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My AC compressor has stopped working recently. I'm currently checking the wiring of my compressor and came across this. Does any one know what this device is in the AC Compressor clutch wiring?
The green wire is the +12V to the compressor clutch and the black wire goes to the switch at the rear of the compressor. The mystery device seems to connect the +ve and -ve connections to the AC compressor clutch.
My XJ-S is a US Spec from January 1987 and has the Delanair Mk III computerized climate control system.
Its the superheat switch, essentially a fuse, to protect the compressor if system pressures go out of range. Its function is in The BOOK, by Kirby Palm.
The device in the wiring is a diode and a resistor.
The superheat switch, when fitted, actually mounts into the end plate of the a/c compressor. Only early Del III cars used the superheat switch. Most used an HSLP switch (High Side Low Pressure).
Cars with the superheat switch used a three-prong thermal limiter fuse (see picture) in the circuit which will blow if the system is low on freon. This prevents compressor damage. There is no such fuse on systems that use the HSLP switch.
The device in the wiring is a diode and a resistor.
The superheat switch, when fitted, actually mounts into the end plate of the a/c compressor. Only early Del III cars used the superheat switch. Most used an HSLP switch (High Side Low Pressure).
Cars with the superheat switch used a three-prong thermal limiter fuse (see picture) in the circuit which will blow if the system is low on freon. This prevents compressor damage. There is no such fuse on systems that use the HSLP switch.
Cheers
DD
The OPs picture is of the thermal limiting fuse. Its part of the Superheat system. You are correct in that the thing that blows the fuse is in the end of the compressor.
Thank you for the insight! Attached is a picture of the rear of the compressor (cruise control module removed for a better view). I don't see anything three-pronged, and I'm wondering if my car has the HSLP fitted to the compressor and remnants of the super heat system in the wiring loom. Previous maintenance perhaps?
The switch itself is under the white cap at the bottom of the compressor in the picture. It is a single pole switch. If it closes, it blows the fuse in the three prong thermal fuse.
The later pressure switch is a much better arrangement. When i converted my 87 to a Sanden compressor and R134a, I went to pressure switch system.
Visually the difference between a Superheat switch and an HSLP switch is very subtle. You'd need to have one of each side by side, IMO, to tell. And the compressors are unique as to which type of switch is used. That is, you can't use a superheat switch in a compressor designed for an HSLP switch, and vice versa.
This illustration shows how the three-prong thermal limiter is fitted
One problem is that the thermal limiter fuses are often simply eliminated from the circuit. Thus, not seeing one isn't absolute proof that you have an HSLP system.
However, FWIW, my Feb '87 build XJS had the HSLP system (VIN 139xxx).
In any case.....my S57 diagrams show a green/brown to power-up the compressor clutch. Are you getting 12v there?
My switch at the rear of the compressor is wire directly to the compressor clutch connector, without any three prong device. There is no bracket at the front of the compressor either. Again not definitive because somebody could have been in there in the past.
I do get 12V at the green / brown wire at the clutch, but if I measure across the +ve and -ve terminals I get around 3 volts only. Not enough to engage the clutch. Maybe the resistor / diode device in my first picture is providing a by-pass circuit to ground, leaving insufficient voltage to engage the clutch?
I think the original mystery device looks like a home made effort to mimic some of the function of the thermal limiter, because its not like the circuit diagram. It does look like I could remove it if I replaced the compressor with a HSLP switch (which I think is the only type now available.)
Doug - Thank you for the confirmation that my "mystery item" might be a an undocumented factory item. I will test it after disconnecting from the harness. I've read somewhere that voltage applied in the wrong place can damage the AC computer.
05-06-2022 | 06:19 PM
AC Compressor Wiring Mystery
