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Two different things going on Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Sat, 15 Mar 2008 13:54:33
In Reply to: when AC is on, low speed cooling fan runs..., aaron, Sat, 15 Mar 2008 12:39:53 Members do not see ads below this line. - Help Keep This Site Online - Signup |
First, when the fan comes on doesn't have a lot to do with the temperature gauge. Huh? The temperature gauge is reading the coolant temperature right by the thermostat housing, on the engine block. The radiator fan switches on and off based on the temperature at the thermoswitch mounted on the bottom of the radiator. Those are different temperatures. If nothing else, the sensor driving the gauge is on the engine side of the thermostat, so that throws in a curve.
Depending on the year of the car, the fan may come on depending on the AC compressor engagement. And about the temp gauge jumping - it doesn't make sense that the temperature of the coolant at the sensor is changing in a split second, at the same time the fan kicks on. The fan pulls a lot of current. The temperature sensor is also electrical - it's nothing more than a resistor that changes value as it gets hotter. The gauge works by applying a current to this resistor, which flows into the engine block and chassis. The measured voltage on the sensor corresponds to the resistance, and therefore the temperature. It's not a large current for the sensor. But the fan is big. You probably have a less-than-perfect ground somewhere, and a small portion of that large fan current is making its way into the same ground path as the sensor. So you see a sudden change in the sensor value when the fan kicks on. Is it ideal? No. Is it a big deal? No.
How high the temp gauge goes depends on the thermoswitch on the radiator, the health of the radiator, AND the thermostat. So you can have a slow thermostat. And given that we've established that the fan operation is confusing the temperature gauge reading, it's even more interesting.
Frankly, it doesn't sound like you've got a big problem. I'm guessing this is a '94 or earlier car. Why? The temp gauge is all over the place, and the owner is worrying about it. What did they do in '95 and later? They changed the software in the EDU (which drives the temp gauge) so that the gauge sits at about 7:30 no matter what is going on in the engine, until the engine starts to overheat.
The engine temperature is going to move all over, depending on engine load. That's how fast you're going, and if the AC is on. It also depends on the flow (and temperature) of the air moving over the radiator. We spend a lot of time obsessing over it. Yes, the temperature is going to move up and down, mostly up when the car isn't moving much because there isn't much air over the radiator.
If you let the car idle, and the fan starts to cycle on and off once the car is warm, it's working.
posted by 75.58.11...
, Sat, 15 Mar 2008 13:54:33 <-- Viewing This Message, Sat, 15 Mar 2008 20:55:00
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