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Solution to cooling fan resister failure???

47K views 20 replies 9 participants last post by  amc49 
#1 ·
Getting warmer here in Michigan. The old Focus I bought over the winter is starting to overheat. All good when moving (runs at 94C), but idling at a stop, it gets up to about 114C before the fan kicks on. I've read a lot of threads here (thanks for the help all) and I'm confident that the problem is the famed resister. Some of the threads have been asking for cheap work-arounds, etc., as the resister costs about $50.

Why not just solder a jumper wire into the burned out resister to bypass the burned out element? Or even cutting off the plug (drastic) and connecting the two wires together. A lot of vehicles don't have two speed cooling fans. Either on or off. The fan will be noisier on high and kick on and off more often, but to me that is not a problem.

Has anyone tried this?[dunno]
 
#2 ·
Not sure if any one has tried it, but thoughts...

With them on High all the time you may have higher current spikes and possibly blowing fuses if your always jumping from static off to full speed...

The life of the fans may drop or fail sooner as they are working at a higher duty cycle for a longer period of time

longer warm up time = longer time of the motor running on a non "optimal" fuel chart

what vehicles are you referring to when "lot of vehicles don't have two speed cooling fans."? Those that don't that I know of have a mechanical clutch fan which is the "low speed" fan....
 
#3 ·
The engine would still maintain operating temperature in the lower range but.....

The fuse may be your number one problem (30A), followed by melting your wire harness 'somewhere' if you try replacing a 30A fuse with a 50A fuse.

Wouldn't recommend it.

Would recommend fixing the original problem correctly.

 
#4 ·
Look at the supplied schematic, the system is already able to take the power, just more noise and fan wears out faster. Possible idle quality issues with bigger current spike from standing start off.

Simply wire direct to miss the resistor will do it.
 
#5 ·
^^^ that assumes that the wire gauge through the low speed relay is the same gauge as that through the high speed relay. what I would do instead is wire it in a "OR" configuration so the low speed relay is actually just turning on the high speed fan relay. that would be the correct way to bypass the low speed.
 
#8 ·
Thanks for all the feedback. I researched a little more and agree that most of the OEM fans are two speed.

Wire gage in the current low speed circuit would be important. I didn't realize that low speed had it's own fuse. I'll check that out and verify juice to the plug. I was thinking that when I pulled the resister out, it would be fried. Time for the VOM. And clean connectors.

Seems like wearing out the motor would be the biggest negative.
 
#9 ·
I think we're missing something here.

The Fan Resistor is a current limiting resistor/ dropping resistor. That may be why the 'low speed' circuit only needs a 30A fuse.

The high speed circuit has no series resistor, the Fan Resistor, and needs a 50A fuse.

If you short out/bypass the Fan Resistor you may exceed the 30A fuse value.
If you replace the 30A fuse with a 50A fuse you may exceed the wires current rating associated with it if it is not the correct gauge wire.
 
#10 ·
ive found tht msot of the resistors fail on my car after a couple years of being blasted with rain and whatever else makes it through the radiator. i jsut make sure i have one NIB at home waiting for the next one to fail. 50 bucks every couple of years isnt so bad. (its more like a preventative maintanence thing. i jsut do it by habit now. )
 
#11 ·
New approach on this issue. I'm just a cheapskate. That's why I bought this old Focus for $2G and patching it up ever since.

I pulled the resistor. Ohmmeter tells me that only the thermal fuse is blown. NAXJA forum (one of my other beaters) dudes have fixed heater fan resistors with Radio Shack thermal fuses for $1.79. That's my next step. I'll try to do pictures.
 
#12 ·
funny to see a fellow XJ owner who also has a Foci on here. funny... wiring up the low speed fan relay to turn on the high speed relay would be the cheapest fix as it would only require a small gauge wire to the +12V coil on the high speed fan relay. most likely you have the 12" or less of 16 AWG wire laying around to do this...
 
#15 ·
Repair for blown thermal resistor

Fixed it by jumping the relays as suggested. Soldering the thermal fuse was a pain, so I took the shortcut. Works great so far. Fans come on full power at 107C.


First picture shows the low and high speed fan relays at the bottom right. Pull the relays out. This is the fuse box right under the hood.



Insert a jumper wire as shown. Re-insert the relays. That's it! The PCM then activates the high speed fan circuit when it signals the low speed relay.
 
#16 ·
Good solution. The same thing can be done underneath the box by simply jointing or bridging the hot wires to the relay coils. Then you don't have relays jammed in there on top of wires.

At least you didn't wire it constant hot like some people do. Then they wonder why their fans and alternators keep blowing up.
 
#18 ·
a_2000_se and others are right. I am so stupid, the lower section of the schematic is only rated at 30 amp, if you simply tie the resistor wires together to bypass and full fan speed, you will have too much amp flowing through the wires.

Looking at this much later caught it, my bad. looks like I'm fixing to experience it first hand, mine started overheat at idle only today. Will be yanking that resistor tomorrow.

Going up underneath the fusebox may not be much of an option, there's a godawful bunch of wires under there and not a lot of room without disconnecting more stuff. I rebuilt one of those entire boxes when mine was shattered in accident. Bought just the plastic box for $10 after cutting out all the wiring in junkyard, the guy at part desk thought I was crazy. The fusebox is part of the main wiring harness. It took maybe 2 hours to rebuild it but I saved a bunch of money doing so.
 
#19 ·
Now that the weather is getting warmer, I am planning to look into the cooling issue that I have had. I've replaced the thermal fuse (resistor) a few years ago. It worked (no overheating) until the weather got really warm. The following spring, I replaced the resistor assembly from dealer and it failed again.
Looking at the schematic diagram, if my connectors were bad, the fan should still not work if resistor was replaced. I am wondering if the sensor, if any, that kicks off the high-speed relay is not working. Where can I find that sensor?

I have an 02 zetec
 
#21 ·
I ended up rebuilding the resistor minus the thermal fuse. The fuse seems stupid if the thing STILL melts into plastic slag, as mine did. I used much bigger spade connectors and positively drilled through them to use small screws/nuts to bolt them together, used bigger wire too. The whole thing works great and I just watch the connector more now. Cost maybe $10.
 
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