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Old 07-30-2003, 11:13 AM
Arthur Dalton Arthur Dalton is offline
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Join Date: Aug 1999
Location: Florida / N.H.
Posts: 8,784
Fan Confusion again ??

Ok , this is all archived, but this is a good time to run it again...

The AUX fan confusion on most AUX. fan post seems to stem from the tricky part of the FAN circuits and applications..this tricky part is the fact that there are 2 [ yes-two] fan circuits , but they SHARE the SAME fan motor/s....
So, without getting into tech details, here are some simple things to observe that will help all in a better understanding of the whos/whats of the AUX fan/s...

The main [High speed] aux fan is there to AID the ENGINE cooling system in the event that the coolant temps start to get in a DANGER ZONE...this usually occures under extreme conditions
and is normal...[that is why they have the cut-in temp so high].
Under NORMAL conditions , this AUX is not needed and the sensors do not call for it.....

The low speed fan is there to aid the A/C systems high-side pressure from getting so high that it is in a DANGER ZONE..[ along with decreased cooling ability]
If the systems pressures get too high , a sensor will shut down the a/c... but before that happens , the sw on the drier sees this pressure increase and it completes the circuit for LOW fan to add
more air-flow accross the condensor to AID the a/c systems cooling ability and keep the pressure in check..
So, what we have here is two INDIVIDUAL fan circuits that accomplish two different functions, but they do it with the SAME fan MOTOR..
Can we have an aux. fan come on without seeing high engine temps ??
..OF COURSE... it will come on when the a/c system tells it that the high-side pressure is getting up there..nothing to do with engine coolant temps.[this a/c will be low fan, by the way]
Or , can we see a aux fan come on when engine temps start getting up around 105-110 , even with the a/c OFF ???
... Sure.... the coolant temp sensor will activate the Coolant Temp AUX fan CIRCUIT and turn on the AUX fan to AID the radiator with more air to check the high engine temp [ this will be high fan].. This will still be The SAME FAN MOTOR..
So, as you can see , when checking your aux fans, you want to know which CIRCUIT you are checking, because the two circuits have there own relays, sensors, fuses, etc... the only common part they share is the FANS themselves..and that is the tricky part that makes for the confusion..
**** NOTE... the reason TECHS jumper the a/c drier sw to look for fan is simply because that is an easy place to jumper for a quick test to verify that the fan motors and feed fuses, wiring, etc are intact is a priliminary test to aid the tech in elimination of parts in short reality, it does not mean that one needs a drier sw., and it has nothing to do with the engine coolant side of ther circuit [ high fan], except to tell that the fan motors are
I guess an easy thing to remeber is...HIGH FAN comes on with HIGH engine coolant temps..
LOW fan comes on when a/c system is cooling [low temp] and the ambient temps are up in the high thermal load conditions to call for aux fan help....
There was a post where one noted that the low fan was on for a few min. at idle and then it switched to high fan...
How come ??
Simple.. the a/c was on and the low fan was being called for by the a/c high side pressure sw [ low fan]. As the car now had a/c on and was at idle a few more min., the coolant temp started to go up , so the engine coolant temp sensor tripped to keep the coolant in check..
So, what we have seen here is BOTH SYSTEMS in operation at the SAME TIME , with the SAME FAN/S.. both systems calling for assistance and both reacting, with the high speed taking prefernce, of course...
A side point here is the fact that if your a/c high side LOW fan circuit is not working properly [ fuse,relay, resistor, etc.], you can still have a/c , BUT, [ note big but] the condensor will run hotter than normal and this will INDIRECTLY affect the Engine COOLANT
temps because the condensor is in front of the radiator and that will affect the radiators efficiency [ this is a passive heat exchange, but often overlooked when one is diagnosing engine temp slighly higher than normal, but not high enough to trip coolant temp sensor] this will be noted only with a/c on and more likely at slow/stand-still conditions..
...Meaning it is important for both fan systems/circuits to be operational, even though they are independant, circuit wise.

I knew this had to get a little techie, but hope it helps anyone wanting some further understanding...and only on this system design...

*An Added PS on Fan SPEEDS.
How does the aux fan get the high/low fan difference with the
same fan motor???
Simple... there is a resistor [voltage dropping] in the a/c fan circuit to lower the speed ...[ this is the resistor you see mentioned in many fan post.... they burn up from high heat /amp draw]...this resistor is in series with the a/c fan relay and the fan motor.
BUT, in the engine temp circuit , that fan relay feed bye-passes this resistor and give full battery voltage to the fan motor/s...
thus, HIGH fan...[ same fan, different feed]

Last edited by Arthur Dalton; 07-30-2003 at 11:42 AM.
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