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Discussion Starter · #1 ·
hi everyone, im new here and also have my first ford, its a 2000 focus se wagon. I have noticed that my fans dont come on at idle, I did my search an learned its due to the fan resistor and the cable burning out most of the time. I checked under the hood and noticed the cable wasnt plugged in i tried to plug it back in but was not able to due to the resistor its self being loose it wouldnt stay on or going in enough for the clip to grab it correctly. Now i ordered a new one with cable connector included, i decided to check the one on it yesterday and there arent any burns on the cable connector and it looks good. So i took out the resistor by unclipping the fans and decided to connect it out of the fans and turn the car on and see if the fans would come on but didnt, will that work? Its really rusted, could it be just that? [?][dunno]
 

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They all look like that because they get hot. The little thingie that you can just see is the thermal limiter, you have to test to see if that part is good. Looking alone tells you nothing. The bigger coil of wire pretty much never goes bad.
 

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The resistor assembly consists of four parts...the resistor (the coil of wire), the thermal fuse (the smaller round item), the protective cage, and the connector.

To test the resistor, use a multimeter on the ohms setting and measure across the two terminals. The resistor is very low resistance and may be hard to read on cheap meters, but if you get a reading of only one or two ohms then it is good.

Testing for fan operation at idle can be a little misleading. Most think the fans should be running, but in reality these engines operate at high temperatures and the fans may not come on when you might think. If you turn your A/C on high, the fans should come on at high speed in just a couple of seconds. Let the A/C run a little bit and then turn it off. The fans should drop back to low speed and then turn off.

When the fans are running at low speed, there should be about 4.5 volts across the resistor assembly if you want to measure that. If you ever read full battery voltage across the resistor, then it is probably bad.
 

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Discussion Starter · #5 ·
The resistor assembly consists of four parts...the resistor (the coil of wire), the thermal fuse (the smaller round item), the protective cage, and the connector.

To test the resistor, use a multimeter on the ohms setting and measure across the two terminals. The resistor is very low resistance and may be hard to read on cheap meters, but if you get a reading of only one or two ohms then it is good.

Testing for fan operation at idle can be a little misleading. Most think the fans should be running, but in reality these engines operate at high temperatures and the fans may not come on when you might think. If you turn your A/C on high, the fans should come on at high speed in just a couple of seconds. Let the A/C run a little bit and then turn it off. The fans should drop back to low speed and then turn off.

When the fans are running at low speed, there should be about 4.5 volts across the resistor assembly if you want to measure that. If you ever read full battery voltage across the resistor, then it is probably bad.
ok i have a really good multimeter ill test and see, does it have to be while its plugged into the car and running?

i dont remember it turning on either with the ac, and if thats another case, what else should i be checking for?
 

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For measuring the resistance, the assembly should NOT be connected to anything (unplug the connector). For measuring the voltage drop across it, then it must be in the vehicle and the fans must be running. If your fans are not running on low speed, then it will not have a voltage drop across it and pointless to measure it.

If the fans don't come on with A/C on high blower (this assumes your A/C is functional), then check the fuses under the hood. There is a 50A fuse for high speed, and a 30A fuse for low speed. Then there are relays too. Start by looking at the fuses.
 

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If your fans are not running on low speed, then it will not have a voltage drop across it and pointless to measure it.
No voltage drop? I'd expect a full 14V voltage drop if the fan isn't turning. Or did you mean 0V on both sides (ie, no voltage going into the harness)?

Reid
 

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No voltage drop across the resistor if the fans are not running. This is measured from one side of the resistor to the other.

E=I*R .....so if there is no current, then there is no voltage drop.
 

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FYI, the fans do not necessarily run as soon as a/c turned on either. Thinking maybe on '00 but on my '02 they do not come on till later. Just last week noticed the fans never came on until like 15 solid minutes driving at 65 mph at night but a/c was on and working. Or, about the same as with a/c off. Car has done that since new. NEVER gets hot. They're playing around with software in there somewhere....on hot day at daytime fans will come on in maybe 2-3 minutes after a/c turned on. In either case fans cycle on/off rather than staying on all the time like older Fords did with a/c on.
 

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Edit: I actually looked at the circuit diagram, and you're right. I was thinking that the measurement was from the supply side to ground, but it's not.

My point about Ohm's law and open circuits is still correct in general, and my comment about about current flowing even if the fan isn't turning still applies.

No voltage drop across the resistor if the fans are not running. This is measured from one side of the resistor to the other.

E=I*R .....so if there is no current, then there is no voltage drop.
That application of Ohm's law only applies to circuits. If it's open (not a circuit), you'll have zero current and a 14V drop across the resistor. That's why when you pull a connector (such as for headlights) and measure the supply terminals, you read full voltage (E = 14V) even though no current is flowing (I = 0A). That's the nature of open circuits ("open circuit" is admittedly something of an oxymoron).

Another way to look at it is to consider this version of Ohm's law:

I = E/R

and remember that open circuit = infinite R, so I is zero even though E isn't.

Also, the fact that the fan isn't turning doesn't necessarily imply that no current is flowing. Rather than an open circuit, there could be a fault that is wasting the current as heat rather than doing useful work.

Reid
 

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My comment about measuring the voltage drop was based on the statement I made in post #4..."When the fans are running at low speed, there should be about 4.5 volts across the resistor assembly if you want to measure that. If you ever read full battery voltage across the resistor, then it is probably bad." ...Full battery voltage would indicate an open resistor.

amc..... the fans should come on with the blower on high speed. This is in the driveway and not going down the road. Most people aren't troubleshooting their fans while going down the road...kinda bad to open the hood at 50mph. I have a dual color (green & blue) LED on mine that indicates high and low speed, so I actually can troubleshoot mine while going down the road.
 

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Discussion Starter · #15 ·
thank you all for the help, the new resistor and cable arrive today but i work 1-9pm so i wont have time to do any testing or what not until tomorrow.
 

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'... the fans should come on with the blower on high speed.'

I will be weeding that out one way or the other, pretty sure I run around 90% of the time on high blower but have not proved out that exact connection there. Report coming shortly.

I use three LEDs, low and high fan and a/c clutch cycling as well.
 

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Discussion Starter · #17 ·
quick update:

I have replaced the resistor and pigtail wire but i have not seen the fans turn on yet even with ac on full blast.

Anything else i should be looking for?
 

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Make sure the relay has power to it and relay is working. Yank the plastic cover and push down on the contact plate with a finger while a/c is on. Fans should come on.
 

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You didn't say why you replaced your fan resistor assembly, but anyway you have a new one now.

Here is a simplified wiring diagram of the fan circuit that should help......
 

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