Where does my fan sensor go?

Ok, I've got a 305 V8 that had a mechanical fan, but the PO wired an electric fan through a toggle switch in the cabin. I'm not keeping that. I'll forget to turn it on.

I bought a JEGs thermostatically controlled fan controller and I plan to install that. It uses a sensor that threads into a 3/8 port with a 3/4 adapter. The only ports on my motor are already in use for the temp gauge and heater hose.

Today I picked up a bizarre T-stat housing on CL that has a 3/4 port and the back of it. I also picked up an aluminum water pump from the same guy. when I got home I realized that the pump has two 3/4 ports on it as well. One is for the heater hose, the other is not in use.

Could I mount the coolant temp sensor for the fan and avoid the weird housing, or does the sensor need to be mounted up top?

I ran one in the unused pipe-threaded hole at the back of the passenger-side head.

Except the headers kept cooking it.

Under the thermostat housing, like you show, would be fine.

Above the thermostat (as per most housings in 70's-up Chevy's) would work too, since ~theoretically~ the thermostat would be opening before the fan turns on. Except if the thermostat doesn't open, and you'll get no fan.

Run the toggle switch in parallel, so you can over-ride the thermostatic switch if you needed to.

I'm thinking, if you threaded into the pump, you'd be getting "cold" coolant from the rad. Which I guess the fan would ensure the coolant coming out of the rad would be truly cold enough.

The most important thing is to try to capture the hottest temperature of the coolant, so that it kicks on before the engine overheats.

snailmont5oh wrote: The most important thing is to try to capture the hottest temperature of the coolant, so that it kicks on *before* the engine overheats.

Actually while that's the best way to install a temperature gauge sensor, for a fan switch, I think the best location is the radiator exit. The purpose of the fan is to boost airflow through the radiator if it's not reducing coolant temperature sufficiently. This location responds to the airflow through the radiator while the hottest point in the coolant system (usually right where the coolant exits the engine to go to the radiator) doesn't.

I put the fan switch sensor on my Samurai on the radiator exit, adjusted it so that it kicks in when the coolant temperature juuust edges above normal (when idling while parked), and it seems to work well.

In reply to GameboyRMH:

In terms of a well controlled system, I believe this is the best solution. Feedback before other noise is introduced.

My choices are thermostat housing or water pump, I don't have an acceptable port on the radiator itself. For packaging, i'd rather have it in the pump itself.

Water pump is a bad spot, IMO unless you can get it right at the return port from the rad. T-stat housing (or a tee in the upper rad hose) is better. A fitting in the lower rad hose would work too.

Get a radiator temp sensor sleeve. It's not actually called that. But it's what I use. Just use the top radiator hose. Put the sleeve near the thermostat. https://www.amazon.com/Aluminum-Temperature-Sensor-Adapter-Silver/dp/B00FY3A2PG

I would say the temp rating of the switch will dictate the best location. A low rated switch would make the fan always be on if located in coolant going to the rad, and wouldn't turn on until you're already overheated if it's a high temp rated switch on the rad outlet.

Thinking about it, the factory e-fan switches in my Jeep are an interesting study in this. It used 2 switches, both activate at 210*. One is in the upper rad hose and turns the fan on "low". The second is in the lower rad hose and turns the fan on "high". That usually equates to about 210 - 212 on the gauge when low kicks on and about 220 - 225 when high kicks on.

I have my fan sensor in a billet aluminum radiator hose connector with a fitting hole in the bottom radiator return hose. The sensor turns the fan on and off but I have also wired a a sensor overide so that I can turn the fan on and off manually. To make this work you need a ground wire so I bared the ground wire and inserted it between the billet aluminum connector and radiator hose. When hose clamped it is secure, tight and doesn't leak. Some radiators have a fitting hole next to the radiator drain which could also be used for the sensor. Don't know for sure but I think OEMs use this fitting hole for electric fans.

I have my water temperature gauge sensor (electrical gauges) in a fitting hole at the front of the intake manifold which is probably the area of most coolant heat. Some cylinder heads also have fitting holes for gauge temperature.

Well, here's where it wound up. in the new weird thermostat housing just before the thermostat itself.

The switch is a 180 degree and will be paired with a 180 t-stat. I'm waiting on some hoses and gaskets before I put it on the car, but it's ready to go.

180 degree switch is too cold for a 180 t-stat. Fan will kick on when it's warm and never shut off. You ideally want a switch that turns on around 195 - 200, off about 5 degrees lower. That'll kick the fan on when the t-stat reaches fully open and shut it off when things cool a bit (so the fan will cycle while sitting still and normally won't run on the highway).

It's actually 185 on, 170 off. Says it's designed for use with a 180 T-stat.

AClockworkGarage wrote: It's actually 185 on, 170 off. Says it's designed for use with a 180 T-stat.

Yeah, that's definitely too cold. It'll kick on once the car is warmed up and then just stay on. You'll never get down to the 170 shut off point. Don't forget, the t-stat starts to open at 180, it's not fully open until about 195.

My analog system used an Auto Meter temperature switch, which was actually for an oil temp light. It turned on at 210° and off at 185°. The switch was on the ground side of the relay, and I had a separate ground path through a manual switch so that I could force the fan on for high heat-soak situations (90° autocross days), but temp would turn it on if I forgot, or in normal situations.