I wired my dual fans in my 1980 cj exactly like the diagram below. But the fans do not come on.

I have confirmed that the fans work.

I have 12 v coming to the battery terminal, 12 v at the switched terminal, and nothing at the fans.

Any idea why the fans won't come on? Bad temp sensor?


***edit to add: i do not have an override switch, not any other ground except the fan grounds. I checked those grounds and they are good.

I have had a bad sender before. One cause the fans to not work, the other failed closed and fans were on all the time. Wire in a switch/jumper to confirm your harness is correctly wired.

Ok I can try that. Just wire in a regular on/off switch on the temp sensor wire?

One other thing I'm uncertain of is which terminals on the relay i need to plug each wire to. Online diagrams have quite a bit of variance. Also, some show a ground at the relay. I don't have that. Do i need one?

Just wire it for full on 100% of the time?

Yes you need a ground for the relay. It's for the coil inside the relay, not for the fans.

Unless the temp switch is switching the ground for the relay.

Is the temp switch in the positive wire or the negative?




In that diagram above the temp sender is switching the ground on or off. What is your temp sender screwed into? Does THAT have a good ground?

No ground on the temp sensor, it's screwed into a metal housing that is only touching the radiator hoses. So do i need a ground somewhere? How would i do that? Temp sensor is single blade.

The realys are wired as follows:

85- temp sensor
86- 12v switched
87-fan
30- battery

Yeah. Pin 85 is where it wants ground.


Essentially you have to provide +12v and ground to the relay coil to make it click. The +12v is wired to the battery, hopefully through a fuse, then the ground has the "switch" (temp sender) that triggers the coil.

That temp sender should be screwed into something that is grounded. Is there somewhere on this metal piece where you could run a ground wire? Or if you can get a ring terminal big enough for the temo sender itself, you could use that. Just run a ground wire to the chassis with a big ring terminal on it, then screw the temp sender in through that ring terminal.

Just for fun you could take a wire and touch pin 85 on one end then touch the other end to any metal in the car and it should turn the fans on.

Ok awesome. Thanks for the help. I'll report back.

Ok,
I clipped a ground to pin 85 and the fans came on, same when i clip a ground directly to the sensor. However, the fans run as soon as the jeep starts instead of waiting for it to get to temp.

I tried just touching the metal (aluminum) piece that the sensor is screwed into to a ground and the fans do not work. Same thing if i touch the metal part of the sensor (part where you put the wrench to thread it in).

Do I need to get a two pronged temp sensor so i can run one to a ground?

Gounding the threaded part or the part that the sensor is screwed in to is correct. Are you sure the temp is getting hot enough to turn the fans on?

The part that the sensor is screwed in to needs to be permanently grounded, and then use an infrared temp gun pointed at the sensor base to see if the temp is getting hot enough to turn on the fans.

Also, do you have the sensor mounted in the upper or lower radiator hose?

It's in the upper hose, which is what the p.o. Had. I've had it for 6 yrs and it hd always worked. I did have a painless dual fan controller that crapped the bed and they won't replace it, so that's why I'm in this boat.

I am certain the temp was high enough for the fans. It's a 180 degree sensor and the engine got all the way to temp. I could feel the hoses get hot when the t stat opened.

Not knowing anything about your setup, I'd say the housing may have a coating on it that prevents continuity like anodizing if its aluminum. The sensor may just be bad.

A single pole single throw (SPST) sensor would work great, which should have 2 contacts on it instead of one.

Yes, essentially any sensor with 2 wires/contacts would work fine. Run one side to ground and the other side to the relay. If you want a manual fan switch you would run it in parrallel with the temp sender. All you have to do is provide a ground to pin 85 and the fans come on.

I agree with Tim.

However, you really do need to know how hot the water is getting to know for sure if that sender is working correctly. If someone put a 160 degree thermostat in the engine, then that sensor might never be seeing 180 degree water.

that's a good point. I put the thermostat in, but I'm not certain what temp it is.

If the water never got over 180 How would the engine ever get to temp? It gets to 210 and then the therm kicks in and it stays around 200-210.

Any ideas how to check the water temp?

Use an infrared thermometer.

Or take the themostat out and drop it in a pan of water and see what temp it opens. Use a meat thermometer to check.


Nowadays you'd be prudent to do this to all thermostats when you install them. There has been a run of very poor quality thermostats on the market. Some open early, some don't open at all. It's worth checking it out.


Also, make sure there's not an air bubble in your temp sensor housing thingy. You may need to burp that fitting if it is located at a high spot.

I've got an IR thermometer i can use.

The adaptor that goes into the radiator hose looks similar to this, but this isn't the exact one:



As far as an air bubble in the coolant, i think I'm good. I burped The system when I put the coolant in and It has never overheated. Until recently, the fans ran just fine as well.