The auxilary cooling fan on my '89 XJ-S no longer turns off when the engine cools. With the engine off, it will continue to blow for hours unless I pull the fuse. Once the fuse is pulled, I can immediatly plug it back in and the fan stays off. My instinct is to install a new thermo-switch but my instincts are often wrong with these cars. Anybody know more about it? Thank you.

 

I'm not sure how the thermoswitch actually operates but if it is stuck "open" then it will be reading a temp that tells the relay to leave the fan on.

I guess that the relay could be stuck as well leaving the fan running regardless.

Personally I removed wires from the thermo-switch when it broke and just wire it together at the diode pack so that it comes on with the ignition.

 

Hmmm.

I think either a stuck switch OR stuck relay could cause this.

The relay looks like the standard accessory relay with conventional 30-85-86-87 terminal designations. You can buy these for $6-$10 at most any auto parts store or online. For that amount of money, shoot, I'd try a new relay and see what happens, IMHO.

Cheers
DD

 

Doug, Can you please tell me which relay it is? The book I have unfortunately does not show the information on my '89. I have 3 relays on the right fender well, and one in fron (centered at the top of the radiator). I pulled the furthest aft of the three and the fan shut off, so I was thinking that might be the one...but, it has five(5) terminals, not four. Thank you.

 

Should be on the left fenderwell, forward-most relay in the bank of relays you'll see there

Click here and go to sections 17.1 17.2 17.3


http://www.captainjaguarscathouse.co...ec%20Guide.pdf


Cheers
DD

 

The aux fan is controlled by a mixture of a diode thingy, that looks just like a relay, and a relay as well. The diode somehow stops the fan backfeeding the electrics when the fan is running after shutdown, and also allows the fan to run after shutdown (I think). Anyway, I think it might be best to be sure exactly how the system works before diagnosing what the fault might be.

My point is that both these devices need to be operating properly for the aux fan to perform as intended!

Greg

 

When the relay is energized voltage from the underhood fuse powers up the fan. This is a constant 12v supply.

The diode pack comes into play on the control-side of the relay.

Normally, "key on" voltage from fuse #11 or #3 (depending on VIN) goes thru the diode pack to the temp switch. When the temp switch closes, the voltage goes to the relay, energizes it, and the fan operates.

Meanwhile, voltage from the output side of the energized relay is also going into the diode pack.

So, if the key is turned off when the temp switch is closed, voltage is still present to keep the relay energized and the fan running. The diode prevents this looped voltage supply from powering-up the rest of the circuits on the fuse which rightfully should be off when the key is off.

That's it, in a nutshell.

Note that the a/c compressor tie-in in not used on the '89 cars.

At first blush, with only one cup of coffee and without any serious head scratching, I don't think the diode pack would cause a "stays on" problem.

Cheers
DD

 

Thanks Doug

Greg

 

Doug,
I second Greg's thanks. Your indepth explainations are both entertaining and helpful. Oddly enough, the symptoms have gone away...at least for now.

 

Hi Folks
Thanks for the very useful info
I have the same problem on a UK (RHD) 92 facelift convertible with AJ6 4L. The aux fan stays on and will run until it drains the battery
I have pulled the aux fan relay from the front left component panel in the engine bay (see pic), swapped the relay to make sure, so it looks like the thermal switch on the radiator is stuck in the closed position (fan switched on because the radiator has reached 80 deg or whatever) which is strange given its mid winter and almost 0 deg

The electrics were simplified on this model so dont have any diode packs to worry about

1) Can anybody direct me to where the radiator thermal switch is, or have a pic / illustration
I have seen some descriptions that it is top left side of the rad (looking forward) hidden by the air filter, but others saying it is right hand side front of rad (looking forward) which will not be an easy job

2) Can anybody identify the mystery red socket in the front left component panel
I cannot find any reference to it a number of manuals, so I assume its a diagnostic socket for the early Jaguar EDS systems prior to OBD-I
Rgds

Aux fan relay & mystery connector

simplified aux fan circuitry

 

Found the radiator thermostatic switch
Top left hand side of the radiator looking forward, hidden behind some shock absorbing foam on side the rad
I have removed the shroud that covers the radiator so had a bit of clear room (I think its called the top crossmember in the manual)
I used an adjustable spanner, but it looks like 30mm
The connector that connects the thermostat to the auxiliary fan relay is the 3 pin triangular "PM5" connector you can see cable tied to the loom in the picture
Cut the cable tie, and with a small flat bladed screw driver depress the locking lug and seperate the 2 halves of the PM5

With a tray underneath the car undo and remove the stat. You will loose some coolant, but not much
You will find the thermostat if original, has the cables moulded into the brass element, and the replacement on has conventional crimp connector male terminals, so you will need to cut the 2 cables pretty close the old thermostat and improvise a new loom connecting the PM% to 2 new female crimp connectors


The replacement was a standard M22 x 1.5 radiator fan switch 82-72C available from loads of places on the internet
test with a bit of cardboard down the front of the rad, and all seems to work

Note if you do take the fan shroud (top crossmember) off as I did, be aware that on the right hand side looking forward is the main earth eyelet for your lighting system, so headlights, turn lights etc will not function until you bolt that down again