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radiator fan


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14 replies to this topic

#1 lonnie

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Posted 16 September 2007 - 06:51 PM

working on 81 brat. updated motor by a year, had radiator tested and it's tight also replaced fan switch. tested fan and it works. let motor warm up and no fan, got close to 3/4 up on temp the guage with overflow tube blowing steam like a train and still no fan. looking at the guage when would i expect to see fan come on. i know if all else fails i can connect manual switch but would like to make it work as intended. any suggestions or help thanks.

#2 nipper

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Posted 16 September 2007 - 07:15 PM

There is a thermo switch in the radiator that switches the fan on and off. The circuit is power to the fan, out to the thermo switch, then to ground.

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#3 lonnie

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Posted 16 September 2007 - 08:12 PM

thanks, i understand about the switch and mentioned i had replaced it. my question is how hot does the guage reading need to be so the fan comes on.

#4 nipper

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Posted 16 September 2007 - 09:37 PM

thanks, i understand about the switch and mentioned i had replaced it. my question is how hot does the guage reading need to be so the fan comes on.


By 2/3 range it shold be on. Usually then fans come on about 1/2 on the temp gauge. Take a meter to the fan and make sure there is power before the motor and after the motor. Check the motors resistance. Ground out the lead to the thermo switch and see if the fan comes on.

CHek your fluid level and make sure its full. Make sure the thermostat is functioning.



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#5 waimaks

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Posted 17 September 2007 - 04:50 AM

Just a quick thing to check,if I'm right, The thermoswitch grounds out into the rad, but if your rubber bushes are all good on the rad and you don't have a jumper from the rad to chassis, no fan there either.
Something to overlook easily and think the thermoswitch isn't grounding out right.

#6 lonnie

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Posted 17 September 2007 - 09:20 AM

thanks nipper and waimaks-

good to know guage position relative to fan coming on. AS per your suggestion ,did check (with key on) grounded power wire to thermo switch and fan runs. having done this, does this also address your question relative to power before and after the motor ? also checked body ground wire to radiator.

am not sure how thermostate would keep fan from coming on but if you think it is worthwhile to check, i am on it.

can not seem to figure this problem out so very much appreciate the help here.

#7 nipper

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Posted 17 September 2007 - 09:24 AM

thanks nipper and waimaks-

good to know guage position relative to fan coming on. AS per your suggestion ,did check (with key on) grounded power wire to thermo switch and fan runs. having done this, does this also address your question relative to power before and after the motor ? also checked body ground wire to radiator.

am not sure how thermostate would keep fan from coming on but if you think it is worthwhile to check, i am on it.

can not seem to figure this problem out so very much appreciate the help here.


A stuck thermostat will not allow circulation of hot coolant to the radiator.

Sometimes it affects the fans coming on and off, sometimes it doesnt.

In general the cooling system shold be serviced every three years, so if it hasnt, now would be a good time. Don't forget a new radiator cap too.

Glad the problem has been found.

#8 lonnie

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Posted 17 September 2007 - 10:30 AM

nipper, again thanks for info however the problem has not been found. currently with every thing hooked up and car running, the fan does not come on.

i will certainly follow up on your suggestions tonight.

#9 lonnie

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Posted 18 September 2007 - 11:42 AM

checked thermostate and it is working. also installed new cap. problem persists, fan simply does not come on when car gets hot. will probably call it a day and simply install a manual switch on the dash. oh well.

#10 waimaks

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Posted 19 September 2007 - 01:50 AM

So at this point... If you ground the wire going to the thermo switch. The fan goes? And you checked the connection from the radiator to chassis?
Then we know that the Thermo Switch is not earthing out at the right temperature.
Try earthing the thermo switch wire onto the rad itself or fan chassis.

#11 lonnie

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Posted 19 September 2007 - 11:40 AM

excellent suggestion, will do tonight!thanks.

#12 lonnie

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Posted 20 September 2007 - 01:12 PM

sorry got side tracked on project. took rig to muffler shop and had new exhaust (from head to tail pipe) installed yesterday afternoon. unfortunately once we started to pull off lift, water heater hose broke and made a bit of a mess. shop has some extra time, so they are installing all new hoses for me. will pick up tonight and hopefully get back on track.

other brat at carb shop, having carb rebuilt. did not do weber upgrade instead opted to have unit completed gone through including jetting on a wet flow bench to optimize performance. not cheap but an interesting excercise in what might be done with the stock unit. will report on this later.

#13 lonnie

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Posted 21 September 2007 - 03:19 PM

grounded wire to radiator and fan chassis, came on both times! since i just bought a new thermo switch, what could it be? thanks.

ps. exhaust came out awsome.

#14 waimaks

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Posted 21 September 2007 - 08:46 PM

All I can think of now is faulty gauge?
I'm outta ideas.

#15 lonnie

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Posted 24 September 2007 - 03:22 PM

did some more clean up work on engine compartment.

wired fan hot lead to switch so i could run fan manually, tried it but no luck. even worse now fan does not come on no matter what i do.

but...a couple of dash lights now come on that did not before. will proceed to replace fan and motor.




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