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Fan Always Running


Fotheringay-Phipps
08-06-2010, 08:12 AM
I have a 1999 Prizm whose ac and radiator fans turn on as soon as the key goes to "on", even in a cold engine with the ac off. Question is why this is so, and this has stumped two mechanics so far. I also have a virtually identical 2000 Prizm without this problem, so I swapped the three fan relays plus the engine main relay between the two cars, and nothing changed.

First mechanic said he was pretty confident it was the ac pressure switch. I put in a new switch and nothing changed. I brought it to another mechanic and they told me they poked around testing switches and the like for about 20 minutes or so and could find nothing. They suggested it was probably the computer. They didn't charge me anything though, so I wasn't able to press them to be more specific on exactly what they did or didn't test. It's possible that they looked a couple of simple things and then decided it wasn't going to be worth their while to track down the source of the problem. (The guy who owned the shop told me it was no big deal in any event. He said the main impact would be that the car would take longer to warm up in cold weather.)

Question is what are the possibilities. From looking around for such cases online I see people suggesting (in addition to the above) things like temperature sensors, but I would think that this would show up on the temperature gauge. But the gauge shows low temperature at the outset, same as any other gauge.

The first mechanic said his reason for thinking it was the pressure switch was that of the two fans, one is related to the ac and the other to both the engine and the ac. Since both have this issue, it's logical to assume that it's one problem related to the ac line, rather than two separate problems relating to both the ac and radiator lines. Both mechanics also suggested it's also possible that there's some sort of crossed wires somewhere along the line, but that it would be brutal to find it.

The Haynes manual suggests checking something called an EGR, but doesn't further specify what or where the EGR is, and I don't know if it's some obvious thing that these mechanics undoubtedly checked or something else worth looking into.

So what do people think? Is it the computer, or are there other possibilities? How do I track this down? I have the advantage of the other car, as above, so if there's something that's easy to remove and install I can switch things around.

And if it's the computer, which computer is indicated? The main engine ECU? (You can apparently get used ones for $35 or so - can you just buy one and plug it in, or is it a major hassle to replace?)

quick69gto
08-15-2010, 10:29 AM
Change the temp sensor despite what the gauge reads.
I've seen the gauge read correctly but the radiator fans stay on at all time.
It's a fail safe built into the car. If the sensor fails or the electrical connection to the sensor fails (i.e. pull the plug), the radiator fans will stay on permanently.
It's a fairly cheap replacement and I would bet it will fix the problem.
Good luck!

Fotheringay-Phipps
08-17-2010, 03:18 PM
Thank you very much.

What I was wondering about was the fact that in a properly functioning car, the fans are off when the engine is off, even if the engine is actually quite hot. This would seem to indicate that the computer is not ordinarily checking the temp sensor without the engine on. But in this car, the fans run even without the engine, which would suggest it's something else. Is this wrong?

Also, can you clarify that you mean the antifreeze temperature sensor? (From looking around I see there is also a air intake temp sensor, at least.)

kazmee
09-22-2010, 03:40 AM
Question: Why is the fan always running, is there any switch that can turn it off, I was reading this thread because I do experience the same situation and still have this problem up to now.

jdmccright
09-22-2010, 12:11 PM
There is a coolant temperature sensor for the gauges and a fan coolant temperature switch that controls the fan. This is located in the bottom tank of the radiator, usually near the center and has a two-conductor connector. When this fails, the fan will come on immediately with key on. You can test it with a resistance check...when the coolant is cold it should be open (infinite resistance), and near-zero resistance when hot. If the switch is working properly, then check the wiring to/from the switch for damage.

Hope this helps!

Freewaymaster
10-12-2016, 11:25 AM
I have to reply here. In regards to to the above thread for a 1999 toyota corolla. The engine temperature switch is not the main problem. The fan relays are. I know because i pulled AC fan no.2 in the small relay box next to the battery and one fan stopped. Then i pull opened the fuse box next to this and pull out "fan no.1" and THE OTHER FAN FINALLY STOPPED. So The problem IS NOW FOUND! Problem number two: is only the dealer has these and the fan no.1 is $66-76 and no.2 is about $47.

somick
10-13-2016, 09:28 AM
I have to reply here. In regards to to the above thread for a 1999 toyota corolla. The engine temperature switch is not the main problem. The fan relays are. I know because i pulled AC fan no.2 in the small relay box next to the battery and one fan stopped. Then i pull opened the fuse box next to this and pull out "fan no.1" and THE OTHER FAN FINALLY STOPPED. So The problem IS NOW FOUND! Problem number two: is only the dealer has these and the fan no.1 is $66-76 and no.2 is about $47.I have not seen the schematics for the circuit so it is hard to tell. But what you have done by pulling the relay is you just interrupted the circuit. The sure way to verify the bad relay is to replace it with the similar one. They suppose to have the same relays in the box. Try just to swap them. If the fan stops - your diagnosis is correct.

Good luck,
Sam

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