Thank you for this reference. Both wiring diagrams I have are incorrect when referenced to this diagram. The previous diagram from xkedata.com illustrates C3 as the center middle terminal, but yours shows a lower left terminal for C3. I need to enlarge your drawing, laminate it, and place in my reference manual. Back to the shop to reconnect the relay properly, then pull the black ground wire, then disconnect one fan. I’ll check the brand while I’m in there. Not to worry, I would never increase the amperage of the inline fuse to solve the problem. Your help is very much appreciated, thank you.
Mark,
No guesswork required. A simple multimeter will allow you to deduce which terminal is which on the relay. With all the wires removed from the relay use a multimeter on the ohms scale to measure the resistance between pairs of terminals. You should find one pair that are a dead short to each other. They will be C2 and C3 (you won’t know which is which yet). Another pair will have a resistance of about 70 ohms between them. They are W1 and W2 (you won’t know which is which, but it doesn’t matter). The remaining terminal which is not connected to anything is C1. All that remains is to determine which of the pair C2 & C3 is which. To do so, you will need to connect 12v between the identified W1 and W2 terminals - connect one to ground and the other to a +12V source (the battery + or any Brown wire). The relay will energize and click. Withe the relay energized check continuity using your multimeter between C1 (identified in the first series of tests), and each of the identified C2 & C3 pair. The terminal found with zero resistance to C1 is C2, leaving the other one as C3. You can now connect your relay to the color coded wires identified in the circuit diagram in confidence that the relay is properly wired. If problems persist, they are elsewhere. Hope this helps.
Thanks Phillip. This Series III ever operates with the AC on without the cooling fans running. When this car is complete it’s going to live in St. George, Utah, where it reaches 115 degrees. No room for error here. Thanks.
Hi David,
This helps immensely, thank you. I’ve ordered a couple of extra relays from SNG Barratt, and I will mark them accordingly once I am back in the shop with the multimeter. I’ll post results ASAP when connected. Many thanks for taking the time to post these details.
Mark
Hi Michael,
I just received an order of new 0-728-20 Relays, and this evening I will install one once I check the resistance. Your diagram is included with the relays from SNG Barrat, and doesn’t match wiring schematics from xke.data. Thanks again for your assistance.
Mark
As I continue work through the cooling fan issue, and begin to replace other wiring harnesses on our Series III Roadster, what multimeter do members recommend for hobbyists like myself? I have basic electrical skills, but certainly need to become familiar with a multimeter, and comfortable reaching for a multimeter as opposed to always working with a continuity checker. The help I have received so far toward resolving the cooling issue has been invaluable, it’s time for the next step. Suggestions much appreciated. Thanks in advance.
A decent mid-price multimeter like a Milwauke 2217-20 or Klein MM600 would probably be a lifetime tool. Look for features you don’t have an immediate need for, because you will need them all eventually: Voltage, amperage resistance, capacitance, frequency, and temperature will give you most everything that you will ever need to measure. It’s probably going to fall short of your future needs for measuring amperage, so a separate amp clamp might be a good idea.
Amp Clamp is on the “to buy” list. Thanks.
Here I am almost two months later from my initial post on jag-Lovers concerning the cooling issues on a '73 Series III E-Type. I have progressed to installing a new wiring harness from MOSS Motors, and receiving assistance from two knowledgeable members from our local JCNA club to sort out the problem. Received great help from Jag-lovers as far as relay operations and locations.The issue is the non-functioning of the Spal fans (one amp meter reads 14 amps, other meter reads .5 amps), having been installed with a new aluminum radiator. The problem is that when both fans come on, the 50 amp in line fuse (above the battery) blows immediately. This occurs whether they are hot wired to come on when the ignition comes on, or they are allowed to start when the otter switch closes. We have by-passed the otter switch, and still when the ignition key is turned the fans come on and immediately shut down, with the 50amp fuse blown.
The ideas are as follows:
1.) Power side dead short when power is directed to fans
2.) Power surge to SPAL fans when they initially come on, blowing 50 amp fuse
3.) The new wiring harness is somehow defective - clean and reinstall old harness to check functioning. Didn’t work.
4.) New otter switch in place. What temperature closes this switch?
The plan now is to create separate circuits for each fan, one functioning when the ignition is turned on. The other fan to function from a power wire from behind dash fuse terminal, thru demister switch, to the other fan, operated manually. The ground side of the fan lead will be wired directly to the frame. We have checked every possible terminal everywhere from the stock system, and can’t locate the dead short. We must be missing something somewhere. We have isolated the problem this far. So frustrating. Other question is why the otter switch black/red trace wire in and ground wire out? Is this operating like a switch? Thanks.
When the coolant that the otter switch is immersed in reaches the defined temperature (sorry, don’t know offhand what that it), it closes and connects the black/red wire terminal to the black wire terminal (ground). It’s called an otter switch, because when the coolant gets otter, it switches… 
I wanted to reply quickly, I have solved one of the issues. I was looking at the wrong wiring diagram! I was accessing the non-air conditioned schematic instead of the air conditioned diagram. I rewired the relays, by-passed the otter switch, and both fans came on. The only problem is they still blow the fuse. When I disconnect one fan, system works perfectly, regardless of which fan I select. I will restore and replace original fans over new radiator, and it should work. (Otter switch is currently bypassed.)
Very good! Having solved the wiring issue, I need to start the car and check the operation of the otter, but it’s 25 degrees outside with a 25 mph wind. Maybe tomorrow. Thanks for your post.
The otter switch that came out of my car was rated for 88C - 94C.
I really don’t know if this is correct for the car but it sure is LUDICROUS!! This meant fans would only come on when coolant reaches 94C ON THE COOL SIDE OF THE RADIATOR!! BONKERS!!
Replaced with 68-72 and now AT LAST my car is running as to be expected with 74C thermostats. A needle width below N going up and touching N when she gets “hot”.
Thanks Philip. I’m going to have to convert from C to F. Great suggestion, one of our club members has a Series II. Thanks again for your response, very helpful. Mark
The saga continues on the Series III cooling fan issues. Because the aftermarket fans installed on the new aluminum radiator continue to blow the 50 amp fuse, the fans have been measured with an amp meter, and found to be pulling 14 amps each. Add the start-up surge when both fans kick in must bring the current over the Lucas 50 amp fuse capacity. I have now wired one fan using a 14 gauge wire directly to the “Demister” switch, through a fused link from the panel, to operate manually, System is working well, but it’s not the way Jaguar intended it to work. I am blasting the original fan shroud, painting, replacing rubber flaps, and rebuilding original fans to reinstall. The unit bolts right to the new radiator. OEM fans pull about 5 amps each.
We are continuing to the air conditioning system by converting from R12 to R134. I have 4 cans of Freon, but they aren’t making it anymore, and I’m afraid I’ll run out. Wiring is a brittle mess! I’ll update when original fans are installed, and otter switch working. Thanks for comments.
You are going to have to hook up an additional relay with a slight delay added to it so only one fan switches on at a time. Same for when they turn off again.
Alternatively, switch them on through a resistor, which then is switched out of the circuit. This is another way of limiting start up current.
kind regards
Marek
Hi Marek,
Thanks so much for your suggestion. I wish I was competent enough electrically to do what you are suggesting. That would be the ideal set-up. Does the resister limit the amount of current that flows thru the circuit? Is it an inline part? How does it delay the flow of current?
Many thanks,
Mark
A resistor wiould slow the fan, and I can’t imagine how you could easily switch it out of circuit. . You could add a delay relay for one of the fans, a bit of a mess wiring-wise. But I don’t think it would work, since you say that when you manually add the second fan, the fuse blows. I’m surprised the fuse is fast enough or the spike is high enough to do this. Could there be an additional load on that line? A 50A fuse is a curious spec for two 5A fans. Perhaps the A/C compressor is also on this line?
Going back to stock isn’t the end of the world.
Good morning Michael,
Thank you for your reply. The horn is also on this circuit, so I suppose if you ever honked the horn(s) while both fans were operating you wouldn’t overtax the circuit. To clarify when both Spal fans come on at exactly the same time the fuse blows within seconds. If they come on separately it takes a couple of seconds to take out the fuse. We have been all over this circuit looking for shorts, open wires, bad junctions or splices, and have found nothing. As earlier reported I wired the relays as your relay diagram suggested (absolutely correct) but I was looking at the schematic for non-AC cars. When I changed to the proper AC schematic, and changed everything, the system functioned just as it should. Only problem, those new fans and that 50 amp fuse.
I really have no issue with stock set-up, but those new fans are so powerful, i.e. moving lots of air! I’m now restoring the OEM shroud, fans, and replacing those rubber flaps. It bolts right in place when finished. I had to alter a couple of connectors to fit the new fans, so I ordered another harness ( they’re on sale right now!). I really appreciate your comments on the resistor and delay, I had no idea how to set up that system. I would like to know at what temperature the otter switch closes. Another contributor brought up the fact the radiator is different temperatures in different areas, and the otter switch is located in the housing in the lower radiator hose. He suggested using a Series II otter switch instead.
I’m most grateful for your comments, thank you for taking the time to respond.
If you have a 28 amp loan and you’re blowing a 50 amp fused we have a mystery that needs solving. I should have asked whether this is a British fuse (rated on spike loads) or an American fuse (rated on continuous loads)> If it’s the former, that’s the problem. A British 50A can only handle 25A continuous. You could evaluate the wiring and if there’s enough capacity, uprate to a bigger fuse. If the wiring can’t handle that, you would need to run a suitable wire.
If you still think the fuse is adequate, then you’ll need to figure out whether there is extra stuff being powered on that line. But maybe the most sensible option is to go back to stock. Sometimes a seemingly simple mod is like pulling on a loose thread…by the time you’re done, the whole suit is unraveled.