Compressor Won’t Shut Off, Freezing Evaporator

[graycenphil]

2006 Jeep Liberty CRD, always had great AC and heat.

I went for a three hour drive today, and noticed the airflow was slowing down. It didn’t matter much because it was a comfortable day and didn’t really need much heat or vent. When I stopped, I opened the hood just to see if anything was amiss and I saw the freon line and the accumulator were covered with ice.

So I assume the AC compressor was on the whole time, not even cycling, and froze up the evaporator and no air could flow through it. I pulled the compressor relay, and the airflow was fine the whole way home.

So what is the most likely problem? What would make the compressor run when it isn’t turned on, and not even cycle as it normally does? Where should I start looking first?

Thanks

[Cusser]

On most modern vehicles, the compressor comes on when in the defrost is switched on. And on some, I've read that that the compressor stays on until the ignition is switched off. See if that happens with your Jeep.

[graycenphil]

Thanks Cusser. The compressor does run in this vehicle in defrost mode. But I only had it in heat (air directed to the floor, temp turned up high) the whole time.

[tbirdtbird]

One thing that comes to mind is a faulty LPCO that lets it stay running, although that would not account for the comp running when not turned on, unless as Cusser mentioned

[graycenphil]

Thanks Tbird. LPCO is low pressure cut off? That makes sense.

I’ve been doing some testing and it seems the compressor runs in almost every mode in this vehicle.Only on vent (air directed at the dash outlets) does the compressor shut off. I don’t like that at all, and may make a cutoff switch. Perhaps just a switch in one of the leads to the sensor on top of the accumulator, unless someone thinks that is a bad idea.

[tbirdtbird]

I expect the sensor on top of the accumulator is the LPCO. I'd replace that, it most likely has a shrader underneath it so you don't lose charge when replacing it.
You don't want to lose the action of the LPCO, since that protects you in case of a refrigerant leak. A system with insufficient refrigerant will starve the comp
of oil and kill it. Manual override cutoff sw I would put in the hot lead to the comp.

As it is with icing up, the system may already be low on charge.
At some point I expect you will have to have an AC shop do a leak check, evacuate, and recharge by weight.

[graycenphil]

Thanks again Tbird. My thought was that putting a cutoff switch in the wire from the LPCO would stop the compressor from running when the switch is open. Does that make sense? It is much easier to get to than the wire to the compressor.

[Cusser]

Thanks again Tbird. My thought was that putting a cutoff switch in the wire from the LPCO would stop the compressor from running when the switch is open. Does that make sense? It is much easier to get to than the wire to the compressor.

Yes, that would work to interrupt the circuit.

Can someone comment if I'm on the right track with the following?
When you run the AC, you should likely remember to switch that to off periodically for a couple of minutes to avoid freezing the evaporator. In other words: if it's too cold coming out the vents, don't add heat, switch off periodically.

I'm wondering if you have a stuck relay, causing compressor to be always engaged or if there's an issue at the AC clutch so it cannot disengage.

[JohnHere]

Refrigerant loss probably is responsible for the evaporator freezing-up and blocking airflow on your 18-year-old Jeep.

Before doing anything else, have a professional A/C shop charge the correct amount of refrigerant into the system, as suggested earlier, by recovering and noting what's in there now, evacuating, and recharging to the specs listed on the under-hood decal. Then monitor its performance and center-vent temperatures on several nice 80-degrees-plus days, and see how it performs.

With a proper charge and the LPCO working as it should to prevent the evaporator from getting too cold, it shouldn't freeze-up.

How many miles has your Jeep logged, and has the system ever been serviced?

[graycenphil]

Thanks JohnHere. 170,000 miles, almost 20 years old, so I really can’t complain. And it has been a great air conditioner all that time, only repair was a new LPCO ten years ago.

I’m in Connecticut, so probably won’t be seeing any 80 degree days for a while. It is low on freon, so I added some and will wait to see if it holds. The truth is the vehicle’s days are numbered (rust) but it should last the summer and would be nice to have AC. I wish I had done the cutoff switch years ago.