[xj] AC compressor not running

I’m new to this list, having read for about the last month. I have a grey
86 XJ6 SIII with about 76,000 miles. The problem I’m having is that the
AC compressor will not run. The system does hold a freon charge, and the
thermal fuse is not blown. The middle wire that leads into the thermal
fuse should have power in order for the compressor to run, but it does
not. (If I jump power to this connector directly from the battery, the
compressor clutch engages and the system works.)

The wiring diagram shows where to go to work backwards from the middle
wire that attaches to the thermal fuse in order to find where the fault
lies, but neither the Haynes manual nor the electrical diagram that came
with the car show where on the car the wires are located. So I know
which wires to check, but I don’t know where to find them on the car.
Any help would be greatly appreciated.

Tom Whitney

Tom-
For starters, the clutch drive routs up to the front of the engine, then
around and bundles in on one of the upper engine harnesses, which includes
the injector harness. From there, it routs ti the two (3?)white,
rectangular moulded connectors at the rear of the cylinder head, intake
side, then into the main harness. The AC controls are distributed between
the left and right sides of the center console. There is a mechanical servo
system on the right, the servo amplifier on the left. I hope that at least
gets you started - I’m on vacation with no access to reference materials
(like schematics), or I’d give suggestions as to where to look.
Good luck-
Hunt

At 09:35 AM 7/19/97 -0700, Tom Whitney wrote:

I’m new to this list, having read for about the last month. I have a grey
86 XJ6 SIII with about 76,000 miles. The problem I’m having is that the
AC compressor will not run. The system does hold a freon charge, and the
thermal fuse is not blown. The middle wire that leads into the thermal
fuse should have power in order for the compressor to run, but it does
not. (If I jump power to this connector directly from the battery, the
compressor clutch engages and the system works.)

The wiring diagram shows where to go to work backwards from the middle
wire that attaches to the thermal fuse in order to find where the fault
lies, but neither the Haynes manual nor the electrical diagram that came
with the car show where on the car the wires are located. So I know
which wires to check, but I don’t know where to find them on the car.
Any help would be greatly appreciated.

Tom Whitney

H u n t D a b n e y & A s s o c i a t e s
Electronic System Development and Design-----------------------------------------------
fax: (714) 540-8131

Tom Whitney wrote

The wiring diagram shows where to go to work backwards from the middle
wire that attaches to the thermal fuse in order to find where the fault
lies, but neither the Haynes manual nor the electrical diagram that came
with the car show where on the car the wires are located. So I know
which wires to check, but I don’t know where to find them on the car.

On mine (XJ6 Series III 86) there is a ten amp fuse in yellow brown wire to
Ranco (anti ice) thermostat. Ranco and fuse are fastened to rhs of climate
control unit. I would start there.

Tom, you will do yourself a big favour by obtaining Jaguar Publication S 57
available from Jaguar Services Illinois (about $20.00) tel 708 331 9151.
The electrical diagrams a superb.

Tony Gardner
XJ6 Series III 1986

Tom,
There are 5 fuses in the AC system.
1 AC compressor thermal fuse…which you have tested.
2- inline fuse on right hand transmission tunnel… Power from Ranco thermostat to compressor.
3- inline fuse holder on left hand side transmission tunnel to amplifier
4- located in main fuse panel - left under dash panel
5- located in secondary fuse panel - right under dash

You don’t state if the climate control system works…or blower motors etc…

I suggest first checking inline fuse from Ranco. Remove two screws from right hand trans tunnel scuttle vent. Remove panel by moving slightly to front of car then pull out. Inline fuse is held in clip…up rather high. Or located rather low. See pictures

**
As Gary says, Tom - what about the system behaviour apart from the non-action of the compressor…?

…the point being to sort out whether there is a particular fault - or the system simply sense that the compressor is not required under the present conditions. Crudely, if the fans do not run; the AC amp is unpowered - usually due to a blown 15A fuse. If the cabin temp is too low, or the system is stuck in ‘full heat’ - the system may decide not to power the compressor. If the ambient temp, as sensed by the Ranco thermostat. is to low - the thermostat will cut power to the thermal fuse…

Generally; the first step is to verify that the AC amp is operating the servo; turn AC to ‘auto’ and set the temp selector ‘65’ and then to ‘85’ repeatedly - allowing some time for the system to react. At change of temp, you should hear the ‘buzz’ of the servo moving. If there is no servo movement - there is no point in further tests; the AC amp has likely failed and must be replaced…

If the servo operates, and the system otherwise works as it should, heating and cooling the cabin; a specific fault affects the compressor clutch only. The point here is that the loss of the compressor, for whatever reason, only affects the systems capacity for cooling - it will try and maintain set cabin temp, and will do so unless ambient temps are higher than set temps…

I’m a bit surprised that you have the compressor mounted thermal fuse - later models usually had the pressure switch, and ‘most’ earlier version have converted to this more convenient set-up. Likewise, the inline compressor fuse may or may not be fitted, but if it is it should indeed be checked as Tony advises…

Basically; the AC amp controls the compressor clutch through the servo connections via the Ranco thermostat (and inline fuse if fitted). If the Ranco has failed, or indeed if evaporator temp is below +2C, the clutch remains unpowered. Indeed, it is normal for clutch power to cycle according to evaporator temps - switching the compressor on and off, usually unnoticed while driving…

Arguably, it is safe to bypass the thermal/pressure switch that protects the compressor itself - PROVIDED that the Ranco thermostat itself works as specified. The Ranco itself must NOT be bypassed (except briefly for testing) - so it is important to verify where the power is lost. The Ranco is basically an ‘on/off’ switch showing ‘make’ above +2C…

Frank
xj6 85 Sov Europe (UK/NZ)
**

84 XJS - I’m having this same problem. I checked inline fuses on both sides of the center console and they are good. I hear the servo motor run when I switch between cold and hot and the air blower changes from top to bottom when I turn to heat. But I get no voltage at the A/C compressor unless I hot wire it in which case it runs and cools. What Else can I check? On the left side there are two wires (1st picture at bottom) that are not connected to anything and I don’t see anywhere they can connect.

**
It may be the Ranco thermostat that’s failed, Don - check for power at its green/brown and purple/yellow…? I assume weather is warm…?

Do you have the thermal fuse or the pressure switch on the compressor?

Frank
xj6 85 Sov Europe (UK/NZ)
**

Don,

Frank certainly points out the appropriate place to start. If your system is responding except for the compressor, with ignition on, climate control system on, check for power on the wires immediately before and after the Ranco thermostat located high on the right side of the heater box. If you have 12 volts in on the yellow brown wires and nothing on green /purple, verify good fuse. Also check continuity across Ranco connections. Lack of continuity, you have a failed Ranco.

Also note, some model years ( I’m uncertain of which specific years) were equipped with a Ranco bypass switch on the front of the servo which could relate to your failure. You can verify if your car is equipped with this switch by simply counting the number of wires in the servo loom. 11 wires = no switch, 13 wires = ranco bypass switch.

If you have power to the Ranco…you will have to trace wires thru the firewall.

Lastly, some members have referenced that the green compressor wire leads to a relay in the engine compartment to activate the electric engine cooling fan…i can’t confirm this as i never had compressor power issues or looked for it. Hopefully other members will advise.

Hope this helps.

Cheers

Gary

I have what looks like a wire wound fuse at the compressor. One wire on this fuse comes from the firewall, one wires comes from the plug on the back of the compressor and one wire goes to the compressor to engage the clutch. I did not find any inline pressure switch in the A/C lines. Should there be one? I just got through replacing the expansion valve and compressor. (with used compressor from a donor car) so I don’t know if there is something I disconnected by accident. The old compressor was froze up and it’s power connector had been disconnected for a while so it’s possible the loose connector got shorted to the engine (the cover on the wire wound fuse is missing) but I didn’t find any blown fuses. It’s been in the 90’s (F) here so yes it’s hot. The A/C blows cold when I hook power directly to the compressor.
I’m not sure how to access the Ranco thermostat / heater box. Can that be seen from under the dash (left or right?) or is there lots of disassembly required.

Don,

The Ranco is located on the right side of the heater box…just to the side of the glove box. You will likely have to remove the trans tunnel vent and panel as well as the under dash panel. The inline Ranco fuse holder is attached to the side of the heater case with a tab and screw.

Thanks! After I took the panel under the glove box down I could see the other fuse and it was blown. What a terrible location for a fuse. I unscrewed it from the wall but there wasn’t hardly enough wire on it to be able to get my hands in there to screw and unscrew the holder together. I’m temped to add some wire length to it and move it over into the fuse box area where it can be more easily accessed. It had a lucas 10a 5a continuous fuse in it. I replaced it with a 5a agc (fast blow) that I had. I don’t know if that is sufficient. The A/C is working now although it is cycling off and on. I can noticeably feel the air get warmer for a while and then cold again and I confirmed that the compressor is cycling. What would cause that?

How many cans of r134a should I have put in it from a vacuum? I added 4 12oz cans r134a and about 6oz of uv dyed ester oil that I read would play better with any residual mineral oil still in the system. High pressure sits about 175 -250 and low is 40 at idle and drops to 25 when I race the engine. But I just discovered a small leak on the fuel cooler connector using the uv light so I will be needing to evacuate the system again. It had a new o-ring. Can that connection be over tightened?

**
If a smaller fuse holds, Don; it’s adequate - if it blows, it is not…:slight_smile:

The compressor clutch deliberately disengages for one reason only; the Ranco senses, rightly or wrongly, too low temp in the evaporator. This cycling is normal - as outside temps drop the on/off cycling increases, as the evaporator cools quicker. Bear in mind that the compressor works to capacity at all times…

However; the cycling of the compressor should not a affect cabin/vent temps much - if the system is working properly. Cabin/vent temps are controlled by flap settings and fan speeds. All air is first passed through the evaporator, then reheated as required by passing more or less air through the heater.

As the set cabin temp is reached; the first system reaction is that fan speeds from high to low. If cabin temps drift off; fan speeds increase - this fan speed cycling is an indication of system ‘health’. The longer the fans stay low, the better the system works. The cycling of the fan speeds does not reflect compressor cycling…

Normal fill, presumably with the original R12 was listed at some 2,75 pounds - but amount to be adjusted using pressures and ambient air temp as guide lines. However; test conditions given in the manuals are very specific, not slapdash - which likely would give adequate results, up to a point…:slight_smile:

But your leak may have depleted the system - have you checked the sightglass,?

Frank
xj6 85 Sov Europe (UK/NZ)
**

Yes, it’s the Ranco Thermostat cycling on and off, and it’s what it’s suppose to do.
It’s adjustable though.

Aristides

When I replaced the expansion valve, the old one was covered in a black tar like insulating putty material, mainly around the temperature sensor that slides in to the holder and is fastened by two screws. I didn’t put anything on the new one. Does it need insulated and could that be affecting the cycling?

adjustable how? Trying to find some documentation on that

**
The valve is temperature (and pressure) sensitive, Don - the outlet temps/pressure, controlled by the valve controls the amount of liquid injected. The amount of refrigerant injected relates to the cooling of the evaporator, but which way requires some thinking - the more the cooler…?

However, the valve is designed to deliver just enough liquid for all of it to evaporate - and the presence of liquid refrigerant is in any case counterproductive. Eventual insulation of the valve inlet pipe will tend to keep it cooler - reducing liquid injection.

Insulation/lagging is mentioned for removal - but is not mentioned for reinsertion, and effect is not elaborated on.

On/off cycling of the compressor is normal and intended - controlled by the Ranco thermostat, it prevents evaporator temp to drop below +2C. How often is ‘normal’ is conjecture - it relates to ambient air temp entering the system to actually heat the evaporator…

Question; how often does your compressor cycle - and how do you detect that it does? In principle, the vent temps should not vary with compressor cycling - and does not in itself confirm cycling…

You may connect a test lamp between compressor ‘+’ and ground to visually see the cycling…

I’m not sure if your symptoms are related to refrigeration or some other problem. If the first; it may be related to a faulty expansion valve itself, or indeed lack of insulation. Or it may relate to system pressures/amount of refrigerant…?

Frank
xj6 85 Sov Europe (UK/NZ)
**

It’s the big counter nut and adjusting screw at the side of the thermostat.
Be careful though, you wouldn’t want your evaporator and lines to start icing up.

Aristides

Mine had that too, it’s been working some years now with out it and without any obvious ill effects.
It’s extra insulation, so I guess you lose some capacity as the refrigerant warms up a little before it enters the evaporator, the valve is brass, a very good heat conductor in a very warm place.

Aristides

The insulation for the expansion valve is readily available from various suppliers. Here’s one example

Cheers
DD