[Tom in Austin]

Recently I was cleaning out leaves under the windshield cowl and happened across a bracket with two round ceramic-looking discs fastened to it. These are ballast resistors, and from perusing the wiring diagrams, it appears they are part of the circuit for the cooling fans.



I think resistors are used with fans to allow them to run at more than one speed. To produce lower speeds, a resistor "burns off" some of the voltage so that less makes it to the fans and they turn slower. I think the climate control fan has one or more resistors that are used to deliver the 1 - 3 speed settings (where 4 is the max speed that takes full voltage).



So, in this application, why are there two of these things? I think in past posts people have said that there's a half-speed mode for fan #1. But doesn't fan #2 run only at full speed when the A/C's on, or temperature reaches a certain level? Do the two resistors relate to just fan #1, or to both fans?



Is there a failure mode for a ballast resistor short of the fan just not turning? For example, could you lose the half-speed mode but retain full-speed if a resistor wore out?

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[BruceWard]

I believe that the fans operate on high all the time when those resistors fail.

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[Greimann]

[quote name='Tom in Austin' date='Dec 13 2005, 11:26 AM']So, in this application, why are there two of these things? I think in past posts people have said that there's a half-speed mode for fan #1. But doesn't fan #2 run only at full speed when the A/C's on, or temperature reaches a certain level? Do the two resistors relate to just fan #1, or to both fans?

[right][post="13355"]<{POST_SNAPBACK}>[/post][/right][/quote]

Both fans operate in tandem (same function for both) so one resistor for each fan.

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[tonyc]

[quote name='Greimann' post='13359' date='Dec 13 2005, 02:16 PM']Both fans operate in tandem (same function for both) so one resistor for each fan.[/quote]





Not sure if I have a ballast resistor problem or thermal switch problem. Fans only come on at 10 o clock position and both together. Isn't one fan supposed to kickin earlier then the second fan kickin later ? Can someone explain the sequence of how the fans come on (low speed and high speed) and whether they are suppose to work together, Thanks



Tony

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[sp4149]

[quote name='BruceWard' post='13357' date='Dec 13 2005, 12:11 PM']I believe that the fans operate on high all the time when those resistors fail.[/quote]



The 944 used the same resistors. Their most important function is to operate the fans after you turn the ignition off. When they fail, your car will dump quarts of coolant when the ignition is turned off (the fans don't run) and the coolant is hot. Clark's garage has a good writeup on how bad resistors can result in loss of coolant after the engine is shut off. At least the resistors are accessible on the 968, they are behind the instrument cluster on early 944s.

-sp4149

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[tonyc]

[quote name='sp4149' post='34734' date='Apr 28 2007, 11:03 AM']The 944 used the same resistors. Their most important function is to operate the fans after you turn the ignition off. When they fail, your car will dump quarts of coolant when the ignition is turned off (the fans don't run) and the coolant is hot. Clark's garage has a good writeup on how bad resistors can result in loss of coolant after the engine is shut off. At least the resistors are accessible on the 968, they are behind the instrument cluster on early 944s.

-sp4149[/quote]

Thanks...this is exactly what happened to me. Car was hot but after turning off the fans didn't come on and I dumped a couple of liter of coolant on the floor. As weel, had trouble starting the car...had to boost the battery so wonder if the resisitors were somehow draining the battery as well. Funny thing...battery is fine now but fans don't stay on too long after I shut the car and temp is still around 10 o'clock on the gauge. Does it cost much to have a garage change these resistors or is there a easy test to see if the resistors are at fault . I'll check Clarks to get more info...thanks again



Tony

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[sp4149]

[quote name='tonyc' post='34852' date='Apr 30 2007, 04:01 PM']Thanks...this is exactly what happened to me. Car was hot but after turning off the fans didn't come on and I dumped a couple of liter of coolant on the floor. As weel, had trouble starting the car...had to boost the battery so wonder if the resisitors were somehow draining the battery as well. Funny thing...battery is fine now but fans don't stay on too long after I shut the car and temp is still around 10 o'clock on the gauge. Does it cost much to have a garage change these resistors or is there a easy test to see if the resistors are at fault . I'll check Clarks to get more info...thanks again



Tony[/quote]

On the 968 the resistors are super easy to find and work on, probably a ten minute job; not so on the early 944s, apparently have to remove the instrument cluster.

-sp4149

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[Tom in Austin]

Tony, I have an extra pair if you end up needing new ones, send me a PM



Tom

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[tonyc968]

[quote name='Tom in Austin' post='34923' date='May 2 2007, 09:01 AM']Tony, I have an extra pair if you end up needing new ones, send me a PM



Tom[/quote]

Thanks Tom..appreciate the offer, I'll let you know.

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[tonyc968]

[quote name='sp4149' post='34734' date='Apr 28 2007, 10:03 AM']The 944 used the same resistors. Their most important function is to operate the fans after you turn the ignition off. When they fail, your car will dump quarts of coolant when the ignition is turned off (the fans don't run) and the coolant is hot. Clark's garage has a good writeup on how bad resistors can result in loss of coolant after the engine is shut off. At least the resistors are accessible on the 968, they are behind the instrument cluster on early 944s.

-sp4149[/quote]





Can't seem to find the Clarks Garage write-up on bad resistors you refer to above...any chance you have the link or more details. Thanks



Tony

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[sp4149]

[quote name='tonyc968' post='35027' date='May 4 2007, 06:15 AM']Can't seem to find the Clarks Garage write-up on bad resistors you refer to above...any chance you have the link or more details. Thanks



Tony[/quote]



COOL-01, Cooling Fan Operation and Troubleshooting

in Clark's Garage, "Troubleshooting"



-sp4149

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