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Cooling fans

gorman

Hard-core CEG'er
Joined
Jun 29, 2008
Messages
1,919
Hey guys,

just replaced one of my radiator fans and thought i would check the resistor next to it on the plastic cowling.

I'm getting two different readings from the two resistors i have (from different fan assemblies).

Anyone know what i should be reading ? thanks ....G.
 
ok fan assembly back in......

does anyone know which fan cuts in first and is there a way to test them or trigger them to run ... thanks ....G.
 
If its the same, both fans are suppose to turn on at the same time. They both run at low and high speeds together.
 
ok, thanks for that ...

besides shorting out the relay terminals, don't suppose you know how to make them run ? .. i want to prove the whole system...G.
 
ok; getting to grips with this problem

The resistor on the fan cowling is for speed control. It is very common for these resistors to fail.

If you look on the radiator side of the cowling you'll see a resistor and a coil of thick wire inside a perforated metal cage.

These parts are stupidly over priced, so i'm going to use a wire wound replacement that comes in an aluminium case.

Could someone please measure the resistance of the small silver component with a red band at one end inside the cage and post up the value .... thanks ....G.
 
Ah, didn't realize you were across the pond. I looked on eBay over there and saw the cheapest one was £52. You weren't kidding when you said stupidly overpriced. Hopefully Amazon won't charge too much for shipping.
 
Ah, didn't realize you were across the pond..
yeah i've tried to update that but couldn't work out how.

£52.... You weren't kidding when you said stupidly overpriced. ..
nope.

Ok for anyone interested; If you have AC, then to get the slow speed function of the fans to trigger, turn on the AC and within a couple of minuites the slow speed relay will switch on the fans.

I haven't found out how to trigger the fast speed relay without getting ng the engine really hot..... which i wanted to avoid..... G.
 
Shop manual and EVTM don't say what the fan dropping resistor value is, so we'll have to measure it, and it's going to be substantially less than 1 Ohm, been down this road before with some other dual speed radiator fans. Measuring really low resistance values is hard, because meter lead and contact resistance flummoxes the measurement, I have a 4-point Kelvin probe ohm-meter, a fairly spendy HP unit, but we'll do this another way.

I made some measurements this morning, pulled the Hi and Lo fan relays and measured the stabilized motor current.
Hi fan = 14.5A
Lo fan = 10.3A

Since I didn't feel like pulling the lower shield and measuring the voltage drop across the resistor, we'll derive it another way.

Battery voltage at the time was 12.5V, so...
12.5V/14.5A = 0.86 Ohm
12.5V/10.3A = 1.21 Ohm

So in low fan mode there is 1.21 - 0.86, or an additional 0.35 Ohm in series with the motors, so the fan dropping resistor is about a 1/3rd of an ohm

Power in a resistor is I^2R, so 10.3A^2 X 0.35 Ohm = 37 Watts.

Good rule of thumb for power resistor derating is 50%, so I'd look for ~0.33 Ohm, 75 Watt resistor.

This one is close, but the power is a little low, 0.33 Ohm at 50W:
http://www.digikey.com/product-detail/en/9-1625984-7/A102447-ND/2366305
 
It's nice when real world and theory match up, my pleasure.

Excuse me then, I need to get back to looking for Higgs Bosons in my basement.
 
I turned the fans on manually by putting a small test wire to the coils of the relays in the under hood fuse box. One relay for each speed. One side of the coils is already grounded. Push the wire in the pocket when re-installing the relay cube. Test which side of the relays coil is grounded whith either a test light or meter.

For the fan speed resistor the coil is the resistance. The little part with the red band is a thermal fuse. Used in microwaves also. THey are crimped in so they don't get heated at reassembly. If it reads open its bad.
 
For the fan speed resistor the coil is the resistance. The little part with the red band is a thermal fuse. Used in microwaves also. THey are crimped in so they don't get heated at reassembly. If it reads open its bad.
excellent; i had my doubts it was a resistor. Do you know what the rating is ? ... the writing on mine cannot be read ...G.
 
I wouldn't bother with a thermal fuse, it was put there to protect a factory component that was barely adequate for the dissipated wattage. The OP is using a component that can dissipate 100W, and it will likely never dissipate more than 40W, the fan does not run continuously, so it's quite safely over-rated. If he mounts the resistor down on the subframe, it will have a nice large chunk of metal to dump heat into.
 
These are the only ones on newark with a 20amp rating. I don't know how to put them in unless you use a crimp connector? some factory fan speed units look like these are spot welded.
thanks for that... i wasn't sure about the temperature rating and how critical it was in your choice. This is going to be trial and error... when i scratched the back of the terminal the t. fuse is connected to, i found a golden/yellow metal holding the wire in place.

I wouldn't bother with a thermal fuse, it was put there to protect a factory component that was barely adequate for the dissipated wattage. The OP is using a component that can dissipate 100W, and it will likely never dissipate more than 40W, the fan does not run continuously, so it's quite safely over-rated. If he mounts the resistor down on the subframe, it will have a nice large chunk of metal to dump heat into.
i was looking at thermal switches with a reset ... now i know the temp rating isn't so critical, i can use one of those and just reset it after it flags up an issue. The old fan was stiff and jambed in one spot.... i think that was what took out the t. fuse ....G.
 
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