure, same goes for high temperature.

Charles' law FTW!

 

[mk378]

In a TXV system such as yours, the compressor cycles off based on evaporator temperature (below about 35 F). This is necessary to keep condensed water from freezing on the coil fins, which would block the airflow.

In normal operation, the high side pressure sensor and PCM usually also controls the condenser fans. When the car is moving, the high side will stay down without the fans needing to be on. As you said, the pressure limits that would stop the compressor are only reached in abnormal circumstances.

 

[Timmastertech]

Chrysler cycles the clutch based on evaporator temperature. The only systems that have a cycling (low pressure) switch are the truck and jeep platforms that are orifice tube systems. Otherwise chrysler has been using evaporator temperature sensors to cycle the clutch since the 80s

 

[Timmastertech]

In a TXV system such as yours, the compressor cycles off based on evaporator temperature (below about 35 F). This is necessary to keep condensed water from freezing on the coil fins, which would block the airflow.

not necessarily, GM and Ford both have TXV systems out there that use a cycling switch as does Volvo and there are orifice tube systems out there that cycle based on evaporator temp. It more boils down to how the OEM wants to control the system

 

[WobblyElvis]

I am aware the evaporator has a temp sensor, as a matter of fact I have a spare sensor that I just dug out of my parts box a couple of hours ago. I have been trying to find a resistance chart for it but so far I can't. My spare measures about 5,000 ohms at room temp. I thought it might be this probe but the cycling is happening so fast I can't see this sensor reacting fast enough. I will change it tomorrow and see if it makes a difference. Going to be cold here for a few days in Toronto. I will post the results.

 

[Timmastertech]

I am aware the evaporator has a temp sensor, as a matter of fact I have a spare sensor that I just dug out of my parts box a couple of hours ago. I have been trying to find a resistance chart for it but so far I can't. My spare measures about 5,000 ohms at room temp. I thought it might be this probe but the cycling is happening so fast I can't see this sensor reacting fast enough. I will change it tomorrow and see if it makes a difference. Going to be cold here for a few days in Toronto. I will post the results.

Whether you can buy the temp sensor controlling clutch cycling doesnt matter, thats what controls low temp cycling. from the factory service manual

EVAPORATOR PROBE​

The Evaporator probe is located on the HVAC. The probe prevents evaporator freeze-up by signaling the Powertrain Control Module to cycle the compressor ON and OFF. The probe monitors the temperature of the refrigerant after expansion.

The evaporator probe is inserted into the evaporator between the coils. The probe is a sealed unit and cannot be adjusted or repaired. It must be replaced if found defective.

www.techauthority.com is the website for the factory service info. If you wish to test the evap temp sensor the info to test it as well as a resistance vs temp chart is in the service manual.

 

[WobblyElvis]

I replaced the evaporator sensor with my spare. Both sensors test identical resistance at room temp and when placed in ice water. To see what is going on, I wired a light from the clutch relay to the interior of the vehicle so that I can see the clutch cycling. I find it hard to believe the clutch cycles so often and sometimes fast. There has been a couple of times the clutch engages for only one second and then re-engages. High load on the cooling system [high interior fan speed and outside air] does force the clutch to engage almost constantly. Vent temp varies and generally doesn't drop to normal [ 45F] very often. I don't know the history of this old vehicle. It could have to much oil in it, some inappropriate refrigerant or air. Low side pressure is a little high and high side a little low according to the manual. Compressor is an unknown.

I am wondering if the system could be undercharged and the evaporator contains mostly gas, not liquid refrigerant during normal operation. The probe is stuck through the middle of the evap core and like a say the clutch cycles extremely fast sometimes. May just vacuum the system and start over, then again, I'm lazy enough to do nothing as it cools acceptably. I've noticed when I disconnect lines from the ac and inevitable lose a small amount of refrigerant I see an oily green/yellow dye residue.

Could low refrigerant [ evap not saturated with enough liquid ] cause very quick clutch cycling? Can't be good for the clutch/compressor. Perhaps measuring the temp of the evaporator outlet line could tell me something. Don't want to over charge it. Questions, questions...questions..

 

[Chris142]

The 1 second on then off is usually over charged. It can build high pressure much faster than low.

 

[WobblyElvis]

Normally I would agree but I've seen the system pressures and I haven't seen more than 325 psi when good and hot. High pressure cut out is over 500 according to the manual. It would be nice to rig up some monitor so you can see all the info while driving. When it did this 1 sec on..1 sec off thing, I was driving around 25mph and I'm sure it had decent cooling. Hard to determine anything, it's all muffed up.

 

[WobblyElvis]

Update

Replaced fan blower resister, blower motor, high pressure AC transducer, evaporator sensor and blend door actuator.

Refrigerant levels/pressures are within limits although the high side pressure tends to hunt/vary about 30 psi. I suspect a lazy TX valve is causing this.

This system cools within spec but there is still no cure for an occasional random fast clutch cycling. [one second on/ one second off/ back on].

It does not seem possible for the Powertrain computer to be cycling the AC clutch based input from the evaporator core temperature probe and occasionally cycling the clutch this fast. Logical answer would be the computer is faulty. I am just going to live with it.