Well, after my son drove the green '85 XJ6 for about a week, he calls me and says the battery is dead. What The @#!@ - that’s a brand new battery!, I’m thinking. I asked if he left the lights on, he said no. (yea right! I’m thinking) I went over to his place to see what was going on
The rest of the story
When I got to my Son’s house, Popped the bonnet (hood) and noticed coolant everywhere. F@%^! I’m thinking. I asked if the temp guage indicated it was hot or if the coolant light idot light came on; or if he could smell coolant; or if he needed to go to the doctor to have his sniffer tested?! He said no. A gallon of coolant and a fully charged batty later, I fired it up and took the car home, then I noticed the fan wouldn’t turn off. Oh, and he didn’t leave the lights on. The fan on the left side of the radiator (Federal cars Drivers Side) wouldn’t turn off. Even with the key out of the ignition. Hmm. didn’t come on when I hooked the battery back up. Weird. (I’m thinking)
As a test, I disconnected the ground and obviously the fan went off, and didn’t come back on when the battery was reconnected (to earth / ground). Hmmm… Ok?
Fired up the car, and the fan came on (again) air con / heater turned off. Engine off & key in my hand. Fan still on. Hmmm… Unpluged from the relay and plugged it back in - the fan stayed off. started the car again - fan came on turned the car off fan stayed on. OK… Cooked Relay…
I dug around in my Jag parts bin; found and replaced the relay, and well that didn’t fix it. So this tells me a thermostat / sensor that controls that fan has failed. The temp guage in the car reported correctly so this tels me it’s not the same thermostat that drives the fan relay. I haven’t looked at a wiring diagram yet, so I’m not sure what where the controlling sensor is located. I figured I’d throw this one out there incase someone else has had a similar problem; and may save me time finding a resolution.
thank you Doug, I will check it out. this makes sense cause the upper radiator hose let go. I did a tune up last weekend including a compression test to make sure I didn’t blow a head gasket 150psi across all 6 happy day
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That the radiator hose ‘let go’ explains the loss of coolant, Mark - though it seems a coincidence.
On the V12 there is a relationship between the AC and the aux cooling fan, which is not mentioned for the xj6 - so the fan is controlled by coolant temp only?
However; on the latter there are two diodes in the circuit - and a failed diode may relate to the abnormal fan behaviour. Diodes are in the light green, light green/orange and green/blue wire interconnection.
Before changing the radiator switch; access and disconnect the wires on the switch - and ohm switch to ensure that it is indeed shorted, and the cause of the problem…?
It’s very easy to check with a multimeter if the thermostatic fan switch works as it should but I agree with Frank that the most probable cause is the diodes.
In my car they are incorporated in the harness just before the relay.
well here’s the kicker. I cracked The Good Book of Green only to be disappointed once again. What!! No Pictures! You mean I have to read?! Like! OMG! It’s Like… I’m in school or something!!
So where are these diods? Are they in the temp sensor that Doug talked about below the upper radiator hose? or the aux fan itself? I would think the diods would be before the relay. right? Come to think of it, I have an '86 Series III radiator under the house. I’ll take a look at it to see what I’m up against. As it stands right now. I’ve just unplugged the relay. It’s cold here in the Pacific NorthWest so no chance of over heating. Hell it barely gets up to with the heater cranked up to singe leg hair off setting.
It’s late here, and I’ll take a look in the morning.
On the 6-cyl cars there is no tie-in between the aux cooling fan and the climate control. There are some diagrams and/or text passages in early versions of the Jaguar ROM which indicate otherwise, causing some confusion
Testing the diodes and the switch would be a good exercise…although the thermal switch is probably the easier place to begin.
I’m only two sips into my first coffee, but I don’t think there is any way for the fan to operate unless the thermal switch is closed. The diodes are upstream of the switch; no matter how they may fail the fan relay cannot be energized unless the switch is closed.
Removing one wire from the switch, thus simulating an ‘open’ switch, might be an easy test. If the anomaly remains, then my theory is disproved, and the diodes move to the top of the suspect list.
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The S57 diagram is a bit confusing, Doug - lots of black wires supposedly grounds. But diagram certainly indicates that the thermal switch controls the relay - switch powered from #11 fuse. And is the sole source of fan motor power is through the relay from #1fuse. And no relay action; no fans - so with fan running; motor is powered through the relay…?
I wonder if the failed radiator hose, spilling lots of coolant, is at the bottom of the problem. Water and electricity makes for odd symptoms…?
OK, curiosity has gotten the better of me so time for a memory refresher.
The relay is triggered by “+” voltage via the thermal switch.
The diodes comes into play with respect to where the “+” trigger voltage comes from.
The trigger voltage comes from #11 fuse, as you say, when the key is “on”. So far, so good. But, for after-cooling with the key “off”, a second voltage source is used…in this case “+” voltage from fuse #1, via the already closed relay contacts.
(After-cooling operation cannot occur spontaneously. If the relay is already closed (via the thermal switch) when the key is switched off, the second voltage source will keep is closed until the thermal switch opens. But, if the thermal switch closes after the key is turned off, there will be no voltage to trigger the relay)
The diodes ensure 1) that trigger voltage goes through the thermal switch, not around it and 2) voltage from #1 fuse cannot back feed into the #11 fuse when the system is in after-cooling mode.
So, I was previously incorrect. It appears that if the diode on the green/blue wire fails it is possible to trigger the fan with the thermal switch open.
Whew ! That was a fun little stroll down memory lane.
Still, pulling one wire from the thermal switch and seeing if the fan still operates seems to be the easiest (but least interesting) test in this case.
If I understand this (that’s a really bit IF)
it appears the diode comes into play to prevent a feed back into the ignition. It appears the “+” comes from the ignition via #11 fuse supplies power to the thermal switch, (normally open) once closed energizes the coil in the relay, which energizes the relay flipping to closed making the fan spin.
it would appear the diodes are there to prevent a feed back loop. So theoretically it could be either the diodes or the switch. I"m going to start with a switch. I’ve got a used one I will pull out of a radiator. I probably have the diodes but I don’t know which box the wiring harness is in.
I still haven’t had a chance to get out there and take a peek.
Right now I’m trying to finish a video that goes over how to diagnose a no start ignition problem. in this case a crusty wire that gave up it’s insulation on the negative side of the coil from the amplifier.
The diode on the green/blue wire creates a loop so an engaged fan will stay engaged with the key off…until the coolant cools down enough to open the switch
I wouldn’t replace it randomly. It isn’t a fun job. Test it. Or, just unplug one wire. If the weirdness you described in your first post continues, you’ll know it isn’t the switch and must be the diode
Thank you for the due diligence on this topic. I will test it first. if it’s right under the upper radiator hose, it’s probably the switch that has been coated with anti-freeze. I just did a tune up on the XJ6 and the inside of the distributor cap had a nice coating of coolant too. Not to mention the air filter canister. The paint is now discolored; and I’m not sure how to clean it without disturbing the OE stickers on it. Ugh!
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At the same time; the #1 fuse, constantly powered, also provides power to the relay coil via green/blue and the closed thermal switch, Mark - actually a double feed (#11 and #1 fuses).
As long as the switch is closed; #1 fuse provides power to both the relay coil and the motor - running the fan even with the ignition off…
This provides the fan to continue to run until the coolant gets down to a proper temperature. I didn’t think of that side while I was looking at the diagram and reading what you and Doug were on about.
Now I understand. Clever.
Thanks again Frank and Doug. you two among many others in this forum have given me the encouragement to take on projects that, in the past I paid to have someone else do with less than favorable results - Specifically my '88 XJ-S. But that’s a whole topic unto itself that just is all bad.
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Did you actually test the function with the engine cold, Mark…?
The thermal switch must open to stop the fan; if it stays closed, as it would with the engine hot, the fan will indeed run on. As it will if the switch is externally shorted by leaking coolant…
I’m not sure of the trigger temp for the switch - commonly 95C for electric fans in general. But the trigger point for the aux fan might be different…?
Yes I did check it when it was cold. I’m pretty sure. Actually now I’m not sure. I will check it again. Thank you Frank. That’s a bug Duhhh!! On my part. I’m almost positive…… I would say a definite maybe.
The fan on the left side of the radiator (Federal cars Drivers Side) wouldn’t turn off. Even with the key out of the ignition. Hmm. didn’t come on when I hooked the battery back up. Weird. (I’m thinking)
