Hey Guys,

Ok, car has the right amount of freon in the system and when you turn on the AC it starts to get cold and then slowly turns warm. The feed line in the engine bay also does the same.

The AC compressor kicks in when you start the AC but here is the thing, it never kicks out. From my experience, the AC compressor should cycle on and off depending on the pressure in the system yes?

Not a AC expert by any means but it seems that if the compressor keeps running there is no regulation of the pressure in the AC system.

Any input is appreciated

Bernie

 

Hi Bernie,


The two leading automotive a/c control system types are "Thermal Expansion Valve" (TXV) and "Suction-Accumulator-Orifice-Tube" (OT)
Our cars are TXV-equipped. As you correctly surmise, in either case, compressor cycling is controlled by system pressure switches. I'm not entirely sure which line you are calling the feed-line. You will have 3 types in the engine bay; Suction line runs from the Evaporator at the firewall to the compressor and should be cold whilst running. The Discharge Line runs from the compressor to the condenser up front and should normally be too hot to touch for very long. The liquid line runs from the rec/drier (mounted atop the condenser) up front to the evaporator at the firewall and will also feel uncomfortably warm to the touch but probably cooler than the discharge line.


The behavior you describe could be a variety of things; Firstly, is the cold to warm transition something very shortly after you turn it on or does that occur after 20-30 mins of operation? If it is after a while of operation with cold air, then your evaporator is likely getting clogged with ice and though you'll still hear the blower, very little air will be coming out of the vents and it will feel warm/humid. However, if memory serves...I believe you'll still have a cool suction line with this failure. This is not all that uncommon with a low refrigerant charge. (Icing can occur almost immediately...but it usually takes a little while to totally block the evap core)
If it is a more rapid transition, you may have a failing or clogged TXV. Remember, all diagnosis assumes a proper charge of the appropriate spec refrigerant and that the system was properly dried (vacuum pulled and held for at least 30 mins before charging)


We have seen several reports of the dessicant beads in the receiver-drier escaping and clogging up the TXV. Some have tried opening the lines in order to back-flush the beads with compressed air. Not sure if that's been successful or not. TXV R&R is a bitter ex-wife as it requires removal of the entire dash.


A failed pressure switch could cause your compressor not to cycle, and you'd have the freezing problem described above. A set of gauges will do you a world of good in your investigation. The compressor should cycle off when suction pressure (Low side dial) approaches the mid 30's (psi) and back on as it increases... though I forget the approximate value. If your low side is never getting that low, then don't look for the clutch to cycle. Rather start looking at your charge level and quality, your compressor...and yes, ultimately, the TXV.

 

Thank you very much for the explanation of the systems and possible causes.

The problem was identified (for certain) directly after the system was vacuumed and charged with a high quality automated machine. So I have to assume that the correct amount of refrigerant was in the system.

The transition from cold to warm begins immediately after turning on the AC. Once on, I can feel cool air and when I check the "suction line" it is cool to the touch. This begins to change after 20-30 seconds. The air flow warms up and the line warms up. The compressor continues to run without cycling.

If I turn the car off and let it sit for a while, start it and turn on the AC again, the symptoms repeat themselves.

The mechanic said its the compressor right away and he may be right but I just need to understand the problem myself.

His comment confused me that the pressure rises when the AC is turned off and then drops to nothing when the AC is operating. This is why I have no AC for sure but why would it drop? It could explain why the compressor continues to run though if there is not enough pressure in the system to trigger the switch correct?

 

When I worked at the Jag dealer years ago there was a young guy that replaced a compressor on an X300 and 'swapped' the Suction and Discharge lines on the back of the compressor.

The system did NOT operate properly and I pointed out his mistake before he got too 'freaked-out' about the problem.

The dessicant 'beads' as aholbro1 pointed out could cause the symptoms you describe.

Feel the line from the rec/drier 'bottle' on the condenser to the firewall/bulkhead and IF the line (small diameter one) is COLD to the touch, you have your answer. The 'beads' act like an orifice tube and the pressures get 'screwed-up' in the system.

If the line is warm/hot then the problem is elsewhere.

bob gauff

 

Thanks Bob,

Here are pics of my hoses.
Two of them are new due to leaking schrader valves and after your post I wanted to be sure they weren't reversed for any reason. Looks impossible as the fittings wouldn't line up correctly if they were swapped but have a look anyway.

The hose with the bottle and the low side schrader is the one that gets cool and then warms up. I am going to check the other lines here this morning to make sure they don't get cold.

 

If the service valves are leaking you can JUST replace the VALVES!!!!!!!!!!!
They are available from any auto parts store that sells A/C/ components.
You do not have to replace the entire line to repair leaking valves.

I keep BAGS (quantity 5 per) of the 'low' and 'high' side valves in stock for repair.

I sold my X300 a while ago and I don't remember the orientation but the compressor rear cover is marked S & D (suction & discharge). I don't have one here for repair to look at.

bob gauff

 

Yup, just the like the valve stem on a wheel, the insert is replaceable. I've had a few leak that replacement cured.