The Question and Answer Board

[Bob Zumbrunnen]

I assume you're talking Celsius here? That temp would be far too low for Fahrenheit.

Electric fans in cars are triggered two ways:

1. Temperature sensor (usually in the block or intake manifold) detects a high enough temperature.

2. A/C is turned on. Keep in mind that using Defrost turns on the A/C since it dries the air.

In my experience, nearly any time the cooling fan(s) quit working in a car have been because the temperature sensor has failed. Oh, and it's not necessarily the same sensor that sends the signal to a temp indicator in the dashboard.

It's possible your temperature sensor is slowly failing or has become so covered in deposits (never use tap or hose water to top off a cooling system -- too many minerals) that it's slow to warm up to coolant temperature.

But 90 Celsius doesn't seem too high a temperature for cooling fans to kick in. Engines like to be hot. The racing phrase is "Hot oil, cold air". That's what makes power, and engines make the most power when they're burning fuel the most efficiently, generally speaking.

Now if you're referring to ambient (outside) temperature, ignore all of the above. Even the hottest of days is very cold by engine standards and ambient temperature isn't a factor when an engine system decides whether or not to turn on the radiator cooling fan.

But it looks like you're referring specifically to engine temperature. I assume you changed your thermostat (you said "thermometer), which is always a good first part to replace when trouble-shooting cooling systems. Personally, I test them, which can be done with a pan of water on the stove and a meat thermometer. If it opens when the water hits the marked temperature, it's a good thermostat.

I've never seen a radiator-mounted temperature sensor for cooling fans (but I haven't been a professional mechanic for years), although it sure seems intuitive that that's where it should be.

If you can fairly easily get at it (I tried to help a neighbor with their fans not coming on in their Ford Windstar, and never could even FIND the sending unit) the temperature sensor is a relatively inexpensive replacement and that's where I'd go next if you feel the radiator's getting too hot. Though it sounds like the fan's still coming on at a decent-enough temperature, it could be signaling the sending unit's eventual demise, which can be a real bummer, especially if you happen to be going up a steep hill when it happens.

On the same subject, if a car starts overheating, it's generally a good idea to keep it rolling at a decent clip if you can do so without using much throttle. Get air flowing through the radiator. Once you come to a stop, the water just starts getting hotter, even if you turn the engine off because it takes forever for the block to cool down. If it starts overheating while going up a hill, then the lesser of two evils is to pull over.

For more/better car advice, add this board to your Favorites: http://www.investorshub.com/boards/board.asp?board_id=1014

Edit: Just noticed you said it *used* to come on at 90 but is now higher than that. If it's much higher, it could be getting dangerously close to boilover, so you probably are right to be concerned. A cooling system is pressurized so the boiling point is actually a bit over 100C.

If you can find the sending unit, and you've got a meat thermometer and a volt-ohmmeter and access to the specs, I'd remove it, test it, clean it thoroughly, retest it, and if it gets within specs after cleaning, put it back in and consider it fixed. If it doesn't get within specs, replace it.

Typically what you're looking for is continuity between the sensor wire or where the wire connects, and the body of the unit (ground) when the unit reaches a desired temperator, and no continuity below that temperator, or if the sensor also sends signal to the dash, a change in resistance as the temperature climbs, within the specs called for in the manual or found online.