
09102014, 10:06 PM

#1

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radiator cooling fan resistor values?
Does anyone know the impedance and wattage of the fan resistor. There can only be two factors, so was looking to buy a pair of ceramic high wattage resistors to do the job and save me dollars on the expensive OE part.
Thanks





09122014, 02:11 PM

#2

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Quote:
Originally Posted by jimk04
Does anyone know the impedance and wattage of the fan resistor. There can only be two factors, so was looking to buy a pair of ceramic high wattage resistors to do the job and save me dollars on the expensive OE part.
Thanks

i have a brand new resistor in my garage, so i can measure the resistance for you.
not sure how can i measure wattage.



09122014, 04:43 PM

#3

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Quote:
Originally Posted by Meir
i have a brand new resistor in my garage, so i can measure the resistance for you.
not sure how can i measure wattage.

Measure the Resistance; then calculate wattage by;
Watts = (voltage*voltage)/Resistance where voltage is nominally 12 volts.
Watts = 144/Resistance
Reference: How To Calculate Resistor Wattage
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09122014, 05:57 PM

#4

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The physical size will also give you some idea of the resistor wattage
I believe Tom's calculation will give you the wattage draw based on the voltage.
You need to have a resistor that is rated higher than the actual draw or It will burn up
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Last edited by JayG; 09142014 at 06:37 AM.



09132014, 11:56 PM

#5

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That's great if you could measure it thanks.



09192014, 11:30 AM

#6

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just got back from a road trip.
will measure and post results this weekend.



09192014, 01:25 PM

#7

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from what I could find on the web, it is around .5 ohm and 100 Watt
It would be interesting to see if your measurements show that
BTW, the resistor value will determine how fast the fan runs
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09202014, 08:42 AM

#8

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Ok
3 wires coming out of the resistor.
Green/white. Goes from resistor to fan.
Green goes from resistor to connector.
White goes from resistor to connector.
I believe the green wire and the white wire are 12 volt input for the two fan speeds.
The green with the white stripe, is the output to the fan.
Another black wire ( ground) goes from connector to fan.
Measured tresistance between the green/ White, to the other Wires.
Between green/white and green 0.8 ohm.
Between green/white and white  0.2 ohm.



09212014, 08:32 AM

#9

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Be careful to "zero" your meter when dealing with such tiny ohm values. It is possible the measured values in the above post are in error by 0.2 ohms each, when the real values would be 0.0 ohms (i.e. a wire) and 0.6 ohms.
What YEAR is the OP's car? The resistor value appears to vary between model years.
The FSM for 2000+ cars shows only a single resistor per fan motor. Both are shown as 0.55 ohms on the schematic diagram.
To calculate the wattage you have to account for the resistance of both the resistor and the fan motor as they are operating in series to ground. Once you have the overall amperage, you can then backtrack and apply ohm's law to the resistor itself to determine its minimum wattage rating.
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Last edited by DennisAN; 09212014 at 08:43 AM.



09212014, 01:18 PM

#10

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Thanks for all the input.
It seems this isn't quite as simple as I thought. What with the twin values and 3 wires!
But this may be worth pursuing, as £65 GBP seems a lot for a resistor period.!
Maybe we can fathom it!



09212014, 06:06 PM

#11

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Quote:
Originally Posted by DennisAN
Be careful to "zero" your meter when dealing with such tiny ohm values. It is possible the measured values in the above post are in error by 0.2 ohms each, when the real values would be 0.0 ohms (i.e. a wire) and 0.6 ohms.
What YEAR is the OP's car? The resistor value appears to vary between model years.
The FSM for 2000+ cars shows only a single resistor per fan motor. Both are shown as 0.55 ohms on the schematic diagram.
To calculate the wattage you have to account for the resistance of both the resistor and the fan motor as they are operating in series to ground. Once you have the overall amperage, you can then backtrack and apply ohm's law to the resistor itself to determine its minimum wattage rating.

That makes sense, that the measured values are off by ~.2 ohm. I believe that on HIGH, the fan get the full 12v and on low it is lowered by the resistor in series
Now the fuse for each fan is 40 amp, if it was a full draw of 40 amps, it would be ~440 watts
12 volts through a .55 ohm resistor is ~260 watts or ~21 amps.
My guess is that the low speed resistor is ~ 300 watts rating if .55 ohm and ~250 watts if .8 ohm
What gets interesting is that the voltage drop on a .55 ohm resistor with a 12 v source is ~11.5 volts. That leaves ~.5 volt to the fan
If we go with .8 ohm as measured, then the current is 15 amps and the voltage drop would be 8.25 volts leaving 3.75 for the fan. This sounds more realistic than .5 volts to the fan
if the actual voltage is higher, say 13 volts than the voltage to the fan will increase slightly and the fan speed would be higher
Maybe a measurement of the voltage after the resistor would shed some light on this
Of course if my calculations are wrong, then fogedaboutit
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10152016, 09:24 AM

#12

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Quote:
Originally Posted by JayG
Now the fuse for each fan is 40 amp, if it was a full draw of 40 amps, it would be ~440 watts
12 volts through a .55 ohm resistor is ~260 watts or ~21 amps.
My guess is that the low speed resistor is ~ 300 watts rating if .55 ohm and ~250 watts if .8 ohm
What gets interesting is that the voltage drop on a .55 ohm resistor with a 12 v source is ~11.5 volts. That leaves ~.5 volt to the fan

Sorry to revive an old thread, but the above is fundamentally wrong as it neglects the fact, that the electric motor has an impedance as well.
If we assume that the motor draws full 40A at 12V, it's impedance would be 0.3 Ohm. But that would be unreasonable, so if the motor drew 20A, the impedance it would be 0.6 Ohm. I would 'guess' that the actual impedance is somewhere in the middle  in fact, I'd say it's equal to the resistor, i.e. 0.55 Ohm.
Since the resistor for low speed operation is connected in series with the motor, the total impedance is 1.1 Ohm, meaning that the amperage draw is 11A and the resulting voltage drop is 6V, leaving another 6V for the fan, cutting the fan speed in half.
The power dissipation on the resistor is 66W, so a 100W 0.55 Ohm resistor should be just fine.



10252016, 01:00 PM

#15

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Based on Danci's calculations, that seems a good choice of specifications. Best of all it avoids the Porsche Parts tax  so keep us informed of the success (or otherwise?) of your experiment.These resistors ae $1.50 on Fleabay but.......
Last edited by Gelbster; 10252016 at 01:03 PM.




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