radiator fan switch

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That is exacty what the guy at the parts store sold me. I ran the engine today for 10 minutes and the rad got too hot to touch, (cap off) and the fan did not turn on, I pulled the wire off the sensor and grounded it to the block and it ran fine. Is it just not hot enough? What temp. do these switches typically trip at? Thanks

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You either have a bad switch or a bad ground! The switch should trip about 200 degrees F. If the fan ran when it was grounded, then run a lug from the base of the switch to a good ground, or right to the engine, or try another switch.

On way to test the switch is to fill a pan with just enough water to cover the sensor end, attach an ohmmeter to the leads, and boil the water. If the ohmmeter doesn't go to about 3-4 ohms or less when the water's boiling, you need another switch.

The ground must be good because the fan will run if I ground the wire to the outer body of the switch. I will try running with the cap on so the temp. will raise higher and I will also test the switch as you said. Would testing for continuity betweeen switch terminal and body at boiling temp. also tell me if it is working?

It's a single terminal switch?

You'll have to get it up to about 200 with a meter on it. It should short from the terminal to the body if that's the case, or it should short the terminals together if it has 2 terminals.

Bad link.

The switch would not close in boiling water so I went to return it, the guy said it was rated to close at 220 and to test it with a propane torch. I tried that and it works. So now I am wondering if that seems high? Doesn't this mean that all the water in the system will reach 220 before the fan will turn on even though the thermostat will open at 195?

There are different temp sensors. Not all are the same. It sounds like that one is meant for fan #2. That fan typically turns on around 224F.

No, it means (if the switch is at the thermostat housing) the water leaving the engine is at 220F. Hopefully, the water entering the waterpump is cooler and the water in the water jacket is at a temperature somewhere in between the inlet and outlet temps. Ideally most of the time there would be enough cooling that the fan would stay off and when it does kick on it would run for only a short while and then shut off. Using a switch at a lower temp isn't going to help much if the fan and radiator aren't able to keep up at this setting. Also make sure the switch and wiring are rated heavy enough to handle the maximum current for the fan.

-jim

It is in the thermostat housing, I was thinking the water entered at the thermostat, but if it leaves there everything makes sense. Thanks everyone.

Right. The engine is cold.... then it gets hotter, and hotter, and when it gets hot enough, the thermostat opens and the rate of flow increases through the radiator... if it continues to get hotter and hotter, THEN the fan comes on to cool it off.

--scott

In message , mark writes

Most modern cars have a pressurised system which allows the engine to run above normal boiling point, the higher the temperature the more efficient the engine.

I think you have a problem with an incomplete circuit. That is a single terminal switch. If you can ground the outer body of that switch and make it work, then you probably are not making a circuit through the thermostat body, engine, whatever, back to the battery.

Just because you have a thermostat body attached to an engine, it doesnt mean you have a good electrical connection back to the battery, necessarily.

A 220 degree switch is okay. That is roughly what your engine should run at.

220 is to hot for "an old 350".

If it were me, I'd run the engine closer to 180. Summitt or Jeggs or Gale Banks should carry the switch...

HTH, Ben

It is still not right. Today we ran the engine and it climbed to around 195 and stayed for a few minutes and then climbed to ~230 before the fan turned on. The fan runs until it is back down to ~200. If the engine sits there idling it will continually climb to 230, fan on ,cool to 195-200 and the fan shuts off and the climbs back to

230....Shouldn't the switch be rated at the same temp. as the the thermostat. It doesn't make sence to have the radiator water hotter than what the thermostat lets out.

Maybe it could be a little lower. It would be nice if it would come on at 220F and go off at 195F. The important point is you have proven the fan can cool it down fairly quickly. You didn't say whether the engine cooling system has any naturally moving air through it or if you are just test running it stationary.

The span of temperature and delay you are seeing in this switch is going to be still there with a colder switch. So if you get a switch that kicks on at say 210F instead of 230F then it will turn off that much lower also, which in your case would be something below 195F. that means the thermostat will start to close to try keep the temp from going below 195F and the fan will run much longer (or maybe not shut off at all) because it will be working against the thermostat. Ideally in operation you should have airflow through the radiator and the fan should only be needed when idling or moving very slow.

-jim

The engine (airboat) is stationary right now, in a garage. I just find it odd that the gauge is always cycling between 195 and 230, shouldn't it just get up to 195 and stay there, the same way my truck does? I tried running the Fan constantly and the gauge stays at 195.