AC Engine fan Ghost
08-23-2004, 06:42 PM
#1
928 Collector
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AC Engine fan Ghost
SO!! I found the killer of my battery. As I was working on my '85 the front engine bay ac fan kicked on in full force. the car was cold and the ac was off as it has been a long time. So ... any thoughts? For now I just unplugged it.
Heinrich
Heinrich
08-23-2004, 10:19 PM
#2
It sounds like either a bad temp switch in the radiator, a bad AC relay or a short in the ground wire that goes from the AC fan to the temp switch. If the ignition was off, I would start with the AC relay.
Dennis
Dennis
08-24-2004, 02:00 PM
#4
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I remembered reading the following a while back, it seems your years have a bargeload of fan triggers.
WallyP said (here):"On the '85 and '86 models, the electric cooling fan is operated by three (5-speed) or four (automatics) sensors.
A coolant temp sensor located in the lower forward face of the radiator end cap. This sensor is open below the set coolant temp, closed above that temp. You can buy sensors with different trigger temps. To test the switch, pull the harness off, check continuity of the switch. Should be open when the radiator is very cool, closed when the radiator is above the trigger temp. To test the fan circuit, pull the harness off, turn the ignition switch on, and short between the contacts in the harness. The fan should run. If it doesn't, check fuse # 29 (25 amp) or relay XVIII.
A freon temp sensor located on the stem of the receiver/dryer. This sensor is open below the set temp of the freon in the receiver/dryer, closed above that point. Tests are the same as for the coolant temp sensor. This sensor does not appear to be very effective on most cars. It doesn't operate when the freon "overheats", but when it gets hot enough to show that the A/C is operating.
A transmission fluid temp sensor located on the torque convertor, on automatics only. This sensor is open below the set temp of the transmission fluid, closed above that point. Tests are the same as for the coolant temp sensor.
These three temp sensors work by grounding terminal 85 of relay XVIII, triggering the relay. The relay contacts are fed from bus X (ignition on) thru fuse # 29 to relay terminal 30, and the output contact (terminal 87) runs directly to the fan, running the fan at full speed. The fan runs when any one of the sensors triggers the relay.
In addition, there is an air temp sensor mounted on the intake plenum. This sensor is fed power thru fuse #23 regardless of ignition switch position, and is open when the air in the intake is below the set temp, closed above that temp. When the sensor closes, power is fed thru a resistance to the cooling fan, causing it to run at a lower speed. This is primarily for cool-down after engine shut-down."
WallyP said (here):"On the '85 and '86 models, the electric cooling fan is operated by three (5-speed) or four (automatics) sensors.
A coolant temp sensor located in the lower forward face of the radiator end cap. This sensor is open below the set coolant temp, closed above that temp. You can buy sensors with different trigger temps. To test the switch, pull the harness off, check continuity of the switch. Should be open when the radiator is very cool, closed when the radiator is above the trigger temp. To test the fan circuit, pull the harness off, turn the ignition switch on, and short between the contacts in the harness. The fan should run. If it doesn't, check fuse # 29 (25 amp) or relay XVIII.
A freon temp sensor located on the stem of the receiver/dryer. This sensor is open below the set temp of the freon in the receiver/dryer, closed above that point. Tests are the same as for the coolant temp sensor. This sensor does not appear to be very effective on most cars. It doesn't operate when the freon "overheats", but when it gets hot enough to show that the A/C is operating.
A transmission fluid temp sensor located on the torque convertor, on automatics only. This sensor is open below the set temp of the transmission fluid, closed above that point. Tests are the same as for the coolant temp sensor.
These three temp sensors work by grounding terminal 85 of relay XVIII, triggering the relay. The relay contacts are fed from bus X (ignition on) thru fuse # 29 to relay terminal 30, and the output contact (terminal 87) runs directly to the fan, running the fan at full speed. The fan runs when any one of the sensors triggers the relay.
In addition, there is an air temp sensor mounted on the intake plenum. This sensor is fed power thru fuse #23 regardless of ignition switch position, and is open when the air in the intake is below the set temp, closed above that temp. When the sensor closes, power is fed thru a resistance to the cooling fan, causing it to run at a lower speed. This is primarily for cool-down after engine shut-down."
08-24-2004, 02:04 PM
#5
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Holy she-ite Batman!!! OK I can see why a relay would be the most likely culprit. How I wish the ex hadn't taken the only Porsche I own with working ac .... 
09-01-2004, 02:41 PM
#7
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Turns out disconnecting the front (ac or aux) fan ........ that was a BAD bad idea. She ran fine on the freeway but got hot in traffic, so I hopped out and reconnected the fan. Runs fine now.