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Diagnosis and Repair of No A/C Fan Condition (And other useful AC Observations)

XJ6 & XJ12 Series I, II & III 1968-1992

Diagnosis and Repair of No A/C Fan Condition (And other useful AC Observations)

Old 04-24-2019, 08:18 PM
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Default Diagnosis and Repair of No A/C Fan Condition (And other useful AC Observations)

I was able to diagnose a No Fan at any Setting condition for the Delanair A/C system. In the event that someone else has the same condition perhaps this post will be helpful in diagnosing your condition. I am no Delanair expert, the diagnosis was the result of lots of research and hours poring over the electrical diagram for the AC system. This electrical diagram may be found at Fig 25.1 in Jaguar Publication S 57. I have a marked up version below.

The symptoms:

1) The AC Fans suddenly/progressively became unresponsive to the Mode Control Switch. In my case, this progression occurred over maybe 5 minutes, revived a day later and then quit altogether.

2) When the fans quit, no mater how much flipping back and forth, changing of the dials, no fan response.

3) Flaps worked under Vacuum Only. There was no “Whir” of servos when changing Mode or temperature.

4) When driving, hot air would “flow” through the system, but not “Blow” through the system. When the mode switch was turned “OFF” the “flow” stopped.

5) There was on a single “Click” relay-activating noise when the mode selector switch was turned. In other words, there should have been at least four (as different setting caused the relays to go off in turn) and I was getting one.

Potential causes:

I learned from researching this forum and others that the system is mysterious and there were no clear (if you don’t have experience or fancy equipment) steps to rule in or out certain expected faults.

A thorough researching revealed it might be any of the following:

1) Gummed Up Fans

2) Blown Fuses (there are at least 5 that might be effected)

3) Temperature Switch

4) Fan Relays

5) Amplifier

6) Mode Control Switch

7) Fan Resistors

8) Broken Wire

I’m sure there are others but these seemed the most common faults. I learned that each of these has it own presentation symptoms that can be checked. I determined to methodically rule out each until I figured out what the problem was.

1) Ruling Out Gummed Up Fans

There is a quadruple Relay that controls the Fans. It is located in the Left Hand Knee Wall footwell just below the footwell vent opening. This is a very important location for diagnosing this condition. Here is a picture:

In the very center of this relay is a Brown/White wire. This wire brings energy from the fuse panel when the car’s ignition is in the “On” Position. The diagram I used to determine this is below. The coloring is mine to indicate where energy went (I am no electrician) when various switching actions occur.

The Orange colored line which is the one from the fuse when the Key is turned to the start position leas to the “C” Mode Selector microswitch. This is a Single Pole Double Throw (SPDT) switch. In this case, its default position means that current flows along the Orange line which you will see eventually ends at a wrong way diode in the servo (no electricity flow there) and to ground. When this switch is activated (by turning the selector switch) the current now follows the Purple path. That path leads to the “A” mode selector switch which is for “Defroster”. The default position for this SPDT switch is to amplifier. When switched off the “Defroster” (to something else besides “OFF”) Power follows the “Green Line” up to the relay containing posts 85 & 86 & 87 + 30/51. This relay is also fed by power direct from the Battery via the light blue line and is independent of the other three relays. In other words this one will “click” when the mode selector switch is turned from Off to Defrost if Microswitches “A” and “C” are working an delivering power. I actually didn’t realize that until just now.

The point of all this is that there should be 12V battery power at the relay regardless of key position at post 30/51 which is the post in the middle with the screw and the Brown and White Wire. As seen on the diagram, power goes from post 2H via a Green and Slate wire directly to the fans.

Verifying the fans function is simply a matter of jumping the Brown & White to the Green & Slate. If the fans come on, then the fault is elsewhere. In my case, doing this caused the fans to come on so Gummed Up fans was ruled out.

2) Ruling out Blown Fuses

Many sites suggest that Blown Fuses are the main cause for this condition. As noted above (I didn’t realize it at the time) If there is power at 30/51 that by definition means that Fuse #6 is good. Checking fuses is easy enough. Just replace them and see what happens. There are 5 in all that might be implicated. #6, #17, #12 and two in-line fuses. One in-line is located on the black ground wire next to the relay and the other is on the opposite side under the passenger side knee wall. I checked all these fuses and replaced them. In the process I learned something that I did not know and that is, I think, very important for us Americans. Actually, ignorance of this is probably the cause of other AC failures I would guess. What I learned is that there is a difference between the Lucas fuses and typical American fuses. American Fuses are referred to as AGC fuses. The difference is that the Lucas fuses are rated at a peak ampererage with a much lower continuous amperage. So, if you put in an 50A Buss AGC fuse you are allowing a lot more amps than the car was designed for. You will see this in writing on a Lucas fuse if you look. It will say 50A and then something like 25 Amps continuous or something like that.

I found a chart of these equivalences at http://www.bernardembden.com website. I ran a paper copy so I can’t paste the chart. If you google his website for “replacing a Lucas fuse with an AGC fuse with the same amperage” you will find the chart.

I’ll reproduce it here in case Bernard's page should disappear.

Lucas -> Buss AGC Equivalent

50 -> 30

35 -> 25

30 -> 20

25 -> 15

20 -> 10

10 -> 7.5

5 -> 3

After replacing all the fuses per the above chart the new fuses didn’t blow and the fans still did not work, so this was ruled out as a cause.

3) Ruling out Temperature Switch

In the picture of the relay above there are two conspicuous Brown Wires with Gray connectors on the end. I was told that if you jumped these (thus bypassing the ambient temperature switch) this might indicate that switch is the cause of the problems. I did this and there was no difference so this was easily ruled out. However, looking at the diagram this fault would only ever rule out the ambient temperature switch and would necessarily involve the Amplifier and should not affect the other fan speeds. So if there are no different fan speeds then I would say this is unlikely the problem.

4) Ruling out Fan Relays

The next most likely target was the Fan relays. These would need to be tested. This can be done in the car, but as a non-electrician, I needed to see them and get them in a place where I could activate them with a battery. I only knew that you could activate it with a battery, not how to do it. So I had to trial and error it. You can take them out of the car, but there is a bolt holding the relay on in the back that is scarcely accessible and available space will only allow you to turn it a millimeter or two at a time. It is also very difficult, nay impossible to get fingers on it, and because of the geometry of a vacuum switch that uses the same nut, you won’t be able to get a ratchet bit on it. So best if you test it in the car if you know which terminals to touch with an independent battery.

In my case, I removed the cover from the relay and everything inside looked pristine. Taking the cover off is easy if one is careful. There are tiny wires barely bigger than a hair that are soldered inside. I understand that these can break loose of the solder and cause the relay not to work. So if you relay is not working (meaning you determine that it is the cause) I encourage you to take it out and see if you cannot repair it. I would say there is a fair chance you can having looked into one.

In my case, when I applied 12V from a spare battery to the posts I did this until I got a click. I was doing this around the 85 & 86 & 87 + 30/51 relay because I suspected that was the one that actually was clicking when I turned the mode selector. Eventually I found the right combination of posts and got clicking when power was applied to all four. This showed that the relays were all working and thus, Fan Relay was ruled out.

5) Ruling out Amplifier

At this point, I was scratching my head. I knew the relays worked. So if the relays SHOULD be clicking when the mode selector switch was turned, and they weren’t clicking when this was done, surely this meant that the fault was between the switch and the relay? There really isn’t much of a direct connection between the Mode Control Switch and the Amplifier and what there was obviously wasn’t working because there was no servo noise. I was strongly suspecting the Mode Control switch or MORE IMPORTANTLY and CHEAPLY one or more of the microswitches that make up the Mode Control switch.

I wasn’t sold yet on it and I understood that getting to the microswitches was no easy task. So I needed to be able to rule out the Amplifier if doing so could be done easily. It could be easy if I had one of the those Jag-Aire manual controllers. These are designed to bypass the amp all together. If I could simulate what that manual controller did and nothing happened then it couldn’t be the Amplifier. I started researching this and learned that I could bypass the Amplifier fairly easily.

If you touch the purple and red wires leading to the servo on the hot side of the amplifier round connector on the driver’s side (there’s a similar one on passenger side that handles the servo connections), in other words complete the circuit through a 12V battery on the servo side of that connection, the Amplifier is bypassed the same as the Jag-Aire manual control. Changing the polarity of the battery would cause the servo to run the opposite way. I didn’t figure this out on my own, I’d read that some people bypassed the Amplifier with a Window Up/Down switch. Whoever had posted that didn’t say “how” to do that, but Bernard Embden’s site did have a write up that explained. I got so excited I went in search of such a switch locally. Of course, no one carries these anymore, but I found one in an ancient package at the local auto parts (non-chain) store. I then spent the rest of the day trying to figure out how to wire it.

The more I thought of how to wire it the more convinced I was that the problem was not the amplifier. According to all I had read, the Amplifier only has an effect on the “Auto” setting. The fans should operate on Low or High. Granted they would suffer from inability to modulate the flaps and hence temperature, but the fans should run.

After a night’s sleep I became convinced that it couldn’t be the Amplifier because the fans didn’t work in any Mode. I provisionally ruled this out. I could always hook up the jerry rig manual control later if further investigation didn’t find the trouble.

That left Mode Control Switch, Fan Resistors or a Broken Wire as possibilities. I was fairly convinced it was the Mode Control Switch but scared to death it wasn’t.

6) Success: Mode Control Switch.

I took apart the dash according to the directions on Bernard Embden’s website. His directions relate to an XJS, but the process was very close and you can figure it out using at least the order of disassembly he sets out.

After Disassembly you will be looking at a loose Mode Control Switch behind a mounting plate with a vacuum switch on the end. The Mode Control Switch looks like this:

Now before I took this apart I researched how this part works and what the fix is if it is broken.

The part pictured is fairly pricey if you can find one. And you don’t really want to disassemble it because it contains cams that turn the microswitches on and off in certain order to make the mode do certain things. The good news is that Sarc (at least a one time member AFAIK) made a video about these microswitches. That video is at
. He goes through this problem in detail. One thing he says is that you can remove the wiring with the switch. I could not and needed to disconnect all the wires. Because I felt pretty sure I had a microswitch problem I researched how I might be able to replace them in the event I found one (spoiler alert-Sarc replaced 10A switches with 16A ones...tsk tsk). Initial research was not good. The parts manual is very weak on this point.

It turns out that there are three different microswitches in use in this car. The parts manual refers to one. A service manual I came across shows P/N’s for all three. Through clever googling I located original versions for sale as used parts. These were very expensive, like $23.95 each. I remembered in Sarc’s video him saying that he bought some, but I didn’t know what size or that there were any differences in microswitches at all.

Turns out there’s tons of things to know about microswitches, such as the difference between SPDT & SPST and On-Mom and On-(OFF) and which pole on a SPST is the one Normally Closed (N.C. - meaning power flows) and which ones are Normally Open (N.O. - meaning the circuit is broken-no power). I learned all this the hard way. By reading and reading til I figured it out and compared it to the wiring diagram. This still didn’t tell me what Amp rating or Voltage the switch should be able to handle. Turns out there’s an easy way for all this and I am going to post it here for posterity.


These old switches have two critical bits of information. One one side is a model number (which is long since deprecated with no reference to a replacement) and on the other is a wiring diagram showing the configuration of the switch. Here’s Ebays version of one of the SPST switches. Don’t bother with the model numbers and P/Ns unless you want to pay $25.00 four a forty year old used switch.


Note: There is a different SPST microswitch also used that doesn’t have that stubby black tang underneath the gold contact, but I couldn’t find a picture of it anywhere. I can assure you. However, that it IS different and it works exactly opposite to the one shown.


I couldn’t find a picture or example of the SPDT microswitch. But I did come across this manual page which very clearly shows three different microswitches.

I was able to deduce that these switches are all 10A 250VAC microswitches as originally designed.

Sorry Sarc. You missed that. These are actually still available in an exact form (I think).

Omron Electrical Components makes exact duplicates of these.

(x2)Omron V-10-1C25 is the SPDT correlating to RTC674 and costs $2.81 per on Mouser.com.

(x1)Omron V-10 -2C25 is the SPST N.C. correlating to one of the SPST microswitches; and

(x1)Omron V-10 – 3C25 is the SPST N.O. correlating to the other.

These switches are all 10A 250VAC.

Honeywell also makes a very near match called the V7 series. These are even cheaper than the Omron ones but they are more difficult to figure out which ones are which. This is really important because if you buy a momentary on (On-Mom) switch it will never work. In any case, likely Honeywell matches are the V7-1B37 D8, V7-2817-D8 and V7, V7B17D8. Check the Honeywell ones carefully and make sure they are right. Also these Honeywell are not exact Amperage and Voltage matches being 11A and 270VAC.

Highly Electric Co., Ltd. Has a series of microswitches called the VT16 line that I believe are the right Amperage and Voltage but do not seem as readily available as Honeywell or Omron.

So before Ordering my Omron switches I thought I’d pull the Mode Control switch and make sure.

That actually took much less time than I worried about. I took the time to make a diagram of the different wire locations so I knew where to replace them. Note that two of the connections have dual Brown/Yellow wires. I marked these locations on both the wiring connector and the microswitch itself with 1 & 2 so I would know which connector came from which switch. In case someone needs it, here it is:

When I finally got the mode control switch out I was amazed by its condition. It looked just like the one way above. It looked brand new! I was shaken because I was worried that my sleuthing had failed me.

On closer examination, though, I noticed that one of the four nuts that holds the switch together was not anywhere near tightened. In fact it, had been cross threaded on just the tip of its bolt. Also, I noticed the power contacts on each microswitch (that’s the one on the top/bottom of the microswitch btw) was kind of discolored. Maybe tarnished is a better word. I tightened down the nut after threading it correctly, scotch brighted the tarnished contacts (they turned colors! From tarnished to bright silver) and connected my multimeter to each microswitch in turn. I learned about this from a Video by an appliance repair guy called Cecil Colvin in a Youtube video called “Understanding a Microswitch”. Basically, you need to keep the microswitch in its habitat. Don’t remove it from the Mode Switch casing for testing. Then set your multimeter for audible continuity and touch one multimeter probe side to the power side of the microswitch and the other to the switched circuit and work the Mode Control so that the switch plunger cycles. If you hear the tone for all contacts the switch is good. Mine was good. All the microswitches tested as good after the tightening and cleaning.

I’d read that small irregularities can affect Microswitches like this, so I decided to hook up the mode control again and try it. Soon as I powered the key, the system came right back. Problem solved. If one of the switches was actually bad it would have been a simple matter to match up the diagram on the bad one with the diagram for the Omron one (or worst case scenario just buy all of them) and replace it.

7) An Afterward about the Resistor

When the microswitches all tested good I immediately began thinking about the Resistor. Someone coming along later might be inclined to try it first since it is slightly easier to get to. My research indicated that resistor failure manifests itself as inability to change fan SPEED rather than out right failure. If I had all one speed no matter which setting I would tend to think resistor. That wasn’t the case with me.

Also, finding one of these resistors looks like it will be very tough. There are many other resistors still available on the market for US cars. Some which look similar to the Jag one. I do not know the specifications of the Jag one, but if someone does then I think it possible to find a domestic substitute and somehow rig it to mount in place of the old Jag one. Happily not my issue.
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Old 04-24-2019, 09:54 PM
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WOW, Tony, this is Stellar!
I remember when SARC was working on his car, and when he dived into his Mode Control, but as mine works well enough for me, I paid little attention at the time.

You have done an Astonishing amount of work to get this far! I'm glad you found the problem was merely a loose nut.
How many Jaguar problems come down to just a loose nut?! (Sometimes the one on the steering wheel.)
Old 04-25-2019, 08:01 AM
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Tony - All that work documenting the intricacies of the system and the fix certainly needs to be congratulated more than just me clicking"Like". Thank you; great job!!
Old 04-25-2019, 08:17 AM
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Very methodical! The engineer in me is suitably impressed.
Old 04-25-2019, 09:06 AM
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Haven't had time to read and absorb the entire write-up....but my hat's off to you!

Old 04-29-2019, 08:19 AM
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This was very timely. I just renewed all A/C parts under the hood, flushed the system, evacuated the system, then recharged with 36ox of R134A. Took a while for the A6 compressor clutch to activate, but it did. Got it down to high 40 degrees at the center vent. But the compressor clutch never deactivated. I did a lot of work under the dash and kickplates this winter so I'm sure I knocked a wire off something to do with temperature sensing. Will now have to troubleshoot that. I have the Delanair mark II Service Manual, so I hope that's a help (in addition to the above well-written writeup). If anybody has any suggestions, I'm all ears.

Thanks, Bill
Old 05-04-2019, 08:00 AM
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Just came across this in my vast collection of files I have stored in my Jaguar folder. I'm sure you all probably have it anyway, but I thought if you don't it sure is helpful.

Attached Files
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XJ6-Delanaire-aircon-mk2.pdf (70.3 KB, 10 views)
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Old 05-04-2019, 10:13 AM
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