Intermittant cooling fan issue

[player1up]

I know a radiator cooling fan should cycle, but mine has some odd behavior.

I replaced the coolant temp sensor in the thermostat housing a few weeks ago and for a few days the fan cycled as it should. Turned on at about halfway between 200 and the mark between 200 and 250 on the gauge. Guessing right around the 213° mark

After a couple of weeks it will not come on by itself BUT ( here's where the odd part starts ) will stay on if I turn the A/C on then back off after it's reached the temp it should normally turn on.

Symptom recap:
Rad fan turns on/off with A/C cycle ( along with condenser fan ) when not up to operating temp.
Rad fan will not turn on by temp alone
Rad fan will stay on if I cycle the A/C switch ( condenser fan turns off when A/C is turned off ) when engine is hot enough.
Rad fan will shutoff automatically when temp returns to normal.
Rad fan will turn on if I unplug the temp sensor.

I've tried searching for this on the main forum but I didn't see anything that matched my symptoms.
I'm hoping the computer isn't to blame...
I'll be checking this as well:
Coolant Temp Sensor Response Chart
110 degs = 4.20 volts
120 = 4.00
130 = 3.77
140 = 3.60
150 = 3.40
160 = 3.20
170 = 3.02
180 = 2.80
190 = 2.60
200 = 2.40
210 = 2.20
220 = 2.00
230 = 1.80
240 = 1.62
250 = 1.45
Temperature to Resistance
32 degs = 30 - 35 K ohms
77 degs = 9 - 11 K ohms
212 degs = 640 - 720 ohms

[168glhs1986]

I have 3 computers if you need one

[player1up]

Thanks! I'll let you know. These symptoms are weird for sure and if the temp sensor checks out I'm guessing its the computer, suppose that's the only other thing it could be.

[player1up]

I tested the coolant temp sensor and the gauge reads higher than the sensor voltage dictates. About 15-20° warmer on the gauge than what the voltage at the sensor says based on the above chart.

So, sensor voltage at 2.70 should be 185° but the gauge is roughly at the 200° mark, therefore the fans should come on when the gauge reads on the right side of the line before the 250° mark...now you can see why I was worried... and I never let the gauge get that hot before kicking the fans on with the A/C.

Unfortunately I didn't have enough time at lunch ( and is unusually cool today ) to check if the fans come on at the correct sensor voltage. I'll have to run it hard on the way home and stop periodically to check the voltage.

Fans should come on at 213° correct? and that would be about 2.16V give or take a thousandth.

Based on what I've observed in the past with fan cycle vs gauge reading, I'm wondering if the gauge is going... wouldn't be the first one to crap out... or maybe I'm senile

[player1up]

I seem to have spooked myself into thinking the fans weren't coming on when they were supposed to. I'm not used to a car that runs this close to the limit. I was able to check the voltage and the fans come on right around 2.15v but I had to let it idle for a loooong time before it was heat soaked enough.
I'll just have to accept that there's a slight discrepancy between what the gauge says and what the temp sensor says.
I just don't like the small temp differential for knowing when it's too hot. Oh well, guess that's the nature of the beast.

[168glhs1986]

Install a manual switch ?

[minigts]

What gauge are you using? Stock or aftermarket? The correct way to calibrate the gauge is to remove the needle, ground the sensor wire and then install the needle back on where the highest indicator is. I won't tell you it will be accurate, but that is the method to calibrate the gauge. Well that's for the oil pressure, but you could do a test with the temp senor and see if grounding it throws it to highest mark. I think you get the idea. Otherwise, go with an after market gauge and switch, as Mike mentioned.

[player1up]

I haven't installed a manual switch yet and not really wanting to add to the unmarked knobs on the dash either, I just use the A/C switch to start the fans.

The gauge is the stock consulier vdo gauge ( sealed unit so no calibration that I'm aware of ) . I'm sure it uses a different sensor than the computer uses and that could be the reason for the temp difference, but I swear that it's reading different than before.
I'll have to see if there's a way to test the sender.

[minigts]

I gotta start looking where these threads are posted. My info was for stock gauges, sorry. :p

[Johnny]

VDO are stock...in his car. :)