Fld120 fan not kicking on automatically

I’m not 100% on your year/model. But other FLD’s of that era usually had a high pressure switch in your A/C system that activated the fan and a switch in the thermostate housing. It sounds like your A/C switch is working. The coolant temp switch on the thermostat housing is probably your issue. There should be another sensor right beside it that operates your gauge in the dash.
The switch you mentioned in your surge tank is for coolant level.

So here’s where I get unsure. I don’t know at what temp your switch should activate. It could be 200 up to 230 deg. And I’m not sure if puts power to the fan control solenoid or if it takes it away to activate the clutch. It was right around that time that they started changing those things around.

Hope this helps, I’m sure others here will fill in the blanks soon, but this might get you started.

 

You mentioned a/c switch was working the fan. I’m guessing you mean it’s no longer working the fan, as the fans not working. ?? What Im getting at is what about the selinoid itself! What about over ride switch? Does fan still work at all?If not it can be locked up meanwhile by nstalling a bolt or by removing air line and crimping it off. Depending on which style you have. I had a92 WS w/ 425c that had a bulliton on it that required moving the selinoid closer ( or maybe further away can’t remember) to the engine for it to work properly

 

Working2party is right first thing to check. The temp sensor on thermostat housing. Not that it matters but is it a peec or mechanical? Peec has other non related sensors in fuel pump ecm that cause other ( non related) problems.Hopefully its a manual. I had a cmodel 425 w/ peec.Unique problems. Either way great engines

 

Not that I’m aware of, I’m not 100% on cats, but I assumed all FLD’s of those years would require the same switches. I was adapting from and 89 w/ Cummins. If the sensors/switch that operated the buzzer has more than 2 terminals, it could be operating your fan also. If that’s the case it could be a broken wire between there and your fan solenoid. You would have to run around it with a test light to figure it out, but I’m familiar enough to tell your right we’re to test.

 

I have a different truck and engine so this might not apply

My engine has three temp sensors. One for the temp light/buzzer, one for the temp gauge, and another for the fan. The fan temp switch has two wires going to it, the other two just have one wire.

If the AC was kicking the fan on and you can also turn it on manually I would suspect the fan temp switch or the wires going to it