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Another Series III Climate Control Modification

Another Series III Climate Control Modification

Tom Bernett

This is a follow-up to my earlier post on a modification to the climate control system. With this mod. it's possible to have fresh air ventilation, a functioning temperature control knob and servo unit, and functioning temperature sensors for system control. The blower fans and A/C compressor are disabled. It is useful in all but extreme temperature situations. The system is easily returned to normal operation for heating or cooling in extreme conditions.

These instructions are based on an '87 XJ6 SIII U.S. federal spec.,left hand drive, with the Delannaire Mk. II system.

In it's simplest terms the mod. can be described as follows:

1.) Remove the left hand console side casing and the underscuttle casing. The Water Temperature Switch is located above the black footwell vent. It's attached to a water pipe and has two connectors attached to it. One connector has two brown wires, the other has two brown w/ yellow stripe wires. Undo the connector with the brown/yellow wires.

2.) Remove the right hand console side casing. Find connector P2. It's located down low and aft. It's a circular white connector, maybe an inch in diameter with about 10 wires. Cut the brown wire between the connector and the servo unit. Cut it in the middle so you'll be able to connect to it later. Careful, don't cut the brown/yellow wire by mistake. You want the solid brown one.

That's it. Try it out. Turn the ignition on and the mode switch to Auto. (If you have the Gardner and/or Napoli mods, switch those off) The servo unit should respond to the temp control knob, the blowers and compressor should be off, and if your moving, fresh air, conditioned according to the setting of the temp. control knob, will be flowing through the dash and footwell vents. Defrost position on the mode switch should still function.

To return the system to normal operation, reattach the connector at the Water Temperature Switch, and connect the two ends of the brown wire you cut at connector P2.

All that remains is to come up with a convenient way of doing the above switching. It can be done with a simple double-pole, single-throw toggle switch or a double pole relay. I'm going to use a relay and control it with the window master switch.


The system is designed such that the heating mode is disabled (no blowers or compressor) until the coolant comes up to temperature. This prevents a blast of cold air in your face when you first start the car on a cold morning. After several minutes of running, the coolant warms up and the system clicks into action. This is controled by the Water Temperature Switch. This switch closes when the coolant reaches 40C/103F and allows the system to operate normally.

Now, just as you don't want a blast of cold air in a cold car, you do want a blast of air immediately when you start up a car that's been sitting in the sun on a hot day. This is accomplished by a Water Temp. By-pass Switch and some fan switches in the servo unit. These are powered when the system is in cooling mode no matter what the coolant temperature is. So they override the above Water Temperature Switch. These switches are powered via the brown wire at connector P2.

So, opening the circuit at the Water Temperature Switch disables the blowers and compressor in heating mode, and cutting the brown wire at connector P2 disables the blowers and compressor in cooling mode. In both modes the temp control knob, in-car and ambient temperature sensors, servo motor, fresh air flaps, etc. still function.

I envision using this mod as my on/off switch. I'll leave the mode switch in Auto, and this switch open in all but extreme conditions. When I need max heat or A/C, I'll close my switch and the system will operate normally with both blower fans and A/C compressor.

If anybody decides to try this, please let me know what you think. I just did mine yesterday, and so far it's meeting my expectations perfectly, but I'd like to know what others think.



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