Series III Roadster Cooling Issues

Series III Cooling Issues resulting from installation of a new aluminum radiator and fans, plus new wiring harness. Doesn’t one of the fans run all of the time, and the other fan come on when otter switch opens? I’ve triple-check wiring schematic and have hooked everything up correctly, but can’t even get the fans to come on. Mechanic hot wired one fan to come on thru demister switch, but when engaged immediately blows the 50 amp fuse. Obviously there’s a dead short when the demister switch is thrown, what am I missing? Many thanks??? Thermostatic switch replaced when new harness installed.

Try using a fan which pulls less than 50 amps when it starts. With a new radiator, it is unlikely that you need new fans anyway. A 50amp fuse is not a fuse - it protects nothing.

I’d simply refit the old fans according to the s3 wiring diagram.

The s3 wiring diagram switches alternately the 12v feed and earth to the fan terminal which is not earthed when the fan is activated, so recheck your wiring as mixing up c2 and c1 could blow a fuse in the way you describe.

Whether the fans run all of the time or not is a function of whether you have aircon. They were originally wired in parallel. There is a wiring diagram on

kind regards

Ok, let’s try to clear this up:

  1. Both fans are either on or off. Unless someone has made some sort of wiring change, there will never be a time where one is on and not the other.

  2. The otter switch controls the fans according to coolant temperature when a/c is off.

  3. The a/c system will latch the fans on whenever it’s being used. It does this by engaging the secondary relay, which in turn grounds the winding on the primary relay.

  4. The otter switch engages the primary relay by grounding the winding. The primary relay completes the fan circuit by passing 12V+ to the fans.

  5. This sounds like a problem with the way the relays are wired (possibly C2 and C3 reversed). They’re under the radiator cowling and were undoubtedly messed with when the radiator was installed.

In addition to the above I’ll add…

One way to wire a manual switch is to provide a path to ground – if the mechanic mistakenly offered 12V to the wiring that something (say, the otter switch) was using that would indeed short when used.

Thank you Marek for your quick response. I don’t think the old fan(s) bracket fits on the new aluminum radiator so some custom work would have to be done to make this work. I also adapted the new wiring harness to fit the new fan connectors. The radiator and fans were produced by a frequent Jaguar supplier, I’m surprised the fans would pull so much power as to blow the 50 amp fuse. I thought the problem might be in the dash switch, or as you suggested, I have mixed up the relay wiring (I thought I was pretty anal about checking the C1 and C2 terminals). Car does have AC. Where is the wiring data on Thanks again for your great help, very much appreciated. Mark

Thanks Michael for your quick response. Okay, knowing both fans work together helps immensely. I was worried the car wouldn’t come up to full operating temp in cooler months (we’re in Kansas City) if they were on all the time. I must have reversed the C2 and C3 connections. Yes, they have been moved to driver side front bracket next to radiator when new radiator was installed. Shouldn’t both fans activate when cooling system hits a specific temperature when the AC is not on? Isn’t that the way the system is designed to work? I replaced the temperature sensor when I installed the new wiring harness, and installed new connectors to attach to the new fans. Surely those fans don’t pull over 50 amps!!! Thank you so much for your help, very much appreciated. Mark

Hi George,
Thank you for your reply. I have ordered a new switch just in case this 50 year old switch might be faulty. But I think the best solution is to correct this system to make it work the way it was designed, then have a safety system back-up thru the Demister switch. The mechanic also ran a wire to the fuel gauge, which might be the problem here.
Thanks again for your assistance, much appreciated. Mark

The fans are fed by an inline fuse that also feeds the horns of all things. That may explain the 50 amp rating.

What I would do is identify the black ground wire on the primary relay. Disconnect it for now, as it’s not necessary for proper operation. Then you can play with the c1 c2 c3 terminals without risking a short. You can reconnect the ground when you have the rest of the wiring figured out.

Wiring diagram here:

And to answer the question you may be already formulating: the way the primary relay is wired, the fan’s power leads are diverted to ground when they aren’t powered. Since these are permanent magnet motors, this causes a little back EMF which reduces their tendency to freewheel. A freewheeling fan reduces airflow. Whether this makes sense or not, disconnecting the ground on C3 doesn’t prevent the fans from operation.

Hi Michael,

Oh my gosh, this helps immensely. This wiring diagram is the best one I have seen, certainly better than the one which is printed in the Series III booklet available from the Jag Suppliers. Thank you. This is the first diagram which illustrates the two relays together which are indeed positioned on top of the radiator (and hence moved to the driver’s side position next to the power steering pump). I will remove those relays and dbl-check the placement of the wiring connections. I’m going to run down to FEDEX print shop first thing and have them enlarge this wiring harness print, it’s so helpful. I have three of these roadsters to restore, I’m going to need larger copy for my 73 year old eyes!!!

I never thought about the free-wheeling fans, but blocking airflow is certainly something to consider. I purchased the textbook from McPherson’s electrical course and have been working my way thru it to understand EMFs. Patience is king when working on electrical issues. Horns now working great, just running out of 50 amp fuses, as is MOSS Motors.

Thanks again for everything, all very much appreciated.




Here’s another option.
The diagram is full color - coded to the wiring harness
Two sizes available: 11x17 and 18x24
Comes laminated

Just a satisfied customer


Thank you Craig, I have already ordered two.


This evening I removed the 5 terminal relay and repositioned the five wires attached, replacing them as shown in the wiring diagram. The wiring on the relay was connected incorrectly. The biggest problem is that none of the terminals on that relay are marked as to "C1, C2, C3, W1, W2. Obviously C3 is the center terminal, and it is the black ground wire. But the placement of the other wires is guesswork. I totally removed the wire from the demister switch (connected by the mechanic), and bypassed the temperature sensor such that the fans would run constantly. When they did come on (once), they blue the 50 amp fuse. The fan wiring harness is new, with new connectors fitted to match those on the aftermarket fans. This has me confounded! Those aftermarket fans must be over 10 amps each. I hate to replace the 50 amp fuse with a larger fuse to fix the problem. Back at it tomorrow.
Thanks to everyone with suggestions.

Again, ditch the ground until you have this figured out. This is the pin layout for a typical Lucas switchover relay:


And under no circumstance should you uprate the fuse. If anything, downrate to 30 amps to protect your wiring.

Still thinking about this…what brand of fans have you installed? It’s possible that they’re just that power hungry. To make this a bit easier, unplug one of the fans. That will reduce the load by half and prevent the fuse from blowing.

And back to the relay…you can identify the W’s with a VOM. There should be about 70 ohms resistance across the two winding terminals.

My understanding is that how the ac system SHOULD BE operating the fans - in those cases where the ac system operates the radiator fans and not a dedicated condensor fan - is as follows:

  1. If AC compressor is on and the output pressure of the compressor exceeds a certain threshold level, the fans will be switched on and remain on as long as that level is exceeded;
  2. When the pressure drops below another threshold(lower than the first to have some hysteresis), the fans will be switched off.

This means the ac system could be operating fine but not require the fans to be on - as long as the compressor output side pressure remains below the threshold due to sufficient airflow through the condensor.

Anyway, that’s how my car is set up and works 100%.

You are describing the functioning of a Mercedes A/C system. On our ancient Jaguars, there is no pressure sensor. The fans are forced always on whenever the A/C is switched on.

OK, I take that to apply to the LHD factory installed versions.

Well, then my S3 has a “Mercedes” AC system :grinning: My car, a RHD, has an aftermarket system, which has a thing called a “trinary” switch. This item is what controls the fans/condenser fan and it also prevents the compressor clutch from being switched on in the event of AC gas escaping (pressure on the output side too low - its needs to be above a certain threshold for the AC clutch to engage).

A more modern control system. The trinary switch is for compressor control. There should be a separate high pressure switch for the fan. If the trinary switch is used for fan control, then the fans will be turned off when the system is overpressured.

I upgraded my radiator and fans to Ron Davis units. Fans are high-capacity and draw more current. I ran a parallel 30 amp fused circuit and added an additional relay to separate the load. Otter switch wires are in parallel to each relay.