Does AC turn on Fans?
06-21-2007, 08:22 AM
#1
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Does AC turn on Fans?
Last night I had a friend over to charge the AC on my 87 S4. Compressor would engage and hi side charge kept rising until the safety shut it down at 375 lbs. This would happen time and time again. He told me the fans should have gone on to cool the condenser to prevent this, but they never did. A look at the shop manuals only provided further proof that I can't read a Porsche wiring schematic. Are the fans supposed to go on? If so, what would be my troubleshooting starting point. (we checked fuses and all were okay, and we couldn't find the relay which might effect this). Could have been "beerfog", but I thought I'd ask here before going at it again (the problem, not the beer).
06-21-2007, 08:28 AM
#2
Race Director
375psi is WAY TOO HIGH.....yes the fans should automatically kick on when the a/c is turned on unless its very cold out......be careful that you didn't overfill it!
Do the fans work at all?
Do the fans work at all?
06-21-2007, 11:00 AM
#3
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On your car, the fans are turned on by (among other things!) freon pressure, so basically the fans should run anytime that the A/C is on. The pressure switch is on the receiver/dryer.
There is a separate fuse for each fan. First thing to check...
There is a separate fuse for each fan. First thing to check...
06-21-2007, 11:47 AM
#4
Racer
the fans are managed by the AC climate control box (pass.side cover) and the power stage (front bumper). It is not directly linked to the compressor clutch signal.
Yes: they do need to go on when you push AC. Even when freon is low I think. (not 100% sure)
The high pressure in the system (hot) will eventually cause leaks, and cooling capacity of the system will be dramatically low. This needs fixing !!
regards,
Theo
Yes: they do need to go on when you push AC. Even when freon is low I think. (not 100% sure)
The high pressure in the system (hot) will eventually cause leaks, and cooling capacity of the system will be dramatically low. This needs fixing !!
regards,
Theo
06-21-2007, 02:41 PM
#6
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Clarification?
On the S4+ cars, there is a pressure transducer on the high-pressure tubibg next to the receiver-dryer, right front of car, in front of the AC condenser. This is a little round cannister-shaped device, maybe an inch and a half diameter and the same long. On my car there are two screw connections on top of the transducer wires with little ring terminals attach there.
In operation, the fans and the flaps in the front grill are controlled by both engine temperature and freon pressure, whichever needs more cooling. The workshop manual provides a couple charts that show the relationships among the two input variables and the resultant flap positions and fan speeds. Essentially though, there are three speeds on the fan that will be selected based on high-side AC pressure. Higher AC Pressure indicates higher cooling load which should result in higher fan speeds.
What happened when you were charging the system? The fans didn't run at all. So neither the engine heat nor the AC pressure were telling the fans to come on.
Does your car overheat in stop-and-go traffic? If it doesn't, it's likely that the fans are working OK.
First thing to consider is that the hood safety switch is actually working, telling the fans not to run because the owner invariably has fingers inside the fan shroud. This function is defined in the WSM, but I know mine ('89 S4) doesn't work that way and neither do others that I've had my fingers in. To test, open the hood, tape or tie-wrap the hood switch (right side of car, bracket on the fender lip, behind the jump-start terminal and forward of the two reservoirs) so it thinks the hood is closed. Start the car, turn on the AC. Fans running?
If that isn't your problem, you get to delve into the rest of the fan functions. Start off with a basic run function. There is an intake-manifold temp switch mounted right up on top of the manifold. Two wires attach, a brown and a brown-green IIRC. With engine off, you should be able to touch the two wires together to force the fan motors to run.
If the fans don't run, look at fuses first. 28 and 29 on my car, IIRC yours are the same. Fuses are in the central electrics panel, passenger-side floorboards. Lift the carpet, and that plywood panel will fold back to reveal the fuse and relay panel there. If the fuses are good, go back to the battery and verify a few connections. Battery is under the rear luggage panel under the carpet. Pull the carpet, the panel, the spare tire and all the stuff in there. Batterys under the cover on the right side. Positive terminal is forward in the car. The power for the fans comes from the battery through a couple dedicated red wires that are nutted to the positive battery terminal. Sometimes when a battery is replaced or a stereo installer gets in there to attach wires, the nut is left loose or the connections get corroded. No matter-- clean all the connections there 'til they are bright, and reassemble. A little Vaseline on the terminals after they are assembled will keep them nice longer.
With primary power verified, goto the workshop manual and go through the troubleshooting steps for the fan controllers. Engine temp signal is from a temp transducer mounted on the front left (right side as you look from the front of the car) bottom tank on the radiator. You can barely see the transducer under the AC condenser, but it's there. Freon pressure transducer described previously. The smart part of the fan controller is under the cover next to the passenger seat. The hard-working part is that black finned unit on the front apron of the car, right side (left when looking from the front) near the hood latch.
If you don't have an answer after checking this stuff and going through the WSM diagnostics, post again with what you have and we'll go at it some more.
On the S4+ cars, there is a pressure transducer on the high-pressure tubibg next to the receiver-dryer, right front of car, in front of the AC condenser. This is a little round cannister-shaped device, maybe an inch and a half diameter and the same long. On my car there are two screw connections on top of the transducer wires with little ring terminals attach there.
In operation, the fans and the flaps in the front grill are controlled by both engine temperature and freon pressure, whichever needs more cooling. The workshop manual provides a couple charts that show the relationships among the two input variables and the resultant flap positions and fan speeds. Essentially though, there are three speeds on the fan that will be selected based on high-side AC pressure. Higher AC Pressure indicates higher cooling load which should result in higher fan speeds.
What happened when you were charging the system? The fans didn't run at all. So neither the engine heat nor the AC pressure were telling the fans to come on.
Does your car overheat in stop-and-go traffic? If it doesn't, it's likely that the fans are working OK.
First thing to consider is that the hood safety switch is actually working, telling the fans not to run because the owner invariably has fingers inside the fan shroud. This function is defined in the WSM, but I know mine ('89 S4) doesn't work that way and neither do others that I've had my fingers in. To test, open the hood, tape or tie-wrap the hood switch (right side of car, bracket on the fender lip, behind the jump-start terminal and forward of the two reservoirs) so it thinks the hood is closed. Start the car, turn on the AC. Fans running?
If that isn't your problem, you get to delve into the rest of the fan functions. Start off with a basic run function. There is an intake-manifold temp switch mounted right up on top of the manifold. Two wires attach, a brown and a brown-green IIRC. With engine off, you should be able to touch the two wires together to force the fan motors to run.
If the fans don't run, look at fuses first. 28 and 29 on my car, IIRC yours are the same. Fuses are in the central electrics panel, passenger-side floorboards. Lift the carpet, and that plywood panel will fold back to reveal the fuse and relay panel there. If the fuses are good, go back to the battery and verify a few connections. Battery is under the rear luggage panel under the carpet. Pull the carpet, the panel, the spare tire and all the stuff in there. Batterys under the cover on the right side. Positive terminal is forward in the car. The power for the fans comes from the battery through a couple dedicated red wires that are nutted to the positive battery terminal. Sometimes when a battery is replaced or a stereo installer gets in there to attach wires, the nut is left loose or the connections get corroded. No matter-- clean all the connections there 'til they are bright, and reassemble. A little Vaseline on the terminals after they are assembled will keep them nice longer.
With primary power verified, goto the workshop manual and go through the troubleshooting steps for the fan controllers. Engine temp signal is from a temp transducer mounted on the front left (right side as you look from the front of the car) bottom tank on the radiator. You can barely see the transducer under the AC condenser, but it's there. Freon pressure transducer described previously. The smart part of the fan controller is under the cover next to the passenger seat. The hard-working part is that black finned unit on the front apron of the car, right side (left when looking from the front) near the hood latch.
If you don't have an answer after checking this stuff and going through the WSM diagnostics, post again with what you have and we'll go at it some more.
06-21-2007, 04:46 PM
#7
Team Owner
Nice write up BOB, this should be archived, Stan
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06-21-2007, 04:56 PM
#8
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First thing to consider is that the hood safety switch is actually working, telling the fans not to run because the owner invariably has fingers inside the fan shroud. This function is defined in the WSM, but I know mine ('89 S4) doesn't work that way and neither do others that I've had my fingers in
So to test your switch run the car till quite hot (high speed run - quick shutdown), turn off - assuming fans continue running - now open the hood - fans go off - depress hood switch - fans go back on...
So it disables the 'after-running' of the fans only
Alan.
06-21-2007, 09:27 PM
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I just got back to check this thread. I can't believe what knowledgeable, articulate, and generous responses I received. I read much more than I post, because as much as I'd like to, I usually have little to add to the discussion. Eventually, I hope to get to the point where I can begin to give back, but in the meantime, I continue to be impressed by the level of discussion and the knowledge base that is this forum. 
I'm printing out this thread and will use the information you've given me to try to resolve this problem. I'll let you know how I make out. Again, Kudos to all of you
I'm printing out this thread and will use the information you've given me to try to resolve this problem. I'll let you know how I make out. Again, Kudos to all of you
06-21-2007, 09:38 PM
#10
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Alan--
Thanks for the clarification. For some reason my hood switch doesn't inhibit fan operations at all. It's still connected OK, because it will trip the alarm if the hood is opened while the car is locked. When my intake temp switch decided to stick closed after an engine cleaning with water, the fans would run continuously even with the hood open. Some others have reported that the switch does in fact limit fan operation but I'm note sure they were specific about which function they were referring to. So maybe my car isn't operating correctly after all. Good news is (knock on wood...) my AC is cold and the engine stays cool. There can be worse failure modes.
Thanks for the clarification. For some reason my hood switch doesn't inhibit fan operations at all. It's still connected OK, because it will trip the alarm if the hood is opened while the car is locked. When my intake temp switch decided to stick closed after an engine cleaning with water, the fans would run continuously even with the hood open. Some others have reported that the switch does in fact limit fan operation but I'm note sure they were specific about which function they were referring to. So maybe my car isn't operating correctly after all. Good news is (knock on wood...) my AC is cold and the engine stays cool. There can be worse failure modes.
06-22-2007, 12:58 AM
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Originally Posted by dr bob
Clarification?
On the S4+ cars, there is a pressure transducer on the high-pressure tubibg next to the receiver-dryer, right front of car, in front of the AC condenser. This is a little round cannister-shaped device, maybe an inch and a half diameter and the same long. On my car there are two screw connections on top of the transducer wires with little ring terminals attach there.
snip
On the S4+ cars, there is a pressure transducer on the high-pressure tubibg next to the receiver-dryer, right front of car, in front of the AC condenser. This is a little round cannister-shaped device, maybe an inch and a half diameter and the same long. On my car there are two screw connections on top of the transducer wires with little ring terminals attach there.
snip
This is probably a good starting point for diagnosis as assuming your fans work properly and cool the engine when it is hot, and the above test switched the fans on regardless of engine temp you have narrowed down the suspects to the HVAC control side of things and eleiminated the fan controller.
06-22-2007, 07:20 PM
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Jon--
Have you done this? The system runs on milliamp-level currents and I am a little shy about testing your method. If you know it works, I'll add your test to the canned-response write-up. Contrast your suggestion with the function of the temp transducer, where higher resistance causes the controller to think higher temps and therefore faster fan run. Hence my question.
Have you done this? The system runs on milliamp-level currents and I am a little shy about testing your method. If you know it works, I'll add your test to the canned-response write-up. Contrast your suggestion with the function of the temp transducer, where higher resistance causes the controller to think higher temps and therefore faster fan run. Hence my question.
06-22-2007, 08:33 PM
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Well, among the 469 things that are wrong with my '85 S2...my AC fan does not automatically turn on when the compressor button is pushed.
-My solution, until I can find an afternoon to diagnose what is wrong: bypass the radiator fan switch. This switch, which is located [looking at the front of the car] on the lower right corner of the radiator, is designed to turn on the front auxiliary fan when the coolant gets hot. I installed a jumper wire a few years ago to help my a/c, and it worked. It isn't that cold, but it is cool enough. I suspect that there are some problems inside the dash that revolve around vacuum issues, along with the problem of the fan not coming on when it should. I went ahead and jumpered the two connectors to the radiator switch, which basically means that this fan runs whenever the key is turned to the "ignition on" position.
I'm not sure if the S4 and later cars with electric fans and no auxiliary fan work this way, but I think they do have a radiator temp switch, and the a/c does depend on the radiator fans to cool the condenser, so I suspect it would have the same result. Give it a try, it doesn't cost much.
-My solution, until I can find an afternoon to diagnose what is wrong: bypass the radiator fan switch. This switch, which is located [looking at the front of the car] on the lower right corner of the radiator, is designed to turn on the front auxiliary fan when the coolant gets hot. I installed a jumper wire a few years ago to help my a/c, and it worked. It isn't that cold, but it is cool enough. I suspect that there are some problems inside the dash that revolve around vacuum issues, along with the problem of the fan not coming on when it should. I went ahead and jumpered the two connectors to the radiator switch, which basically means that this fan runs whenever the key is turned to the "ignition on" position.
I'm not sure if the S4 and later cars with electric fans and no auxiliary fan work this way, but I think they do have a radiator temp switch, and the a/c does depend on the radiator fans to cool the condenser, so I suspect it would have the same result. Give it a try, it doesn't cost much.
06-27-2007, 10:55 PM
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Originally Posted by Normy
Well, among the 469 things that are wrong with my '85 S2...my AC fan does not automatically turn on when the compressor button is pushed.
-My solution, until I can find an afternoon to diagnose what is wrong: bypass the radiator fan switch. This switch, which is located [looking at the front of the car] on the lower right corner of the radiator, is designed to turn on the front auxiliary fan when the coolant gets hot. I installed a jumper wire a few years ago to help my a/c, and it worked. It isn't that cold, but it is cool enough. I suspect that there are some problems inside the dash that revolve around vacuum issues, along with the problem of the fan not coming on when it should. I went ahead and jumpered the two connectors to the radiator switch, which basically means that this fan runs whenever the key is turned to the "ignition on" position.
I'm not sure if the S4 and later cars with electric fans and no auxiliary fan work this way, but I think they do have a radiator temp switch, and the a/c does depend on the radiator fans to cool the condenser, so I suspect it would have the same result. Give it a try, it doesn't cost much.
-My solution, until I can find an afternoon to diagnose what is wrong: bypass the radiator fan switch. This switch, which is located [looking at the front of the car] on the lower right corner of the radiator, is designed to turn on the front auxiliary fan when the coolant gets hot. I installed a jumper wire a few years ago to help my a/c, and it worked. It isn't that cold, but it is cool enough. I suspect that there are some problems inside the dash that revolve around vacuum issues, along with the problem of the fan not coming on when it should. I went ahead and jumpered the two connectors to the radiator switch, which basically means that this fan runs whenever the key is turned to the "ignition on" position.
I'm not sure if the S4 and later cars with electric fans and no auxiliary fan work this way, but I think they do have a radiator temp switch, and the a/c does depend on the radiator fans to cool the condenser, so I suspect it would have the same result. Give it a try, it doesn't cost much.
Harvey
06-28-2007, 12:34 AM
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Originally Posted by dr bob
Jon--
Have you done this? The system runs on milliamp-level currents and I am a little shy about testing your method. If you know it works, I'll add your test to the canned-response write-up. Contrast your suggestion with the function of the temp transducer, where higher resistance causes the controller to think higher temps and therefore faster fan run. Hence my question.
Have you done this? The system runs on milliamp-level currents and I am a little shy about testing your method. If you know it works, I'll add your test to the canned-response write-up. Contrast your suggestion with the function of the temp transducer, where higher resistance causes the controller to think higher temps and therefore faster fan run. Hence my question.
I should have been more specific and stated it was correct for S4+. It may be correct for pre S4 but I don't know.
Yes I have done this on my 88SE, while I was diagnosing non running fans (motor bearings) - didn't have any issues with it at all. With functioning fan motors the cooling fans and flaps all now work perfectly. (And have done for the last 5 years since I did the fix).
My understanding is that the pressure switch on the TOP of the little canister only connects to the fan controller - basically tells the fan controller - Hey the AC is on and there is enough pressure in the AC system for it be operating. The switch or is it a transducer on the bottom of the little canister is the one that controls the AC compressor.