XJ12 AC run without ranco

Hi I have a 1983 XJ12 and am in the UK. I bought the car without functioning air con and I’m trying to get it working.

When I connect the ranco thermostat it blows the 50 amp fuse in the drivers fuse box.

I’ve tried sourcing a new ranco but cannot find one in the UK.

Theee questions:

1 any idea why it would be shorted to ground?
2 can I run the air con with the ranco bypassed without major damage as long as I run it at over 20 degrees c ambient?
3 anyone in the UK have a ranco that works or a lead on where to buy one!

Thanks in advance!


Did you do the test with the Ranco disconnected?
It should have no path to ground, it’s just a two pole switch.
Maybe open it up and see inside?

You could run the system without it, but there will be nothing to stop the A/C from working if for some reason the evaporator ices up.

The Ranco is just a standard AC Thermostat. See if these guys will ship you one of their preset ones.

If not, try searching your area’s version of Amazon for “preset automotive ac thermostat”.

1 Like

Here is a good discussion on the Ranco switch:

This is what I keep trying to figure out with regard to your 50A fuse blowing when the Ranco is connected (provuded that I read your issue correctly): the Ranco switch de-energizes the compressor clutch when necessary, which also means that it energizes the compressor clutch all other times.

Thus, unless the Ranco switch itself is shorting to ground (which is a possibility, and can be checked for continuity from each terminal to its case) the wiring to the compressor clutch or the compressor clutch itself may be shorted to ground?

The Ranco switch is a single pole single throw normally closed temperature sensitive device acting as a limit switch for evap low temp monitoring.

No voodoo magic here. Bypassing should reveal more, but I think Mr. Peck has a good handle on what is going on here,

Yes I did check. It has continuity to ground. It’s right up in the hvac case and I think I need to take the dash out to access😩. What’s the % risk of damaging the evaporator without the ranco??

No nothing works with the ranco switch connected. Fuse just blows. I bridge the terminals of the ranco to get the compressor working.

Great. Thanks for the lead. Will try and find something.

Not clear on this. If a Ranco terminal is shorted to ground, bridging them would still leave the short and blow the fuse. Do you mean remove the wires from the Ranco, and tie them together?

If so, then IMO there’s no problem with running sans Ranco. For it to be needed, you have to be running the AC under conditions where the evap is so cold that it freezes water in the air, blocking the fins, and permitting refrigerant to pass through without flashing, which could then “slug” the compressor. With most use, you never get that cold (running fan very slow, using AC at 60 deg ambient etc). If you do, you will feel the AC from the vents getting warm. If so, just turn it off for a while and then back on (like the Ranco does).

The Ranco switch is just that: a switch.

Are you familiar with relays and micro switches that have NO (normally open) and NC (normally closed) contacts?

If so, think of the Ranco switch as a thermally operated micro switch connected to the NC terminal.

The Ranco switch is “closed” until the evaporator gets close to freezing, then it switches to NO (i.e., it switches “off”) and the compressor clutch disengages.

If you “remove” the Ranco switch, you effectively turned the switch “off” and just disengaged the compressor clutch … i.e., no air conditioning.

Instead “bypass” the Ranco switch (because it is apparently shorted to ground) and install a manual toggle switch to the Ranco switch wires, for testing purposes.

If the 50A fuse blows … the wiring to, and/or, the compressor clutch, is shorted to ground.

If the 50A fuse doesn’t blow, the compressor clutch should be pulled in when ac is turned on.

Thanks. I’ve bypassed the ranco and the compressor runs fine and the fuse doesn’t blow. I assume that means that if I leave it like that there’s a risk the evaporator will freeze and get damaged. My question is what is the likelihood of that? Almost certain, so don’t run it without the ranco? or unlikely as long as ambient temp is above 20 degrees c?

quote=“Scaphan, post:1, topic:448276, full:true”]
When I connect the ranco thermostat it blows the 50 amp fuse in the drivers fuse box.

That’s very odd, Russell - the 50A fuse (#3) is exclusive for the fans; the Ranco/compressor is fused by a 15A fuse (#6). A PO may have put in a 50A there attempting to ‘solve’ a problem, but verify fuse number! The Ranco/compressor has a green/brown wire, the fan fuse has a brown/white - this may be important; since the AC is just installed, there may be some misconnections…

The Ranco has a brown/green (power ‘in’) and a purple/yellow (to compressor clutch). On the Ranco; ohm between the two prongs - you should have some resistance, but I don’t know the specs. If there is a plain short; the Ranco is indeed defective. Then ohm each prong to ground, both should be infinite - if not; the Ranco is again defective…

The reason Ranco is shorted is sort of immaterial (s… happens), but inspect for eventual damage to insulation or any other possibility for external short.

There is no safe external temp limit for operating without the Ranco. I would advise against it; the system is set up to ‘always’ run the compressor, the Ranco is the back-stop. You have no continuous reliable indicator of outside temps - and your memory may fail…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

Yes, if left running long enough … i.e., long drives with the ac on. ‘How long of drives’ depens on conditions, but I’m guessing that even a drive long enough to fully bring the engine to operating temperature would be okay.

“Unlikely”? I wouldn’t count on it not icing up at any temperature.

Remember, I said bypass with a switch for testing.

Okay noted. Will have to fix!

Sorry I meant fuse 3 blows. The ranco fuse is fine and doesn’t blow. Weird eh? Will check the wiring colours and report back. Thanks.

Somehow the Ranco seems indeed to be wrongly connected/related to the 50A fan fuse, Russell - not correctly to #6 as it should be. Remove the #6 15A fuse and see if the servo/AC operates…? It should not…

And I had a senior moment! The correct ohm reading between the two prongs on the Ranco is of course ‘make’ - it ‘break’ when the capillary content cools and contracts at +2C (60F). Unless either (or both) the Ranco prongs is grounded, causing a short; the Ranco is likely working as it should. It is, however, difficult to check, in car, that the Ranco is fully functional…

As it takes heaps to blow a 50A fuse; you have a short. And if the compressor runs properly with the wires disconnected from it - the short should be detectable at the Ranco…

But the compressor, and indeed the AC system with it, is not meant to be fused at 50A…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

Okay thanks. the #6 fuse is the one in series with the ranco by the HVAC unit right? That one is good and weirdly doesn’t blow. Its the one under the steering wheel (UK) that blows rendering the whole system inoperative with no fans.

Bizzare as the 50A fuse has nothing to do wit the Ranco, it’s for the blower fans as Frank said.

On the other hand, on the diagrams I have the 50A fuse is #16, there is also #6 - 35A that seems to feed the mode selector switch and servo and a third one 10A (no number) in series with the Ranco that feeds the compressor clutch.

Thanks Artistides. Your diagram is for the 4.2 and I have a V12. Assume its the same?

The fuse that blows is the 35amp #6 fuse. Sorry for the confusion with the 50amp! The 10amp fuse in series with the ranco doesn’t blow.

As Aristides implies, Randy; fuses are model year and market dependent. And manuals are not entirely trustworthy - all my manuals show AC (mode switch) served by a 15A fuse. In some versions; a 10A fuse is fitted inline between Ranco and compressor clutch - but it is not universal…

Car wire colours are the same for the 6 and 12 except where engine management require differences. Another factor is PO interventions - he (or you, when reactivating AC) may have connected the fuses in different ways.

The main problem is that the fuse (whichever) blows when Ranco is connected - and not otherwise. Do check Ranco for ground of any of its prongs; if grounded it will blow the stoutest of fuses. And the cause, likely the Ranco itself, must be sorted…

While working on this; the safest is to leave Ranco wires unconnected - and using AC without the Ranco is unsafe…

The respective wire dimensions reflects the fuse size; the fans’ wiring is dimensioned for the higher current. The AC’s are more ‘flimsy’,
So it is unsafe to run both the AC and the fans on the same fuse - the 15A blows before its wires burn. Nominally, the wire at the fuse (15A) and function con*trol is green - the fan’s wire at fuse (50A) and relay bank is brown/white - as a start.

xj6 85 Sov Europe (UK/NZ)