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Old 08-11-2019, 01:07 PM   #1
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Should Cooling Fan Come On When AC is On?

Hey All, Can someone answer for me: [2002 Focus SE wagon, 2.0 Zetec] I'm having an engine overheating issue, and cooling fans are not turning on (I think). My question is: When I turn on the AC, do the engine cooling fans turn on immediately, or is it still engine load/temperature dependent (meaning that cooling fans don't turn on immediately after you turn the AC on, but rather only turn on after the car warms up or the radiator/condensor heats up)?

[I've seen it mentioned that the engine cooling fans should come on when the AC is turned on, but it did not specify whether they should come on right away.]

[I've already tested the fan fuses and relays, and fan motor for operation - all are good and fan motor works with 12VDC applied directly. The cooling fan resistor measures .25 ohm, which is typical according to specs - but I would need to test it further before ruling it out. I have not tested the temperature sensor yet.]

Thanks much.



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Old 08-11-2019, 09:30 PM   #2
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Originally Posted by johnishere View Post
Hey All, Can someone answer for me: [2002 Focus SE wagon, VIN type 3] I'm having an engine overheating issue, and cooling fans are not turning on (I think). My question is: When I turn on the AC, do the engine cooling fans turn on immediately, or is it still engine load/temperature dependent (meaning that cooling fans don't turn on immediately after you turn the AC on, but rather only turn on after the car warms up or the radiator/condensor heats up)?

[I've seen it mentioned that the engine cooling fans should come on when the AC is turned on, but it did not specify whether they should come on right away.]

[I've already tested the fan fuses and relays, and fan motor for operation - all are good and fan motor works with 12VDC applied directly. The cooling fan resistor measures .25 ohm, which is typical according to specs - but I would need to test it further before ruling it out. I have not tested the temperature sensor yet.]

Thanks much.
On my 2002 SE wagon Zetec 5-speed the low speed fan runs as soon as I turn on the AC - even if the engine is cold. Interestingly if I turn off the AC and the engine is still cold the fans will continue to run for about 30 seconds before shutting off.

The fan relays control the fans and the PCM controls the relays. The PCM is signaled from the HVAC control unit in the dash.

Paul
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Old 08-11-2019, 10:09 PM   #3
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Originally Posted by Paul1958 View Post
On my 2002 SE wagon Zetec 5-speed the low speed fan runs as soon as I turn on the AC - even if the engine is cold. Interestingly if I turn off the AC and the engine is still cold the fans will continue to run for about 30 seconds before shutting off.

The fan relays control the fans and the PCM controls the relays. The PCM is signaled from the HVAC control unit in the dash.

Paul
That helps me out. Thanks much.
(The 30-second run-out time makes sense. Just because you turn off the AC, the AC condenser is still likely to be warm or hot.)
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Old 08-11-2019, 10:32 PM   #4
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Originally Posted by johnishere View Post
That helps me out. Thanks much.
(The 30-second run-out time makes sense. Just because you turn off the AC, the AC condenser is still likely to be warm or hot.)
I believe the low speed fan runs whenever the AC is on to maintain airflow across the condenser even when the vehicle is not moving.

If your issue is overheating and yoru fans are not coming on in Low of high speed I would suspect a failed cylinder head temperature sensor (which is what signals the PCM to activate the fans for cooling), although I have read where folks with similar issues as yours have replaced the sensor and it did not fix the issue.

Just out of curiosity how did you test the relays and the fans? Did you bypass tests of both relays?

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Last edited by Paul1958; 08-11-2019 at 10:51 PM. Reason: typos
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Old 08-11-2019, 11:50 PM   #5
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Originally Posted by Paul1958 View Post
I believe the low speed fan runs whenever the AC is on to maintain airflow across the condenser even when the vehicle is not moving.

If your issue is overheating and yoru fans are not coming on in Low of high speed I would suspect a failed cylinder head temperature sensor (which is what signals the PCM to activate the fans for cooling), although I have read where folks with similar issues as yours have replaced the sensor and it did not fix the issue.

Just out of curiosity how did you test the relays and the fans? Did you bypass tests of both relays?

Paul
Yeah, I've heard of horror stories trying to trace down an overheating issue.... Where folks have replaced thermostat, radiator fans, water pump, fan resistor, temp sensor AND ECU [PCM]..... and STILL overheating and/or no fans.

I tested the relays manually by pulling them out of circuit. For anyone not familiar: Apply 12vdc to the relay coil pins (+ and - to each of the 2 pins) to make sure it is pulling in (clicking) the switch contacts and making continuity across the switched contacts [test with multimeter]. Both relays check out. (I even tested the fan diode (D2) [test with multimeter] in the fuse block.)

I tested the fans manually also. Again, for anyone not familiar: Run 2 wires directly from the battery to each fan motor just to confirm that the fan motor itself works. [Unplug the connector at the fan and clip a (+) and a (-) to the spade pins of the fan motor connector.

I just haven't had time to dig deeper into the other possible causes, like the temp sensor. Also, I haven't gotten around to checking the circuit logic to determine whether the temp sensor has any effect on the fans not turning on when the AC is turned on. Logically, the temp sensor should not.... but ??? I believe that I've seen somewhere that a bad temp sensor would show up as an error code [DTC] on a OBD scan. So, I'll probably have one of the dudes at Autozone do a free code check to see if that 'might' narrow it down. Or I might use that as an excuse to just buy a scanner since I don't have one.

*NOTE: In an absolute pinch/emergency if all else fails, you can wire the fan directly from battery and put in a toggle switch and run the fans part time* manually ..... kinda like a Chitty Chitty Bang Bang or somethin.
* If you run the fans continuously, this puts a heavy strain on the alternator which could potentially burnout your alternator.

Last edited by johnishere; 08-15-2019 at 06:54 AM.
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Old 08-14-2019, 09:22 AM   #6
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Originally Posted by johnishere View Post
Yeah, I've heard of horror stories trying to trace down an overheating issue.... Where folks have replaced thermostat, radiator fans, water pump, fan resistor, temp sensor AND ECU [PCM]..... and STILL overheating and/or no fans.

I tested the relays manually by pulling them out of circuit. For anyone not familiar: Apply 12vdc to the relay coil pins (+ and - to each of the 2 pins) to make sure it is pulling in (clicking) the switch contacts and making continuity across the switched contacts [test with multimeter]. Both relays check out. (I even tested the fan diode (D2) [test with multimeter] in the fuse block.)

I tested the fans manually also. Again, for anyone not familiar: Run 2 wires directly from the battery to each fan motor just to confirm that the fan motor itself works. [Unplug the connector at the fan and clip a (+) and a (-) to the spade pins of the fan motor connector.

I just haven't had time to dig deeper into the other possible causes, like the temp sensor. Also, I haven't gotten around to checking the circuit logic to determine whether the temp sensor has any effect on the fans not turning on when the AC is turned on. Logically, the temp sensor should not.... but ??? I believe that I've seen somewhere that a bad temp sensor would show up as an error code [DTC] on a OBD scan. So, I'll probably have one of the dudes at Autozone do a free code check to see if that 'might' narrow it down. Or I might use that as an excuse to just buy a scanner since I don't have one.

*NOTE: In an absolute pinch/emergency if all else fails, you can wire the fan directly from battery and put in a toggle switch and run the fans manually..... kinda like a Chitty Chitty Bang Bang or somethin.
You have checked individual components but not checked the circuit as a whole. I would do relay bypass tests on each fan speed (using a jumper wire, etc) which checks the entire fan power circuit from the fuses to the fans. And I would also check that the control side of the low speed fan relay is getting a signal (ground) from the PCM when the AC is on (using a low amperage test light, etc). You could do the same on the high speed relay but you would have to know when the PCM is activating the high speed fans.

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Old 08-15-2019, 06:05 AM   #7
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Originally Posted by Paul1958 View Post
You have checked individual components but not checked the circuit as a whole. I would do relay bypass tests on each fan speed (using a jumper wire, etc) which checks the entire fan power circuit from the fuses to the fans. And I would also check that the control side of the low speed fan relay is getting a signal (ground) from the PCM when the AC is on (using a low amperage test light, etc). You could do the same on the high speed relay but you would have to know when the PCM is activating the high speed fans.

Paul
Yeah, to check the total circuit, I just unplugged the CHT sensor connector. Both fans came on. I haven't taken the time yet to locate and analyze the exact circuit operation characteristics (specs) for the CHT sensor to PCM - but, apparently a disconnected CHT sensor is the equivalent of a high enough resistance [infinite resistance in this scenario] to bring the PCM fan control to high. (I seem to recall that the CHT/PCM on the Ford uses some sort of split/dual resistor in a low-voltage range to accomplish reading 2 ranges, blah blah.)

Now, that does indicate that the circuit as a whole is functioning - however, I'm not sure as to whether the PCM "should" be signal-high (for the fans) with an open CHT loop (disconnected CHT sensor).

To complicate matters, I'm not 100% certain that the AC circuit is actually working - so the AC test to see if the cooling fan(s) turn on is questionable [the indicator light on one of the AC switches is not illuminating]. So, now I'll probably want to check that out also just to eliminate another variable.

My schedules are pretty extreme, but it looks like I'm gonna have to clear out a good portion of the day to fully dig into this thing once and for all.

Moral of the story: This is gonna be a PITA (pain in the ass).

EDIT: Yeah, I knew something was bugging my brain with the whole 'disconnect the CHT' thing. I just refreshed my memory on the circuit theory for a ECT/CHT sensor. As I thought I remembered, a ECT/CHT sensor operates (typically) by negative temperature coefficient resistance. So, on cold temp (engine cold) sensor has high resistance which leaves high voltage (5vdc) on the PCM input. On hot temp (engine hot) sensor has low resistance which leaves lower voltage on the PCM input. If this is correct, an unplugged ECT/CHT sensor would be high (infinite) resistance with near 5vdc on the PCM which means the fans should be 'off' (?). Well, obviously I'll have to spend some more time with..... as said before: PITA.

EDIT: Ok, it looks like it is the PCM which is outputting the 5vdc signal voltage... So, I guess with the CHT sensor disconnected there would be 0 voltage to the PCM input - and I have to assume that the PCM would turn on the fans in that event. So, nevermind... carry on.

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Old 08-15-2019, 03:18 PM   #8
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REPLACE the relay, you can have one that works and tests fine all day long and then when it gets just right it does not come on when needed. If you test 100 times OK and it does not come on at 102 then it is still bad. Relays are cheap and silly to melt down a car over one.

My '02 the fan comes on with a/c in summer but does NOT with a/c in winter, I've driven for as long as an hour before it comes on and no issues at all. That is with an indicator light wired into the power to fan. I have one on high and low as the cooling electrics are commonly what send the cars to the scrapyards.

FYI, PCM activates high fan around 230 F. or over 270 psi highside pressure with a/c on. Low fan on at 220 F. and a/c on; both numbers can vary by 5 degrees and even at each application.
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Old 08-15-2019, 05:48 PM   #9
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REPLACE the relay, you can have one that works and tests fine all day long and then when it gets just right it does not come on when needed. If you test 100 times OK and it does not come on at 102 then it is still bad. Relays are cheap and silly to melt down a car over one.

My '02 the fan comes on with a/c in summer but does NOT with a/c in winter, I've driven for as long as an hour before it comes on and no issues at all. That is with an indicator light wired into the power to fan. I have one on high and low as the cooling electrics are commonly what send the cars to the scrapyards.

FYI, PCM activates high fan around 230 F. or over 270 psi highside pressure with a/c on. Low fan on at 220 F. and a/c on; both numbers can vary by 5 degrees and even at each application.
I finally just now had the time to drive this thing around for awhile. And of course, nothing happening.... Meaning temp gauge right where it's supposed to be, although no fans either. As far as the relay is concerned, I've never been a fan of "diagnosis by maybeology": 'maybe' it's bad even though it tested ok out of circuit, or 'maybe not'. The relay is easy enough to bypass test (can even do it live while driving) to determine if it's the relay or not. Besides, if it was working 100 times - or even once in my case - before failing then I'd be ok to replace it on a guess that it might be intermittent. But in this case, the relay doesn't seem to be turning on ever - so before I replace the relay I'd like to know whether the relay circuit is getting a signal from the PCM.

For me to believe that the relay tests perfect every time I test it 'out' of circuit,
but then that it just happens to be failing at exactly the same time that I put it back 'in' circuit,
AND that it then just happens to start working again exactly when I disconnect the CHT sensor....
it would take a whoooole lot of vodka for me to believe in that much of coincidence.

I managed to locate some specs for the CHT sensor (resistance/voltage specs), so when I have some more time tomorrow I'll probably test it to confirm whether it's bad. The relays have proven to be working at least during testing. However, the CHT sensor has proven that when it's connected the fans don't work; when it's not connected the fans work. So, although that is not conclusive, it does confirm that the PCM, wiring, relay, fuses, fan resistor and fans are working (generally speaking) - so therefore between the relay and the CHT sensor, it would be more likely lean toward being a bad sensor in this case.

Yep, I also planned on likely putting in a couple of indicator lights for the fan operation (after I get things working) until I've regained some confidence in the car again. I do appreciate the input and the info.

Last edited by johnishere; 08-16-2019 at 12:01 PM.
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