Fan Always Running
Fotheringay-Phipps
08-06-2010, 08:16 AM
[Note: I've crossposted this to the Corolla forum. I hope this is not a violation of forum rules.]
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?)
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?)
n1ckml007
08-20-2010, 01:20 PM
I would check the grounding, I had a very strange issue with the DRL module for the headlights and it turned out to be a bad ground.
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