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My 2000 Pontiac Grand Am GT (6 cy/3.4L) has been having cooling issues. I started by replacing the thermostat and performing a flush & fill on the radiator. This didn't seem to resolve the issue -- the engine still overheats when at a stop.

This lead me to suspect the fans, so I tested the fans themselves (independent dual electric cooling fans). They do not start when the engine is warm... this is the problem.

I wired each fan directly to the battery, both function correctly. Next, I checked the relays, both tested operational. If I disconnect the temperature sensor, both fans come on immediately. So, I replaced the temperature sensor.

The problem persists; the engine will reach 200, the thermostat opens (determined by watching the temp. gauge), but the fans do not start. If I disconnect the (new!) temperature sensor, both fans start. I have read that turning the air conditioning on should also cause the fans to turn on, this does not happen (with the temp. sensor plugged in), not when then engine is cool or hot.

I am at my wits end as to what to check next. I am prepared instead to hardwire the fans to the ignition switch. Can anyone give me any ideas of any further diagnostic steps before I give up and take the "duct tape solution"? This is my wife's daily driver, so the pressure is on! Thanks in advance.

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Did you bleed the air out of the system when doing the coolant change? –  Mark Johnson Sep 30 '12 at 21:12
@MarkJohnson -- Thank you for pointing out this requirement. It was not the main problem, but armed with the knowledge that the bleeding is necessary saved me time later when I did the larger repair. –  Chris Jan 9 '13 at 12:52
Chris Ah, I see you did have a bad temp sensor. Confusing that it would feed a believable signal to the dash gauge yet be off enough to screw up fan activation and cause the engine to overheat. Sometimes that's hard to diagnose. I've driven multiple vehicles where the powertrain controller waits so long before turning on any fans that I find myself turning on the A/C to force it to engage a fan. –  Mark Johnson Jan 9 '13 at 19:49
Sorry, I'm a little unclear as to the answer on this one. Did your problem end up being a defective temp sensor? –  user3870 Oct 22 '13 at 3:42
@Brandon Yes, I believe so. I exchanged the temp sensor for a new one and installed it, and the cooling fans engaged. The overheating problem, however, was a larger issue -- we had coolant leaking into the cylinders (with little evidence other than constant need to refill) and I had to replace the lower intake manifold gasket. It is, apparently, common on this car. GM used a "plastic set" gasket, and the plastic warps then degrades over time. The replacement gasket is metal set. I did the work myself and I am a third-rate home tinkerer -- it wasn't hard with a repair manual. –  Chris Oct 22 '13 at 15:04

4 Answers 4

up vote 3 down vote accepted

Don't be discouraged by a new part not working. My family runs a shop, and we get defective parts all the time.

There is a table located here that shows diagnostic steps you can take. It's for a 1999 model, but 1999-2001 are virtually the same (I own a 2001 G/T model).

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Thanks, I'll check out that link. The thing about the part being defective... I don't think that's the problem. When I disconnect the temp. sensor (the new part), the dash temp. gauge drops to zero. When I plug it in, it springs back to the proper temp. That leads me to believe the part in question is functioning, and the trouble is elsewhere. I guess nothing's impossible, though! –  Chris Sep 30 '12 at 2:50
It could be working for reading the temperature and displaying it, but not working when it comes to turning the fans on. If that's the case, I would say it's probably something with the PCM. If you're lucky, you might can find a junked car with a used PCM that still works, rather than having to buy a new one. They can be pretty expensive. –  Sly Sep 30 '12 at 2:56
...or I'll run some wires from the ignition switch to the fan relays and call it a day! hahah –  Chris Sep 30 '12 at 2:57
That works, too. It wouldn't be the first or last time someone has done it. :P –  Sly Sep 30 '12 at 2:58
I'm a computer programmer, this car stuff is driven only by necessity/wifey. That link looks like a great resource, other than the computer scanner thing, I'll start at the top there. If the solution is on that chart, I'll be sure to accept this. Thanks again. –  Chris Sep 30 '12 at 2:59

Hook your temp sender up to a meter and drop it in some boiling water. This should tell you if it's working or not. Personally, I'd just disconnect it until you get the issue figured out -- running the fans continuously isn't going to hurt anything, it'll just be loud.

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your issue is with the temp switch not the temp sensor. the sensor will only operate the fans when it fails to avoid a possible overheating issue ,where as the switch is the fans main controller via ecm. the switch should be on the radiator itself or near the water pump.

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The problem might be with the body control module (BCM). The PCM sends the signal to the BCM, which turns the fans on. Or you can buy a separate temperature fan relay system with a temp probe and bypass the system. It will work as normal cooling when needed only.

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The BCM in most vehicles only controls things like the interior lights, radio, and the like. The PCM will switch the fans on as needed through a relay. This is how GM vehicles do it, anyway. –  Paulster2 yesterday

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