Let’s make things interesting. Once again I am starting the epic troubleshooting process kindly provided by Karl. (KR98664) If you have never read this fine article and are experiencing any HVAC issues, I highly suggest grabbing a beverage of your choice and reading throughly.

As you may know, I recently solved my A/C performance issue by performing the Jaguar TSB modification on the compressor. Air is still ICE cold (except when it’s not, continue). Behaviors below started happening today.

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DCCV valve verification passed. However, I will test again tomorrow morning for good measure.

Vent Temps (both sides) in Manual Mode LO: 38F

Place temperature on panel to 65F, in manual mode. At random intervals, the vent temperature (both sides) can range from 40F to 75F.

In short, while in manual mode (set to 65F or other) vent temperatures will swing wildly on both sides. In automatic mode, the behavior and temperatures between ducts can vary. Example…driver side is set to 65F. Raise passenger side to 75F. The system starts adding heat to the drivers (wrong) side and vice versa.

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I believe (correct me if I am wrong)….I can safely rule out the cabin temperature sensor behind the steering wheel, as unwanted temperatures change even in manual mode. Recently (within the last 6 months) I cleaned the sensor throughly, including vacuuming, electronic safe spray and rubbing alcohol with a q-tip to remove the “lacquer” that builds up over time.

Tomorrow morning I will test…

- The DCCV again

- The driver/passenger/evaporator vent temperature sensors

- The sunlight sensor (a quick test on the way home seemed fine, but I need to verify. With such a large swing in temperatures, I am making an educated guess that the system wouldn’t make such large adjustments based on sunlight. Could be wrong)

As of now, I believe there is an issue with at least one of the duct sensors. If I understand correctly, these sensors still report data back to the control unit, even in manual mode. (Except LO and HI)

Place your bets.

 

Thinking a slight tweak of your troubleshooting logic may be needed. When LO (or HI) is displayed, that's Manual mode. When any number is displayed, you're back in Auto mode, or close to it. I think you have to press the Auto button to get back into full Auto mode, which also controls the output doors, Fresh/Recirc, and fan speed. But I'm pretty sure when anything but LO or HI is displayed, the system tries to regulate the output temperature.

See my previous comments above. I think you were taking the system out of Manual mode without realizing it. I'd suggest working through all of post #3 (Automatic Mode Faults) in the troubleshooting guide:

https://www.jaguarforums.com/forum/s...2/#post1714571


The sunlight sensor could certainly be part of the problem. For troubleshooting, try covering it with opaque tape or something similar. See if the symptoms improve.

 

Ah, good point, I was confused by this, but my hunch was this is how it worked.

Is it fair to say there are 3 modes? i.e....

-Full Auto (Auto LED on) - Controls temperature, fresh/recirc, air flow and speed

-Temperature Only or Semi Auto (Auto LED off, numbers displayed) - Regulates temperature based on chosen value (using duct sensors), all other controls manual

-Full Manual or Override (LO or HI) - No changes made by car (except "backend" controls such as heated wiper park, compressor cycles etc.)


Agreed; I was able to verify that today.

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Having made a career in technology, I am forced by habit perform the old "did you restart" methodology on anything controlled digitally.

Last night, I pulled the battery and left out. This morning I replaced the battery and followed the "power restore macarena" (Allow to warm up to temperature with no accessories to set idle speed, sunroof sync*, clock, sound settings…)

Naturally, everything is now working perfectly and all the tests passed. I'm not holding my breath...more to come.

*For anyone who hasn’t heard of this before, it’s a GM thing. After restoring power you need to cycle through each position of the sunroof for the controller to learn set points. It does nothing on other cars except make me feel good.

 

The (potentially huge) snag with disconnecting the battery is you clear all the OBD monitors (& fuel trims etc) - and then need them to set again for such as "smog" tests.

Hopefully they will, but if they don't the car is also inhibited from flagging many codes so you can end in Catch-22 (and no "smog" pass).

 

Yeah, I guess you could call that middle one Semi Auto, or Not Quite Auto, or I'm Not Auto Yet. Hadn't really thought about it too much. The main thing for troubleshooting is using Full Manual (LO or HI) to bypass most of the automatic controls.

 

Good point.

Luckily I do not have to smog (yes in even California); there are workarounds.

I also have a Launch x431..which is an AWESOME scan tool. Highly recommended.

Makes sense for sure.

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Everything continues to work properly for now. Hopefully it stays that way.

 

Karl...I feel like I owe you a bottle of Johnny Walker Blue or something similar.

You were right; it was the sunlight sensor.

I put it together when I noted the warm air blasts came during or immediately after going under an overpass at a relatively low speed. As the sun started to set, going through areas dark enough to enable the auto lights were also followed by the momentary switch to hot air. While the sensor was technically working, it was either sending bad information to the control unit causing it to overcompensate or the control unit was interpreting the data incorrectly.

I unplugged the sensor and it hasn't happened since.

In my case, the system actually operates better without it. Any temperature settings are now much closer and stable to the desired level. While in standstill traffic today on the 101 with 97F ambient temp; I actually had to turn the a/c DOWN; it feels like a brand new R12 system. The TSB modification on the R134a valve still blows my mind on how much difference it has made.

I tip my hat to you sir.

 

You do realize forum protocol, don't you? There's no need to state when I'm correct, as it's just a given around here. Think of all the bandwidth bogged down if this was posted every time I was correct. Big online vendors would go dark. Facebook, Instagram, and Tik Tok would go down. Riots and mayhem would ensue.

Groovy, glad you got it fixed. I am surprised this sensor was responsible for such large swings in the temperature output. I'd have thought the response would have been limited to a few degrees at most.

Which sensor on the dash was it? Two sensors sticking up, but am not positive which is which.

There's a raised oblong sensor, closer to the windshield. I believe this is the autolamp sensor for the headlights.

There's a round sensor that doesn't stick up much, further from the windshield. I believe this is the dual solar sensor for the HVAC system. Was that the one you unplugged?

To help the next guy, did you ever try blocking the sensor with tape or similar? I want to make sure that would actually work. Supposedly the plastic panel is very fragile so would hate to see it damaged if covering up the sensor would work just as well.

 

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LOL well even still, I greatly appreciate it.

The round sensor is indeed the solar sensor. The learning document you mentioned originally (which is a pretty darn good read, explaining even the basic physics of heat) has a diagram which points to the round sensor.

FoMoCo has used the oblong sensors (which include the blinking PATS status LED when locked) since the 90s across all models with auto headlamp control. My auto headlamps continue to work with the round sensor unplugged.

To test the sensor, I threw a hoodie over it; easiest way I found to block all the sunlight.

 

I think I may have discovered the problem here.

Before you make fun of me…there’s a backstory here. Nothing drives me more crazy on a car than rattle noises.

In short, my defroster (dashboard) vents/panel was cracked when I first bought the car. (Cracked like pieces had broken off and fallen underneath) Every parts car I’ve come across has had the same issue; the plastic is so brittle it breaks with the slightest pressure. I spent 2 weekends trying to fix, using superglue and baking soda in hopes of providing some rigidity. Nope, still snapped like a twig in several spots. (No bending or tolerance, a slight pinch snapped any part of this panel clean off, like the white support plastics inside the headlights)

As a workaround, I bought a large 36x60 thin black rubber mat. I cut it to size and to my amazement, it didn’t look half bad. I then cut holes for both sensors and glued them into place. The mat is heavy enough that I could just lay it in place, no need for glue or fasteners. It looked okay and most importantly, no rattle noises since it’s rubber.

Well I used the same superglue I was using on the original piece to hold the sensors against the rubber, not realizing traditional superglue trashes certain types of plastics. After removing the rubber cutout, I discovered that both sensors were in rough shape. The superglue had whitened and melted the mating surfaces to the rubber mat. From what I can tell, it looks like it may have damaged the sensor internally. It’s been very hot recently and I’d bet this started simply from getting too hot one day. (Hence it started to be an issue out of nowhere)

I’ve ordered an OEM replacement defroster panel (including the sensors); will be interesting to see if everything works properly. That is of course if it arrives intact or I don’t break it by merely trying to install it. I’ve read you can help to revitalize plastics by either placing them in boiling water or soaking them in water over a few days. If all else fails, I can cut a new piece of mat and try again with a different glue.

Either way, I will still test the sensor for experiments sake. However, I might just leave it unplugged. I never use the dual climate control and it has still performed better, even when the sensor was still installed in the OEM plastic panel.

 

very interesting read