I did another ambient/duct test this morning on the commute to work and the results were more stable.

External Tenp; 79-80
Left and Right Center Ducts: 80

So I believe the DCCV is holding hot coolant back properly. I’ll test temperatures again tomorrow morning with my new Infrared Thermometer.

And thanks, SteveCat, for the info on the Scroll Compressor and the internal valve. I am aware of this possibility, as I had researched information on this procedure to force the compressor to run at maximum cooling. After working through KR98664’s cooling guide and also checking for a slipping clutch, if every else checks out, i’ll turn my attention to this compressor valve modification as a final effort.

Thanks for for your help. More testing results to come.

 

I agree. We can safely rule out a problem with the DCCV. Thank you so much for humoring me and double-checking this.

Have you inspected or replaced the cabin air filter? I put that near the beginning of the list of things to check, as that could be part of the equation. A clogged filter will reduce airflow over the evaporator, and reduce the amount of cooling available. It may not be the only item at fault, but still should be checked. And in my overrated opinion, if going to all the work to access the filter, I'd just replace it.

In this thread, I hadn't previously suggested the compressor modification yet. The vast majority of cars out there work just fine without this mod, so I didn't want to send you there before ruling out other (easier) stuff yet. We can get to that later, when it's time to check the compressor performance using a pressure gauge test set. I'm not sure what you have for equipment and how comfortable you are doing refrigerant work. If you plan to pay a shop for that, let's definitely rule out the other stuff first.

Have you swapped the compressor clutch relay yet? I think you had mentioned the relay was good, but am not sure exactly what that means. I prefer to swap in a known-good relay from another system that has been verified to work properly, such as the fog lamps.

Please do the paint mark test on the clutch, as you have already mentioned.

Please measure the electrical resistance of the compressor clutch, too. This can be easily done from the relay socket under the hood. You don't have to crawl under the car for this simple test.

Please check the serpentine belt tension. The tensioner has little marks to show when tension is correct. You may need a small inspection mirror to see these marks. On my car, when the belt started slipping, the tension was fine but the belt had become slick after a coolant spill and was slipping. I added a little bit of belt conditioner spray and the AC operation immediately returned to normal. Some sources say not to use spray on serpentine belts, but there was no such warning on this can. I tried it, saw immediate improvement, but a few days later decided to change the belt to be on the safe side as it was already getting long in the tooth.

After that, I think we're down to checking the compressor performance with a test set. Will you be doing this yourself? It's not difficult at all. For what a shop will charge, you can typically purchase a test set and vacuum pump for yourself. Your call.

If doing this yourself, make sure the compressor can draw the low side down to approximately 39psi. Don't worry about cycling rates or the high side pressure for now. If the low side gets down to 39psi, even if just momentarily, the compressor performance is adequate and you're probably looking at low refrigerant. You wouldn't need to do the compressor modification, but it also wouldn't hurt. Once again, your call.

 

With my new Infrared Thermometer in hand, the top radiator and heater hoses were not what I expected for the performance of the DCCV.

After 8 minutes of idling to engine fully at operating temp and two 2 minutes at 1500 RPM:!

Radiator Hose: 176 degrees
Top Heater Hose: 140
Middle Heater Hose: 160
Bottom Return Hose: 138

These don’t coincide with the ambient duct temperatures recorded earlier and point to DCCV issues. But I observer the engine compartment had gotten really warm by the time I measured the heater hoses and the surrounding manifold and heat shields were also at around 140 degrees. So i’m wondering if the heater hoses were influenced by the other temps.

Also the ambient temp under my carports was already up to 77 degrees at 6 am. Southern summers. So I don’t think I needed a full 8 minutes to teach operating temperature and a full two of idling at 1500 RPM.

I think i’ll repeat these test again tomorrow morning and watch the temp gauge in the car instead of an eight-minute timer for reaching the half-way point to full operating temperature. Then idle at about a minute instead of two minutes to compensate for warm ambient temperatures at the beginning of my test.

KR98664, let me know what you think.

 

Well, after doing the Infrared Thermometer test on the radiator and heater hoses for a second time, I kind of believe the middle heater hose is getting some hot coolant from the DCCV when it should be closed and hot coolant shut off from the heater core. Here are my results after warmup to operating temp (about 5 minutes), and a 1 minute idle at 1500 RPM. I also checked it at various intervals and you can see the temp of the middle heater hose increase at a faster rate than the other hoses;

AT OPERATING TEMP, 1 MINUTE IDLE, 1500 RPM

Top Radiator Hose: 163 - 178 - 180. - 187
Top Heater Hose: 117 - 128 - 134 - 135
Middle Heater Hose: 127 - 140 - 143 - 152

So this might mean a possible DCCV replacement and I still need to do the clutch slipping test mentioned in KR98664’s Cooling Guide.

The life of a Jaguar owner is never boring.

 

Nice detective work! A tip of my orthopedic hat to you.

I got to thinking about how radiant heat (from the adjacent exhaust manifold) might skew this test. I designed that test to do it quickly on a cold engine to minimize the effect of underhood heat. Even if both sides of the DCCV were properly closed, those three heater lines (2 supply, 1 return) will absorb some underhood radiant heat. However, if coolant heat was not a factor (good DCCV fully closed on both sides), those three lines should warm up evenly. If one line warms up faster, that would mean the DCCV is indeed letting some hot coolant pass through.

The big question is how much of a factor is this. Your previous test of the vent temps (post #21) show both sides to be even. Perhaps a little bit of hot coolant is getting through on the one side, but it may not be much of a factor. It's really hard to say.

 

As I continue on my sojourn into the depths of the S-Type cooling system, I came up with an idiotic idea, just for the heck of it:

i decided to to give the DCCV a little exercise on the commute from the office yesterday.

In manual mode I turn up the temperature to “HI” fo force the DCCV fully open and allow hot coolant to flow through the heater hoses. It was quite an experience blasting heat when I needed cold.

After I could no longer tolerate the heat, I lowered the temperature down to “LO” to command the DCCV in a closed position. Since I was on the Freeway, the temperature quickly fell to 40 degrees.

But here’s the interesting part. After I got on inner streets and stop lights at lower RPM, the climate system held steady at 40 degrees all the way hone. It felt so good. So i’m wondering if fully relaxing the DCCV (default mode) and then fully closing helped in any way?

So I couldn’t resist grabbing the Infrared Thermometer (wonderful gadget) this morning and recording more temps at idle to mimic the Freeway experience:

HEAT AT “HI” ON IDLE

Radiator Hose: 160
Top Heater Hose: 143
Middle Heater Hose: 138
Bottom Return Hose: 125
Center Duct: 120***

AC ON “LO” AT IDLE

Radiator Hose: 180
Top Heater Hose: 153
Middle Heater Hose: 149
Bottom Return Hose: 142
Center Duct: 60***

Note that with the AC on LO, and with the now-trapped hot coolant between the DCCV and the Heater Core, the Heater Hose temps didn’t change much.

But ithe Center Duct temp in the cabin dropped from 120 to 60 degrees. So I still kind-of-sorta believe the DCCV is working fairly well, if not perfect.

Does anyone the inner structure of the DCCV? I assume there is a spring or something inside that relaxes when open for heat and 12V from a “make cold” command contracts it to shut off the hot coolant? Or is it the other way around?

No testing today, as my wife felt sorry for me and let me drive her Subaru, where the AC and everything works.

Hopefully this weekend, i’ll be able to mark the compressor pulley and clutch for the “clutch slippage” test.

 

This is an interesting development. All along, I've figured we're probably dealing with low refrigerant, but wanted to run some free tests on other possibilities before incurring the expense of a recharge. But now, after manually cycling the DCCV, all seemed well.

Assuming (Danger! Danger!) the DCCV was the variable, that would rule out low refrigerant. Even if the DCCV itself was fine and some other problem child coincidentally behaved at that moment, that would still point to an adequate charge of refrigerant.

Are you able to duplicate the scenario and regain the low/slow cooling? Or was that a one time thing?

Don't place much stock in the heater hose temperatures once the engine is warm. There's so much radiant heat from the adjacent exhaust manifold.

 

Well? Any updates? Inquiring minds want to know...