View Full Version : Cooling fan help - switch and wiring (no build docs)
Matt P
08-04-2025, 10:48 PM
New owner of a roadster that needs some TLC. I do not have any of the build docs, but it was a complete kit purchased in 2014 and completed not too long after that.
The fan doesn't come on when engine is hot nor manually when using the switch (the one dangling in the picture for troubleshooting purposes).
I am unable to get power up to the connector by the fan, indicating some issue prior to that point.
I am curious if anyone can identify the wiring harness or help with a potential schematic.
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cv2065
08-04-2025, 11:37 PM
This is from 2017 but should help identify what is going where in the harness.
https://www.factoryfive.com/wp-content/uploads/2017/06/Chassis-Harness-rev-T.pdf
The fan power is configured in the Ron Francis harness and is triggered by completing the ground connection on the fan relay.
Is the fuse good?
Is the relay good? You can try swapping the fan and horn relay and testing with the horn button.
There are two Fan Thermo Control (green) wires in the harness. One near the top of the engine with the choke and some other wires, and the second near the fan power/ground wires. (blue/black). If the fan runs when powered with +12v on the blue wire, then it should run when either of the green Fan Thermo Control wire are grounded.
Does the car have EFI or a carb? If EFI, check to see if one of the Fan Thermo Control wires is connected to the EFI fan control. If using a carb, check to see if a temperature control switch is installed in the radiator and connected to a Fan Thermo Control wire. If the latter, try grounding that wire, bypassing the switch to test if it is the problem.
CraigS
08-05-2025, 07:07 AM
Alternatively to a fan temp sensor Papa mentioned in the radiator there might be one in the coolant passages in the intake manifold.
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In this pic either of the 'sensor' locations can be interchanged.
Matt P
08-05-2025, 04:05 PM
Thank you everyone for the help thus far, greatly appreciated!
It is carbureted and the sensor is in the engine. Relay and fuse are all good.
Based on the wiring diagram that was linked to above, I was able to decipher that I was missing a ground, but I am still a bit perplexed.
The orange wire goes to my manual switch on the dash.
The green wire (presumably) goes to my engine-mounted temp switch (I did not 100% verify this by tracing the wire, but based on the wiring diagram and what I could visually scan, that looks accurate).
I was able to get the fan to turn on by temporarily adding a jumper from the Green wire to a ground. When I flipped my manual switch (orange wire), and the Green wire was jumped to Ground, the fan kicked on.
Am I missing a second Green wire that needs to run to a permanently connected Ground?
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It sounds like the temperature switch isn't closing the ground. The orange wire is likely going to pin 86 (trigger) on the fan relay.
With a switch on the trigger pin, if that's what you actually have, and a temp switch, both are needed for the fan to work. The switch would need to be on, and the temperature switch would need enough coolant temperature to close ground for the fan to come on.
Matt P
08-05-2025, 04:28 PM
I would like the manual switch (orange wire) to be able to turn on the fan without the temperature switch being within its temperature range (so, the manual switch it a bit of a backup if temperature switch goes out, or a manual override if you will).
I need to study the wiring diagram and my plan a bit better, but looking for input on the proper wiring for such a setup.
All you need to do is connect the wires together that are currently tied to the toggle switch. Then run one of the Fan Theromo Control (green wires) to the switch with the feed connected to ground.
weendoggy
08-06-2025, 10:28 AM
What is controlling the toggle? Ignition or ground? That is what you have to find out. Is the Orange wire HOT with key on? Does it go to ground? Same goes for the Brown wire.
Your switch picture shows Brown and Orange, with Brown saying it's "switch feed", so you have to figure out if it's HOT when key-on or not. If not, find out if it goes to ground or is activated by ignition.
For simplicity, let's say the Brown wire is hot with key-ON. The Orange wire would then be hot when toggle flipped to ON, then hopefully go to a relay (86) to trigger it, provided terminal (85) goes to the thermo sensor (grounded). If all that's good, chances are the sensor is bad, and you can test that easily by just grounding the sensor lead by just connecting a wire from the terminal to ground. If the fan works, problem solved, thermo sensor is bad. The relay itself should have a 12v battery lead to (30) and the fan lead to (87). These can be reversed, but standard wiring is how I put it.
I only say “simplicity” due to not knowing how it’s wired and going by the wire name. You just have to figure out what’s going on. The example I gave is ignition activated.
Having an ignition activated relay is not my favorite thing and I do all mine ground activated. You can see how it’s done here, with redundancy.
https://www.weendoggy.com/wiring.htm#diagrams
Matt P
08-07-2025, 01:19 AM
Thank you both Papa and Weendoggy.
Weendoggy, thats exactly what I plan to do - make my manual switch a 'ground' switch rather than 'hot' and move it over to be on the same pin as the sensor, just like your diagram. I have seen another diagram that jumpers the constant power from 30 to 86 and plan to do that as well.
However, I have another newbie question about 'depinning' the terminals from the relay. I have watched numerous videos and tried every possible tool (actual depinning tools, screwdrivers, pics, bobby pins, etc), and I can't get it to release. I figured I would ask if there is anything unique about the ones used in these kits, see above picture for reference.
weendoggy
08-07-2025, 07:30 AM
Weendoggy, thats exactly what I plan to do - make my manual switch a 'ground' switch rather than 'hot' and move it over to be on the same pin as the sensor, just like your diagram. I have seen another diagram that jumpers the constant power from 30 to 86 and plan to do that as well.
However, I have another newbie question about 'depinning' the terminals from the relay. I have watched numerous videos and tried every possible tool (actual depinning tools, screwdrivers, pics, bobby pins, etc), and I can't get it to release. I figured I would ask if there is anything unique about the ones used in these kits, see above picture for reference.
Keep in mind that going 30 to 86 will always be live to the relay even with ignition OFF, unless you have your 30 power delivered by a master shut-off from the battery.
Depinning depends on what connector. Most of the standard relay block terminals are very easy. They have a "tab" attached to the spade, which you will depress and release while pulling on the wire/spade to remove. Some come out the front end so you have to know what you're dealing with. Same thing goes with the fuse panel. If you're talking about the relay itself, that won't happen, and not sure why you'd want a terminal off the relay to begin with. I'd need to see a good picture of what you're attempting. Metri-Pack uses a special tool, but don't think you have them.
Matt P
08-07-2025, 12:31 PM
Just looking to remove the wire currently on 86. From watching videos I assumed it was the tab kind like you said, but I’ve tried so many ways to feel for and depress any sort of tab and I can’t get it to budge. Do these pics help?217338217339
weendoggy
08-07-2025, 02:16 PM
I believe you'll need to remove the plastic lock tab (white) as shown. You may have to pry up a bit as shown by arrows. Then you'll see how the terminal comes out. It should remove from the back side and will have a small tab on the narrow part of the connector that should be pressed to release. Once you do one, it's easy.
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Matt P
08-07-2025, 08:45 PM
Always so obvious once you do it! I can't believe I didn't come across that setup at all when researching relays and specifically wiring them. I had to push down on the little tab right between where you have your arrows, then simultaneously pull it out.
Now I just need to figure out where to get those terminal/connectors, or, a decent way to T into the existing wires, given its a very tight area to work in.
weendoggy
08-08-2025, 10:37 AM
If you plan on "T" to the wires, DO NOT use those stinkin' connectors. Best way is to strip the wire on the one you want to "T" into (think of cutting out a section of insulation), then spread the wires and insert the wire you want to connect, wrap and solder, then tape the connection. Another way is to cut the wire, install a non-insulated connector on one end (cut a long piece of shrink tube to cover the section) and put the other end along with the new additional wire, crimp and solder. Don't forget the shrink tube before crimping/solder!
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BornWestUSA
08-08-2025, 03:45 PM
This is how mine is wired (schematic by original kit owner) either the temp sensor on the manifold grounds the green wire at the set temperature internally or the dash switch grounds it manually.
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Nigel Allen
08-08-2025, 05:47 PM
This is how mine is wired (schematic by original kit owner) either the temp sensor on the manifold grounds the green wire at the set temperature internally or the dash switch grounds it manually.
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To the tune of Dire Straits Money for Nothin'
'That's the way you do it'.
Matt P
08-10-2025, 05:50 PM
Well, thought I got it, but now it looks like the green wire going to the thermo switch is getting back fed with 12v. I’ve seen others wire it this way so I am perplexed on why it is happening. Here is an example of how mine is wired (although I do not have the light wired in like this diagram). the 30 and 86 are jumped, and the orange manual ground switch is soldered to the green (should be ground but is 12v) thermo switch. 217466217467
Did you verify that orange wire wasn't hot?
Matt P
08-10-2025, 06:20 PM
Yes, sorry for not including that. Orange wire is switched ground and confirmed it is ground. Also confirmed that the green wire is not being backfed until the jumper and relay are connected.
I'm confused by the use of pin 87a for the dash indicator light. It looks like the light will get power when the fan is off. Could that be the source of the back feeding?
Matt P
08-10-2025, 06:57 PM
Sorry I mislead you a bit, that is someone else’s diagram and I do NOT have the light, the rest is wired the same way
Well, I'm not sure what to recommend other than going back to the original configuration on the relay.
Nigel Allen
08-11-2025, 02:43 AM
Well, thought I got it, but now it looks like the green wire going to the thermo switch is getting back fed with 12v. I’ve seen others wire it this way so I am perplexed on why it is happening. Here is an example of how mine is wired (although I do not have the light wired in like this diagram). the 30 and 86 are jumped, and the orange manual ground switch is soldered to the green (should be ground but is 12v) thermo switch. 217466217467
NOTE: All voltage measurements with the ignition on.
With multi-meter black lead grounded, and red lead connected to thermo-switch terminal:
When the thermo-switch is open (fan not required) there will be +12volts at the terminal on the thermo-switch. This +12volts is coming through the coil from the ignition+. A multi-meter does not put enough load on the circuit to pull the voltage down. This is a perfectly normal and in your case, good situation. When the switch is closed (fan required) there should be zero volts at the thermo-switch terminal.
Next step:
Have you measured for volts at the fan connector?
1. Ignition on. (This will apply +12volts to one side of the relay coil)
2. Ground the green wire from the thermo-switch (This will apply ground to the other side of the relay coil. The fan relay will now be energised)
3. Unplug the cooling fan and measure the 2 terminals in the socket with the multimeter. 12 volts should be measured across the 2 terminals.
No volts at the socket:
1. measure between chassis ground and the positive terminal in the socket. This will check for a bad ground terminal in the socket. If you get volts, then ground terminal is not sufficiently grounded.
2. measure between battery positive and the negative terminal in the socket. This will check for a bad positive terminal in the socket. If you get volts, then positive terminal is not getting +12volts from the relay. Replace relay and or fuse.
Voltage at the socket:
1. re-connect fan and see if voltage is maintained. If not, then you have a high resistance connection somewhere. (if this is the case let me know and I will 2 finger type a fault find procedure)
2. If volts maintained then fan is most likely stuffed. Unplug fan and set meter to ohms. measure the fan plug, you should have a low reading. If open circuit, fan is dead.
Probably worth tracing out the ground connection for the fan, it is probably nearby to the fan socket. Could be poor. Fan draws plenty of current, so connections need to be sweet.
Hope this makes sense. Don't hesitate to reach out if it doesn't.
Best of luck. You will get this.
Nige
Nigel is the electrical guru!
Nigel Allen
08-12-2025, 07:47 AM
Thanks Dave!
Cheers,
Nige
Matt P
08-12-2025, 07:52 PM
Nige, can’t thank you enough for the thorough details! Right when I saw 12v I stopped further investigation and thought that can’t be right. So glad you pointed out it’s normal.
I was able to shortcut right to the problem. I grounded the wire at the sensor and the fan kicked right on. I took the switch out and did some tests and could not get continuity out of it after heating it up multiple ways, must just be the switch.
If anyone has a good replacement one can find at a local auto store, let me know, otherwise I’ll order from factory five parts directly and wait on the shipping.
BornWestUSA
08-12-2025, 08:51 PM
Nice, can’t thank you enough for the thorough details! Right when I saw 12v I stopped further investigation and thought that can’t be right. So glad you pointed out it’s normal.
I was able to shortcut right to the problem. I grounded the wire at the sensor and the fan kicked right on. I took the switch out and did some tests and could not get continuity out of it after heating it up multiple ways, must just be the switch.
If anyone has a good replacement one can find at a local auto store, let me know, otherwise I’ll order from factory five parts directly and wait on the shipping.
Amazon has a few, I'm not sure what size or temperature kit came with, this should work, measure yours to be sure:
https://www.amazon.com/Thermostat-Temperature-Switch-Electric-Cooling/dp/B08B8QCT6K/135-4496816-5637506?pd_rd_w=a7MBB&content-id=amzn1.sym.2cd14f8d-eb5c-4042-b934-4a05eafd2874&pf_rd_p=2cd14f8d-eb5c-4042-b934-4a05eafd2874&pf_rd_r=G9ZWQ7CJHWQQE5XAP5FP&pd_rd_wg=BbJIC&pd_rd_r=54247307-2275-4dfd-995c-2b314e83e41a&pd_rd_i=B08B8QCT6K&psc=1
Nigel Allen
08-12-2025, 08:55 PM
I think these thermo-switches come with the kit. So there are probably some available from forum members. I suggest you reach out.
Cheers,
Nige