Climate problem

I have a xj40 sovereingn 1994 .
I have problems with the temperature in the car. Sometimes very hot, sometimes very cold. (automatic and manual mode, equal)
I don’t have a minimal comfort.
I noticed that in my car at medium fan speed I have air cold from the right outputs and air hot from the left ones (frotal output) . incredible!
Instead in condition of high speed fan the air is equal from right to left .

Subsequently I did a test: I removed the left blower fuse to see if the right one works…and it works.
Same test with the right blower fuse and the left fan works.
They both work well at all speeds (low, medium, high).
What problem could the system have? Maybe a cold solder? what’s playing pranks on me?

Hopefully this will give you a basic idea how the climate control system works …

The left and right fan assemblies force air into a big box that contains two blend flaps that direct that air through the evaporator and heater matrix according to what’s selected on the climate control panel.

That air which is now hot, cold, or some temperature in-between continues up into a long
box (that is actually the top of the dash) and exits out of the three dash vents or the two windshield demisters on top of the dash, again according to what’s selected on the climate control panel.

Because of this ALL the air that is entering the long box for distribution out of the vents is the same temperature (be it hot or cold), in other words it’s impossible to have hot air come out of one vent and cold air out of the other. The only thing that I can think of that would be remotely possible to cause something like that would be that the long box with the vents has somehow partially split apart allowing the conditioned air to escape at one end.

The good news is both your fans are working at all the speeds. When troubleshooting always use the “manual” position. This bypasses all the sensor’s inputs (I believe there are five) and will give you the most direct control of the temperature selected.

thanks for the drawing.
During the test, removing the fuses, the fans had work well at all speeds. I probably have a false contact somewhere (On the famous Darlington?). Because at maximum speed the temperature is the same on all outlets.
Do I have to dismantle too things to access the Darlingtons?

Darlington only controls fan speed not direction of air flow. If fans work at all speeds, the darlingtons are good.

Does this temperature imbalance occur when recirculation mode (or max cooling) is turned off? My thought is if the vacuum operated actuator for the recirculation flap for one of the blowers has failed one blower could be pulling in warmer outside air whilst the other is only recirculating cooler inside air.

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Interesting. I did not think about it. Now what tests can I do to understand if it depends on this problem? do I have to dismantle something?

You can use a hand-operated vacuum pump (Mityvac) to test the actuator. Look under the dash and you will see the climate control blowers, one each on left and right sides:

The white arrow points to the actuator. Remove the vacuum line from the actuator, attach the line from your vacuum pump, and give it several pumps. This will close the flap, causing the blower to recirculate cabin air rather than bringing in ouside air. The actuator should hold vacuum.

thanks, I’ll try

Mike I thought of that also but remember that the air being blown in the evaporator/heater matrix housing by the two fan assemblies (red arrows) is blown/mixed together before always going through the evaporator (and then through the heater matrix if “heat” is selected) before exiting the rectangular opening (white arrow) into the dash assembly containing the outlet vents.

Screenshot 2024-07-11 at 11.31.58 AM

I’m fairly sure by now that all the thoroughly mixed air will be the same temperature entering the dash assembly.

Indeed. UNLESS, if the two fans are delivering close to the same CFM, is the air flow more like A, B, or C (neither A or B)?




Hi Mike …

You bring up some excellent points to consider …

Screenshot 2024-07-11 at 11.31.58 AM copy

The air from both fans enters through two oval holes in the bottom third of the housing. The two air streams shoot straight in from either side and literally collide with each other where they mix and with no other place to go the air is forced upward through the evaporator / heater matrix and out the top.

Because of the turbulence this design creates my guess is that the air is pretty well mixed by the time it blows up into the dash assembly. But as in all things with this car, nothing would surprise me.

Boy oh boy, where is Mr. Wizard when you need him? :grin: Sounds like an interesting science experiment…we need a cardboard box the size of the Jag airbox, some ducting, several fittings for the ductwork, two different colored smoke bombs, two smaller boxes (left and right side of larger box) to contain the smoke, a variable speed/volume fan, a Y fitting to insure equal airflow is routed to both sides of the airbox and - what else?

Mike …

This would make a PERFECT High School science fair project ! Remember those ? Nobody understood what they were about and the more nerdy the topic the better the grade, such as …

Screenshot 2024-07-15 at 3.50.08 PM copy

I remember one time I made a miniature wind tunnel out of a small table fan. And although a tad crude, I thought it was pretty cool. Right up until I plugged it in (no on/off switch) and it somehow vibrated off the side of the table, hit the floor and parts came flying off. The class loved it … my teacher not so much (no certificate of achievement for me). So you see I was into aerodynamics at a very early age :sunglasses:

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