Fans on non stop..
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From: Boulder Creek, CA
Fans on non stop..
87 S4
Not the sensor on the intake manifold, unplugged it, no change.
Unplugged the lower radiator temp sensor..no change.
Sounds like the radiator sensor..agree?
And..does that drain coolant to replace?
Not the sensor on the intake manifold, unplugged it, no change.
Unplugged the lower radiator temp sensor..no change.
Sounds like the radiator sensor..agree?
And..does that drain coolant to replace?
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From: Oman
Jeff,
There is nothing in your post to suggest the temp sensor is at fault. The sensor has a modulating output and until the temperature reaches 79C the fans do not switch on- at that point where they do switch on resistance should be 1931 ohms and the fans should be running at something like 60% of rated speed. As the coolant gets hotter the resistance drops and at 95C the resistance should then be down to 1128 ohms and the PWM output should drive the fan at maximum speed noting that if the fan were connected directly to the battery/alternator the fan will run a bit faster- exactly how much faster I do not know- probably a conseqeunce of some efficiency loss.
The system also has an override in that when the ac solenoid is energised the fans drive irrespective of engine temperature and as I am aware they drive at full output. So if your fans are driving at full speed it may well be something connected to the ac solenoid circuit being energised even if it should not be so you could try measuring that line to see if voltage is present irrespective of whether the ac is running or not.
There is nothing in your post to suggest the temp sensor is at fault. The sensor has a modulating output and until the temperature reaches 79C the fans do not switch on- at that point where they do switch on resistance should be 1931 ohms and the fans should be running at something like 60% of rated speed. As the coolant gets hotter the resistance drops and at 95C the resistance should then be down to 1128 ohms and the PWM output should drive the fan at maximum speed noting that if the fan were connected directly to the battery/alternator the fan will run a bit faster- exactly how much faster I do not know- probably a conseqeunce of some efficiency loss.
The system also has an override in that when the ac solenoid is energised the fans drive irrespective of engine temperature and as I am aware they drive at full output. So if your fans are driving at full speed it may well be something connected to the ac solenoid circuit being energised even if it should not be so you could try measuring that line to see if voltage is present irrespective of whether the ac is running or not.
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From: Boulder Creek, CA
Jeff,
There is nothing in your post to suggest the temp sensor is at fault. The sensor has a modulating output and until the temperature reaches 79C the fans do not switch on- at that point where they do switch on resistance should be 1931 ohms and the fans should be running at something like 60% of rated speed. As the coolant gets hotter the resistance drops and at 95C the resistance should then be down to 1128 ohms and the PWM output should drive the fan at maximum speed noting that if the fan were connected directly to the battery/alternator the fan will run a bit faster- exactly how much faster I do not know- probably a conseqeunce of some efficiency loss.
The system also has an override in that when the ac solenoid is energised the fans drive irrespective of engine temperature and as I am aware they drive at full output. So if your fans are driving at full speed it may well be something connected to the ac solenoid circuit being energised even if it should not be so you could try measuring that line to see if voltage is present irrespective of whether the ac is running or not.
There is nothing in your post to suggest the temp sensor is at fault. The sensor has a modulating output and until the temperature reaches 79C the fans do not switch on- at that point where they do switch on resistance should be 1931 ohms and the fans should be running at something like 60% of rated speed. As the coolant gets hotter the resistance drops and at 95C the resistance should then be down to 1128 ohms and the PWM output should drive the fan at maximum speed noting that if the fan were connected directly to the battery/alternator the fan will run a bit faster- exactly how much faster I do not know- probably a conseqeunce of some efficiency loss.
The system also has an override in that when the ac solenoid is energised the fans drive irrespective of engine temperature and as I am aware they drive at full output. So if your fans are driving at full speed it may well be something connected to the ac solenoid circuit being energised even if it should not be so you could try measuring that line to see if voltage is present irrespective of whether the ac is running or not.
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From: Oman
Jeff,
The fans are triggered by the logic controller that considers what is going on in the head unit and decides what signal to send to the final element PWM controller. Once the controller senses that the ac is running AFAIK it tells both fans to drive at full whack. The PWM is a high speed switch that by switching the power on and off enables the motor to run at the desired speed. If the PWM unit fails the first question would be whether it is likely to fail on both channels and if so does it automatically drive to full power as a fail safe feature given it is way better to have the fans running flat out rther than not at all- I suspect it will do that but I do not know that for a fact.
When the ac unit head unit fails it typically does not switch the power on to the ac clutch solenoid so I doubt that will be the problem. If the fans run at full speed from the moment you switch the ignition on and the ac unit is switched off then that may suggest a failure of the final element [the PWM].
The fans are triggered by the logic controller that considers what is going on in the head unit and decides what signal to send to the final element PWM controller. Once the controller senses that the ac is running AFAIK it tells both fans to drive at full whack. The PWM is a high speed switch that by switching the power on and off enables the motor to run at the desired speed. If the PWM unit fails the first question would be whether it is likely to fail on both channels and if so does it automatically drive to full power as a fail safe feature given it is way better to have the fans running flat out rther than not at all- I suspect it will do that but I do not know that for a fact.
When the ac unit head unit fails it typically does not switch the power on to the ac clutch solenoid so I doubt that will be the problem. If the fans run at full speed from the moment you switch the ignition on and the ac unit is switched off then that may suggest a failure of the final element [the PWM].
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From: Boulder Creek, CA
Jeff,
The fans are triggered by the logic controller that considers what is going on in the head unit and decides what signal to send to the final element PWM controller. Once the controller senses that the ac is running AFAIK it tells both fans to drive at full whack. The PWM is a high speed switch that by switching the power on and off enables the motor to run at the desired speed. If the PWM unit fails the first question would be whether it is likely to fail on both channels and if so does it automatically drive to full power as a fail safe feature given it is way better to have the fans running flat out rther than not at all- I suspect it will do that but I do not know that for a fact.
When the ac unit head unit fails it typically does not switch the power on to the ac clutch solenoid so I doubt that will be the problem. If the fans run at full speed from the moment you switch the ignition on and the ac unit is switched off then that may suggest a failure of the final element [the PWM].
The fans are triggered by the logic controller that considers what is going on in the head unit and decides what signal to send to the final element PWM controller. Once the controller senses that the ac is running AFAIK it tells both fans to drive at full whack. The PWM is a high speed switch that by switching the power on and off enables the motor to run at the desired speed. If the PWM unit fails the first question would be whether it is likely to fail on both channels and if so does it automatically drive to full power as a fail safe feature given it is way better to have the fans running flat out rther than not at all- I suspect it will do that but I do not know that for a fact.
When the ac unit head unit fails it typically does not switch the power on to the ac clutch solenoid so I doubt that will be the problem. If the fans run at full speed from the moment you switch the ignition on and the ac unit is switched off then that may suggest a failure of the final element [the PWM].
So..when the fans fired up on their own, car off and cold for _hours_ overnight..no key..no nothing.
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From: Oman
Are you sure both fans were running during the event you witnessed?
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The fans take power through dedicated cables from the battery, through 30 amp fuses and then live power hits the PWM unit- if the fans are powering up randomly then the only possible explanation is a failed PWM unit that is passing power straight through it- as I am aware there is no relay for the fans - why they did it that way I have no clue- maybe they viewed the PWM unit as equivalent to a relay given it is a switching function but I doubt such would have the reliability of a relay.
Are you sure both fans were running during the event you witnessed?
Are you sure both fans were running during the event you witnessed?
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The fans take power through dedicated cables from the battery, through 30 amp fuses and then live power hits the PWM unit- if the fans are powering up randomly then the only possible explanation is a failed PWM unit that is passing power straight through it- as I am aware there is no relay for the fans - why they did it that way I have no clue- maybe they viewed the PWM unit as equivalent to a relay given it is a switching function but I doubt such would have the reliability of a relay.
Are you sure both fans were running during the event you witnessed?
Are you sure both fans were running during the event you witnessed?
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From: Oman
Another point to consider is that the PWM is basically a slave unit in that there is no logic- it just does what it is told to do by the logic controller module. If the logic controller determines that one of the two fans is not running correctly it drives the single "good" unit at full speed. What this suggests to me is that if the logic controller develops a fault one wonders if that could drive the PWM unit to full speed even if the logic controller is not powered up. Although PLC units are very reliable once they do go wonky who knows what condition they may default to.
The obvious thing to do given your location and connections would be to swap out the logic controller and/or the PWM module if you can borrow such.
The obvious thing to do given your location and connections would be to swap out the logic controller and/or the PWM module if you can borrow such.
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From: Oman
I don't know how this system works completely, but will the 928 run the fans even if the car is off if the temp sensor is warm enough, like to cool things down after the car is shut down? If it works that way, could be a faulty temp sensor that's always closing the circuit.
When the car is shut down there is a temperature switch on the S4 inlet manifold- with the heat in the engine creating a heat soak condition if the switch detects a certain level of temperature increase it flips and this cuases the fans to run at something like 60% of full speed and they run until the switch senses a temperature drop back to the set point of the switch. Thus if that switch were faulty and there is no other controlling logic like a delay timer after shutdown,
The test that Jeff did was designed to prove whether the thing is working or not however that test would depend on whether the switch opens or closes. As I recall shorting the two cables causes the run on condition- either way it is easy to rule that one out.
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I went through this last Fall. Did you wash the car or was there a possibility anything in the engine bay got wet? It happened to me after a power wash to the exterior and I'm guessing some seepage in the engine bay. Was ready to send off the final stage unit for a fix (intake sensor didn't change anything, as you said). After 2 days of maybe drying out the problem went away and never returned. Just a guess.