This is a plea to Mike Morrin or anyone else out there who has
working knowledge of the '72-'87 Jaguar A/C amplifiers. Mike, thanks
for the excellent post of May 25 on this subject. It took me a step
further, but I’ve still got problems.
My '85 XJ-S supplies only cold air. To tackle the problem
initially I followed the flow chart in the Factory Service Manual. When
I checked the amplifier wiring, I found that it was supplying a constant
12V to the servo via the Red wire. When I measured resistance in the
Blue wire from the temperature control switch to the amplifier, I found
that it varied with movement of the control knob as it should. The only
other step suggested in the flow chart was measuring the resistances in
the circuit fo servo feedback (Green-Yellow and Yellow-Orange). Both of
these were appropriate at about 2000 ohms. According to the Manual this
indicated an amplifier problem.
I purchased a new amplifier and installed it, only to find it did
the same thing - supplied only cold air. I figured I’d blown the
diagnosis, but when Mike posted his letter, I went back to recheck.
What I found was that, although the results were the same, the new and
old amplifiers were doing different things.
Mike's note indicated that the Blue and Green wires should have
voltages of 5-6V, varying with temperature setting, with relative
voltages determining heating vs. cooling modes (Green > Blue = 12V on
Purple wire = heat; Blue > Green = 12V on Red wire = cold). The old
amplifier produced uniformly low voltages (2-4V) that varied with
temperature setting and amplifier adjustment, but never allowed the
voltage of the Green wire to exceed that of the Blue, resulting in
The new amplifier kept the Green wire around 5.5V, but put the Blue
wire at 12V with no variation through the entire traverse of both the
temperature control knob and amplifier adjustment, again resulting in
For those of you who've stuck with me this long: Am I missing
something wrong elsewhere in the system, or could my new amplifier be
faulty? Any suggestions would be greatly appreciated.
65 EType Cpe
56 XK140 FHC