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cooling Fan info by Mike C.


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#1 importwarrior

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Posted 21 November 2015 - 11:15 AM

Provided by:
Mike C.

There are two fairly independent wiring circuits to the fans on 87-later cars.  One is for normal engine cooling mode and the other is controlled by the a/c computer:  The primary and secondary fans each have two relays that can run them: one relay for the engine-side cooling function and a second relay for the a/c function.  The pusher fan has just one relay but two different things can activate that relay.

Engine cooling: simple temp sensors and relays... no computers involved:
* temperature sensor switch underneath "primary" fan trips a relay which in turn makes that fan run when radiator water temp reaches a certain point, roughly middle of the dash temp gauge.
* temperature sensor switch underneath "secondary" fan trips two relays which makes that fan AND the front pusher fan run when radiator water temp gets even hotter, roughly 2/3rd to 3/4s on the dash temp gauge.  Ergo all three fans should be ON at this point...

A/C mode:
* A/C computer uses two relays to run the primary and secondary fan whenever the a/c computer cycles the compressor clutch ON; these are different relays than the ones triggered by the radiator temp switches for engine-cooling mode.
* a pressure sensor in the a/c refrigerant lines triggers the pusher fan relay IF the refrigerant pressures get higher than normal - indicating a/c operation on a hot day.  Because of the engine-cooling relay wiring, there is feedback that causes the secondary fan's engine-cooling relay to trip (running the secondary fan) even though the secondary fan should already running from an a/c controlled relay just described.

Look at the a/c refrigerant lines between the headlights and near the a/c receiver/dryer tank... you'll see two different sensors in that piping.  One is the "dual pressure switch" that protects the a/c system components from too little or too much refrigerant
* if there is too little refrigerant, the compressor OFF pressures will be really low and the switch will OPEN preventing the compressor from running; this protects it from running without the oil that's also in the refrigerant.
* if the a/c refrigerant pressures get dangerously high for some reason - too much refrigerant, blockages in the system, lousy airflow through the condensor (front radiator), etc - the high-side of this switch opens to cut off the compressor before the pipes burst.

The other sensor switch - the smaller one if I remember correctly with black and black+yellow wires in the harness side of the connector - is the pressure switch that trips when the a/c is working hard on a hot day... i.e. when refrigerant pressures are normal-high.  You can unplug the connector on this one and jumper across it as a test, that should run the pusher and secondary fans.  Running the a/c on a cool day though generally won't generate normal-high refrigerant pressure so the pusher fan won't run.  So I wouldn't worry about "pusher fan doesn't run when air conditioner is on" as that's normal if the outside air temp isn't high.

mike c.

edit: the little temp sensor switch on the thermostat housing is the ground for the compressor clutch relay; this switch is normally closed unless the coolant temps are dangerously high - then the switch OPENs to kill the a/c in an effort to help save your engine from overheating.  This sensor tends to rot internally (the connector tang will spin easily when this happens) rendering the a/c clutch/compressor totally inoperative.  Very common failure.

B-71 87 TSI ~ RIP

Black 87 Starion ~ Mess SOLD!!!

Proud New 89 slightly Rusted Fiji Owner !!!








#2 importwarrior

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Posted 21 November 2015 - 11:16 AM

some extra links
http://www.starquest...nt#entry1456525

http://www.starquest...howtopic=149644

B-71 87 TSI ~ RIP

Black 87 Starion ~ Mess SOLD!!!

Proud New 89 slightly Rusted Fiji Owner !!!







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