Your switch might very well be faulty Greg, but I have seen many a times in modern cars the electric fan to kick in for just 15sec or so, and do that again a minute or so later.
You have a much more powerful aux fan now, so the cooling effect is much more immediate.
Personally I would prefer this, i.e. the aux fan keeping a steady temp in the block and having a steady reading on the temp gauge rather than cooling it way down, then wait until it warms up again and so on.
What happens when you leave it idling for a long time? How often the aux fan kicks in and what does your temp gauge do?
Donāt know if this pertains to your inquiryā¦ I moved the wire on the relay located on left fender wellā¦ so the fan stays on as long as it is hotā¦ I believe the directions to do this are in the BOOKā¦ someone who remembers might chime in on this.
itās not been warm enough for me to see a cycle happening, I had to wait a good 10 minutes of idling just to see it go from 195F to 200F. In the meantime, Iām replacing the switch (easy and only $30), and hopefully in a month or so weāll get our first 85F day. Thatās when my engine will easily hit 200F when idling in traffic, so Iāll know then.
OK , I think youāve inspired me to take a few temp readings of my V-12 at idle now that weāre getting up to 20C today. Iāve changed the coolant , replaced the tāstats and Iāll check the temperatures when the auxiliary electric fan switch energizes the fan. Like I said in an earlier post. itās a lower temp switch thatās meant to start the fan sooner than later but Iāve never confirmed the actual temperature. Mind you my temp gauge stays well below the N so Iām not sure if it will get hot enough to get the fan going without a good run. The joys of living in the great white North as compared to California I suppose.
The readings on the barrel gauge are anybodyās guess. I am reading from my actual aftermarket elec gauges, one for A and one for B, that I installed myself.
The actual temp at the thermostats can vary compared to when the actual temp switch clicks on/off on the coolant coming out of the radiator.
I think the more important thing is, how long does the Aux fan have to stay on to move the coolant needle X amount of degrees for the block, and how long can it go until it has to come on again. Based on all my other non-Jags, I would expect a good 1-2 minutes of cooling, followed by a good 5 minutes of not needing the fan. (if idling)
I also want to shut down with the aux fan on, and see how long it takes to switch off without coolant flowing, so that the battery doesnāt get taxed too heavily, or else I need to wait for aux fan shuts down before I shutdown engine.
Remember, the temp readings that are relevant to fan operation are radiator left side lower tank and bottom radiator hose. The temp of the engine or thermostat housings is not relevant, and will only lead to confusion as you try in vain to figure out why the fan is on sometimes and off other times unrelated to the thermostat temps.
do you think a laser reading on the lower radiator hose is sufficient? Or should it be on the waterpump?
OK Iām going in today with my fancy temperature taker. So lower left tank and bottom hose ? Are they not in the same vicinity? Also what about at the top hose or the engine block where the fan switch is located. I guess Iāll try and get a temp when the fan turns on and close the hood in order to time how long for it to shutdown?
The testing instrument will be a Ryobi (no affiliation) for these tests.
I doubt if a reading on the hose itself will work simply because the wall of the hose probably has some significant insulation value so the outer surface will be cooler than the coolant within. The housing of the water pump should be good, though, provided it doesnāt get heated too much by its connection to the block. Can it look at that inlet elbow itself that the fan switch is screwed into? Thatād obviously be ideal.
Well I must admit trying to take engine temps so I could get the picture and the temp in the same picture requires a lot more hand eye coordination than what I apparently have.
I let the car idle for forever and a day and the electric fan wouldnāt really come on.
I went for a brisk run which seemed to make the fan switch on and off at idle. Iām thinking itās coming on around 77C, and seems to drop out around around 73-74C. After shutting the car off it runs for approximately 56 seconds.
The highest temp I recorded was by the top rad hose at 90C.
I tried to measure around the fan switch and water pump 77C -83 and about the same for-the lower rad hose.
Not sure this is very conclusive as I needed about three more hands. Iāll attach some pictures of you can make sense of them.
The barrel temp gauge picture is just idle and then after a run.
At the end of the day I read this book written by some guy who lives in Florida. I think the car is within a good operating temperature range. No issues here.
I did some measuring today too, as we hit 74F/23C ambient. I think my switch started working again! I took wires off yesterday to test, and put them back on last night. Perhaps I wiggled something, or it just started working again. Regardless, Iām replacing it, as I donāt want an intermittent switch.
SO, first off, I donāt think there is a reliable way to measure the coolant coming out of the radiator with a laser gun. The readings near the water pump at the sender are I think heavily influenced by the block. My readings by the sender were about what the thermostat temp was. The outside of the water pump was 205F/96C, when the coolant temp was only 195F/90C. The lower radiator hose was all over the place, with 20F difference depending on how I pointed laser.
BUT, with the switch working hopefully as it should (Iāll test it in a pan of water once I take it out), I was able to get a range and time on getting my coolant at the thermostats down.
My Aux fan turned on when Thermostat temp hit 201F/94C. It took 4 minutes to get it down to 195F/90C, and then shut off. Unfortunately, I could not get it back up to 201F/94C, because at this point I opened the hood! Goes to show how much that hood contributes to baking.
That sounds a little odd. Do you still have the original radiator set up.
Not sure how much influence the block or the hoses affect a laser gun reading. I even measured at the water rails. All were fairly close. I was trying to measure at the front and back of the water rails to see if there was much of a difference. Always toying with the idea of the Lutz modifications .
Iāve never had a problem with overheating or the fan not working. Sometimes I should just leave well enough alone.
Not sure if you remember these pictures I posted on the V-12 category a few years back.
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There is some possibility that the inner liner of that lower hose has collapsed. This might also explain other symptoms, as the collapsed liner might close off flow and then suddenly let it move, resulting in dramatic changes in the temps seen by the fan switch.
(temp in Celsius x 9/5) +32 equals fahrenheitā¦ (Temp in F minus 32) x 5/9 equals Celsiusā¦ just a reminder of how its calculatedā¦
No, thatās not it. Cāmon, theses V12s are work enough without assuming everything is a catastrophe 
I edited how I worded it aboveā¦was more about the angle of the laser too.
Close approximation: (Ā°C x 2) + 30 = Ā°F, or
(Ā°F - 30)/2 = Ā°C.
You can do it quickly in your head, and itās close enough for most things.
Example: (200Ā°F -30)/2 = 85Ā°C
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I just think the rubber hose does not contribute well to a reflective surface for the IR laser.
Iāve had problems using it on shiny metal too.
The main thing, my Aux fan was able to move the coolant temperature coming out of the block/head from 201F/94C down to 195F/90C in 4 minutes at idle. Those temps I know are correct because Iām using my actual coolant temp sensors mounted in the water rail just before the thermostats on both A and B bank. Quite happy with itās performance. I think my older aux fan took a good 6+ minutes.
So I get just what I hoped for, engine will normally run between 190F/88C to 201F/94C. The thermostats and aux fan hold it there. Nice tight range.
Without trying to over complicate things, I use my temperature sensor as a tool to gain knowledge as to what the normal operating temperature in my car is telling me. Not all these gauges are made the same and are reading the same for all JaguarXJSās.
It helps to establish a baseline that you can take on the road and help you understand youāre own car ( the difference between running hot and overheating essentially). It also helps establish what normal operating temperature is.
Idling for an hour in stop and go traffic with the AC on your car will creep up and run hot but does not necessarily mean youāre overheating.
Especially on these V-12ās as your temperature sensor for your car gauge and your ECU are on opposite banks using two different sets of information. It can also help you determine how efficient your radiator is.
Generally speaking with 88C tāstat you should be showing 88C-94C after a good run and back to idle in your driveway. They should read at the housing within 1C-1/2C at each tāstat.
If you have 90C at your top hose going in your radiator than you probably should have a min. 20C - 30C (range) lower coming out of your radiator if itās working efficiently.
Also make sure you have new correct pressure radiator caps on, people seem to miss that.
This is my baseline that has served me well over the years, others can chime in if Iām out to lunch.
Hmmm, according to my readings, assuming my aux temp sensor is working at itās correct temps of 88C/79C, my radiator is not cooling that well?
When it triggers on: Top hose = 94C, Bottom hose = 88C, difference of 6C
When it triggers off: Top hose = 90C, Bottom hose = 79C, difference of 11C.
But then that would mean, when my radiator cools down the coolant to the required 79C to shut off the aux fan, my coolant temp at the thermostats should be 99C-109C? That doesnāt sound right.
I am still running original radiator. Because Iāve never seen temps that high, I donāt worry about it. I also live in a mild climate (Seattle). But next year I plan on pulling it and having it cleaned/recored regardless as part of my rolling restoration.