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Old 04-01-2010, 01:31 AM   #1
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Default Fans not coming on, car getting hot

I dug around for a bit and didn't find anything relating to this specifically, so I thought I'd give an ask. The car is a stock 1.6L; after about an hour drive (longest since I bought the car last week; yes, I checked that coolant level was proper when I got the car), I hit suburban congestion, sat at a traffic light for about 4 minutes, and noticed the temp. gauge, which seemed to never get past 3/8ths, climb ever-so-slowly to 5/6ths. I'm not sure what units the stock gauge is hashed out in, but generally, 6/6ths means steam, overheating, and possibly an oil/coolant smoothie from a blown HG.

Being the past proprietor of hordes of shitboxes with cooling problems, I remembered to make a mental note of if I heard the fans kick in while stopped at the stop light. I did not hear a fan. As soon as I started moving again, the temp dropped back down to normal after a minute or so.

After I arrived at my destination, I let the car idle for a few minutes, and sure enough, the temp started to rise again. This time, I turned on the A/C, which forced the fans on, and the temperature rectified itself.

So I'm new to these cars, but I'm guessing that either the coolant temp. sensor for the ECU is bad (if it isn't the same as the probe for the gauges, which does show heat), or there is a separate relay for the fan from the ECU (not A/C) that may be bad. Any advise on diagnosing this? Also, does anyone in Maryland know of a pick and pull that has Miatas? I went to two Crazy Rays over the last week, and not a single Miata. Thanks!
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Old 04-01-2010, 01:58 AM   #2
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Check the t-stat to see if it is oppening. It is the neck that comes out of the front of the engine. The sensor in that housing id the ones for the fans. The sensor for the guages is in the rear of the head.

Unplug the conector for the front temp sensor and ground it. It should kick the fans on.

I had the same thing happen to me when I swaped a used engine in to my old spec. started the car and let it get to temp and it started to over heat. I swaped out the sensor and it worked.

Have a great day,
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Old 04-01-2010, 10:20 AM   #3
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the thermoswitch on the front thermostat housing has likely failed, or the thermostat isn't opening.

two tests:
  1. unplug the thermoswitch and ground the contact, fan should kick on.
  2. open the diagnostics box (driver fender by shock mount) jump TFA and GND, fan should kick on.

report back.
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Old 04-02-2010, 12:11 AM   #4
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Thanks for the help guys! Yes, it ended up being the fan switch sensor; the fans came on when grounding out the sensor lead. The old one had seen better days; in fact, upon touching it, it literally fell apart:

I went to Pep Boys on my way home from work to get a replacement, and they gave me the wrong part; not wanting to drive 40 miles round while fighting daylight, I decided to go to Advance Auto which was closer by, and I installed it (along with new wiper blades) in the parking lot. Upon returning home, I let the car idle for a couple of minutes, I popped the hood, and low and behold the fans came on and off as they should!

As a bonus, the last jackass who was turning wrenches on this car left a wad of balled up paper towels under the master cylinder (no leaks), and in the spot where the cam that drives the windshield wipers is, I found a very nice magnetic pickup wand, and one of these: Astro Pneumatic AST6320 Rechargeable 60 LED Work Light: H . Luckily I had a 12V DC power supply laying around, so I was able to charge it up. It looks like a lightsaber:

That will definitely come in handy, and I was actually in the market for something similar.

Thanks again!
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Last edited by hotspoons; 04-02-2010 at 12:12 AM. Reason: Grammar is not for pussies
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