My AC will start out cold cold, then at some random time later it will get warm, and it may go cooler for a little while then get warm again. The mechanic said the pressure actually goes into a vacuum sometimes (when warm?). He surmises it is a faulty (wrong resistance) 10397366 Evap Temperature Sensor (discontinued, of course). Even if I had a new one, you have to pull the dash to replace it > that tells me big $$.

Any other known probable causes for such behavior? If that sensor ohmed out right, he'd then suspect the expansion valve which has no direct test.

 

What's the likelihood it is the TXV?

 

Take it to an automotive A/C shop, they will fix the problem.

 

The expansion valve regulates the flow of refrigerant. If it's blocked or malfunctioning, it can cause inconsistent cooling and lead to the AC blowing warm air!

 

Yeah, I already took it to a shop. Yes, it can cause this type of problem. I asked about likelihood. I'm starting to lean that way. I just don't know how accurate the assessment of the temp sensor is.

 

Someone who is an expert can correct me if I'm wrong, but I don't think a bad temp sensor will cause the system to pull vacuum on the low pressure side. All the temp sensor does is turn off the compressor if the evaporator output temperature is too cold.

I had a recent negative experience with a shop telling me I needed to replace everything in my AC system (they call it an "A/C Kit") because of weak cooling performance and the compressor maybe sounding a little noisier than I remember in the past. Instead, I went to O'Reilly, rented a set of gauges and followed the procedures in the factory service manual HVAC part 2 section to measure my system's performance and isolate my problem based on the FSM step-by-step diagnostic procedures.

What the gauges told me is that the shop was selling total BS and looking to make a quick buck selling me a bunch of stuff that wasn't needed. My return for watching a few YouTube videos about how to use the AC manifold guages to diagnose AC problems and doing the work myself was about $500/hr.

If you want to see procedures for isolating electrical problems and troubleshooting DTCs you can find those in Part 1 of the HVAC service manual on the web here. This section also includes information on all the things that affect engagement of the AC compressor clutch (like the evaporator temperature sensor).

If you want an interactive version of the service manual, you can find it here. Just follow the prompts for your year and engine then click on the "Repair and Diagnosis" link, then find the "Heating and Air Conditioning" section and then find the link for "Initial Inspection and Diagnostic Overview" for the steps to test the system. I found this interactive manual super helpful, and it actually includes some images missing from the pdf files in the links above. As a bonus I came across a TSB in the interactive manual addressing an annoying buzz behind my dash that's been bugging me for the past three years! 15 minutes of work and that problem is now solved too.

Good luck with the troubleshooting!

 

Wow! Thank you for that; that is an awesome find!

 

Yes, thanks @NMH3 for the info. I was looking at my own set of paper FSMs this weekend, but that digital version is nice to have also. What was your cooling issue? I'd like to hear about that buzz you fixed

 

I had weak cooling performance and the compressor maybe sounding a little noisier that unsual while working overtime in 95F+ heat. I used the gauges to observe low and high side pressures and followed the FSM steps to learn the system was likely "slightly low on refrigerant." So I bought a $12 can of R134 and slowly topped off my system by monitoring pressures until they were closer to specified mid-range. Cooling performance improved 10 deg and was right back to factory specified performance. Clearly not a "bad compressor" like the shop professed, which of course they said meant I also needed to change the condenser, accumulator and TXV. Bunch of crooks.

The buzz is specific to to the V8 and sounds like its coming from behind the radio while idling or pulling away from a stop. It's been bugging me for years. Required one zip tie and to loosen then retighten one easily accessible bolt, all in the engine bay, and the buzz is now gone! You can read the TSB here.

 

I think my H3 has the same symptoms as the OP (and half the symptoms described in other posts, too. )

If I stop at a traffic light, or just idle for a while the air coming out the vents increases. When I go again, or rev the engine, the AC air duct temperature drops.

(I should note that it is over 96 Deg F here. 101 on the indicator on the Mirror while moving.)

In my experience this indicates either a weak compressor or just low Freon, as @NMH3 states.

When I can do a "vehicle shuffle", I will pull the H3 in the shop and do the procedure as described in the manual that @NMH3 linked above.

Note that they say to do the test in the shade or inside. In my experience, it's impossible to get a full charge from the little cans when the ambient temperature is so high. I know there are tricks like the pot of warm water, but if you don't have a proper freon recovery/loading system, I suggest either doing it either inside or early in the morning.