XK-140 - Dynator and Radtec fan switch wiring advice


First, my questions are very basic, but electric devices and I have a bad relationship caused by my ignorance and lack of confidence in what I might know. So please be patient.

I have a positive ground system on my XK-140 and just installed a positive ground type Dynator, in part due to adding an electric fan with a new aluminum Radtec radiator that has the thermal switch/boss in the bottom of the radiator. (And yes, all original parts are being carefully preserved.) The following are my questions.

  1. Radtec does not suggest any power connection location for the wire to the switch. Any suggestions that work well for you?

2.In connection with the installation of the internally regulated Dynator, they instruct me to rewire my three-post (B, F, & D) RB310 voltage regulator to remove the regulating function by connecting the B and D wires directly. They do not specify what, if anything, should be done with the F field-wire connection. Can you give some guidance on this?

  1. On the Dynator, is the case grounding adequate or do you suggest something else for grounding?

  2. Do you suggest any upgrade of the wiring between Dynator and battery or is the original wiring heavy enough?

Thanks for any help.


I got help on the Radtec wiring issue direct from Radtec and the grounding question, so all that remains is the question of how to rewire the RB310 voltage regulator box connection, specifically the F wire.

Power the fan from a relay. Use the thermal switch to trigger the relay. You can power the relay from the battery connection on the starter. This means that the fan will run any time the thermo switch is closed. Put a fuse, or better, a breaker between the battery and the solenoid.


Thank you for all the time and effort in this response… I really appreciate it. Radtec sells the thermal switch/boss as an integral part of the radiator. I did just receive a response from them, which was as follows. They don’t suggest use of a relay, but I see the merit in your approach and think I will follow it.

You will need to ground the blue wire from the fan as this the positive, take the black wire from the fan to the switch, the other side of the switch will be your power from the car, you can take this direct from the battery if you would like the fan to run on when the car is switched off (fan will switch of once the radiator has cooled) or you can take a feed from the ignition side of the fuse box, this will allow the fan to only operate once the ignition is on.

This is the thermal switch in the Radtec unit.

Hi there,

It’s peculiar that they use a power switch without relay.

I would think that the fan will draw like 15 Amps, and would not recommend you to wire to an existing fuse, but rather use a new fused connection directly to the battery post. Check the current of the fan, and add a healthy 30% for the fuse. Radtec will give you the specs.

The switch is also at the bottom? Would you know the switch on temperature? Must be pretty low, usually the switch is placed near the radiator coolant inlet.


The Radtec kit uses what seems to be a straight bladed thin line Spal. 14 to 16 Amps continuous draw. The switch supplied must be right for the power and simplify the installation. Do use a fused feed independent of existing ones: in case you use an existing one, the draw is significant.


Does preserving the functionality of the Ammeter matter to you? That is, do you want it to reflect the inflow/outflow of current to/from the battery - including when the radiator fan is running? If so, where you connect the Dynator into the system, and where the fan gets its power from matter. It is easy to overlook this (I did) when connecting an electric fan into the system with the result that the ammeter “lies” when the fan is running. I pondered how to connect a newly-purchased Dynator into my positive ground XK140 for some time, and ended up fixing the generator so I never resolved the issue to my satisfaction. Please let us know what you decided to do, and how it works out.


A bit of additional information. The RadTec switch is designated as 86-76, which I take to be the fan on-fan off temperatures. The switch is mounted right down at the bottom of the radiator close to the lower hose connection. I assume that is to make sure the switch is always in coolant even if the level dropped.

Also, surprisingly, they say the Spal fan only draws about 8 amps.

I will use a new fused or breakered lead. The Ammeter consideration is so far over my head a this point that I think I will just get it all working first.

I found some more regulator wiring instructions for a Dynalite, which I believe is functionally the same as the WOSP. They are:

  1. DIsconnect the earth (ground) from the control box (regulator)
  2. Remove Field wire from the F terminal of regulator and the warning light wire (small one) from D terminal of regulator and attach together
  3. Remove dynamo lead from D terminal of regulator and the battery lead from B terminal and attach together to create a direct battery feed (40 amp minimum)

Regarding the relay recommended in Mike’s post, I see they come in various shapes, sizes and amp ratings and that some are dirt cheap Chinesium. Any recommendations for what you use as relays here?



Rob in case this Spal is 8 Amps and the switch is rated for that current, the relay is unnecessary.

I commected (purposefully) mine after the battery, not to see the ammeter flicker and with a switch. Mine though is back up, so very rarely used.

I have a Radtec ali rad in my 140, which was supplied with a blanking screw for the thermo switch hole as well as the switch. I have chosen to use a Fosseway thermo switch (modern equivalent of an Otter switch) in the old thermocarb switch hole mounting on the inlet manifold to trigger my fan. The thermocarb is now switched to earth from the cockpit. If you wish to stick with the Radtec switch, these are pretty standard and a whole range of temperature values are available. I have always found it trickier to set the range I want by placing the switch in the bottom of the rad, so prefer the inlet manifold position.
I’ve never really understood why some wire a fan to a permanent (non-ignition) live. Once you switch off the engine, you are only really cooling the radiator rather than the engine as the water is no longer circulating (unless you have removed the thermostat and combine the electric fan with an electric water pump, also wired to permanent live via the fan thermoswitch). I have seen so many cars with flat batteries after sitting for a while, not charging but with the fan running…

Agreed to a large extent but not all: my fan is 14 amp, used only in the heaviest of conditions. I did not wire after an existing fuse (lights, horn, etc…) because it would overload. I did not wire to ignition (thick brown) because 14amps are quite a few (and more when start up of the fan), and was worried about the total load on the ign. Circuit.

This said the Radtech substitutes the original fan, and is 8 amps: I would probably switch as you propose Roger, but it’s not my case.

The otter elbow is ideally reachable and would be my choice for a support fan. However, for the permanent fan, in case your coolant gets low, the switch may not detect temperature and they you have no air cooling.

In my XJSC , the huge mechanical fan is gone permanently. I installed a modern PWC (which is as simple an install as a relay, but much more expensive): it gives a variable fan speed from 86 degrees to 95, where it goes max. It is simply great as the fans are 95% of time turning at low speed, barely audible.

That sounds like a very good idea. I too have no mechanical fan in the 140 - the Radtec installation is very nice with built-in fan mounts on the reverse of the radiator, so no unsightly electric fan through the grille, and ‘pullers’ are more effective than ‘pushers’.
I’ve tried to google PWC relay but don’t get anything - could you give a bit more info please, Lluis?

Yes, Roger: I even have bought 3, as in the to-do list is subjecting the XK and the Midget to the same treatment. I cannot go to the garage now, but before the evening I will take some pics.

That would be great. If you have a manufacturer name, or part number or something to help identify that would be very helpful. I don’t know what PWC stands for so can’t find anything about them! Are they a kind of relay, or a kind of thermo switch, or?

Still not in the garage but typing: Pulse Width Control or your can also try Pulse Width Modulation (PWM) fan controller.

I had a look at one or two sites for these but I think they may require a little more electronic knowledge than I have, unfortunately! I was rather hoping for something that I could simply replace the thermoswitch with… I think these are control boxes requiring a particular type of sender, perhaps?

Concerning the factory ammeter functionality with add-on accessories, something I have on the boat that would work here is a Bluetooth “Smart Shunt”. It gets inserted between the battery and the chassis, so it “sees” every load without regard to how or where that load is connected. You then use a smartphone phone app to get the voltage and amperage data. Just make sure you connect the Bluetooth processor so it has negative ground.

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Roger: my apologies for the late pictures it was late when i came home. This is the one that will go in the Midget or the XK. Its very very straightforward, the multiple wires are there to control up to two fans. You juat have ground, switch (variable resistance), fans positive and power (high and low current). Two more wires are there for a bypass (full power). You programme the ramp up with two buttons

I did not use the ac input nor the diagnostic led.

Best regards,