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H0ME
Newbie


Joined: 21 Feb 2018
Posts: 38
Location: Peak District - South Yorkshire


PostPosted: Thu May 02, 2019 7:28 pm    Post subject: Now keeping charge - replaced heater blower resistor Reply with quote

I replaced 96461655002 the heater blower resistor the other week as I had an error code for it. It was pretty black. Forgot to hook up the CTeK. Went in today a week later and found that I still have full charge. Would a knackered one of those cause battery drain?
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1996 993 C2 Tip
 
  
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9xxnick
Monza


Joined: 29 Jul 2014
Posts: 184



PostPosted: Thu May 02, 2019 8:48 pm    Post subject: Reply with quote

I suppose it's possible, if there was a very high resistance short to earth and the circuit that feeds the blower is live with the ignition off.

Might be worth checking the inputs to the resistor with the ignition off in case there's 12V present, which would indicate a relay that's failed in the closed position.

I'm digging deep for the electrical thinking here, and am happy to be corrected if there's a logic fail.
 
  
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Luddite
Spa-Francorchamps


Joined: 18 Dec 2018
Posts: 328
Location: Scotland


PostPosted: Thu May 02, 2019 10:53 pm    Post subject: Reply with quote

HOME, I know nothing much about modern cars but I would be surprised if your HEATER resistor would discharge your battery when parked up with neither the heater switched on or more significantly your ign switched off, as I would not expect the heater circuit to be live in that situation ...?

Were the resistor part of a COOLING circuit in a turbo which I believe can be live after the ign is switched off and the fan(s) can run on dependant on engine temperature.... then perhaps, IF the engine temp sensor or the relay it controls or it`s wiring had an intermittent fault causing the fan to come on sporadically when the engine was cool...??

As for kn*ckered.... in terms of a resistor, it would either be open circuit (broken internal connection) or shorted out (perhaps physically damaged or burned out) ...

9xxnick a poor connection at the resistor or anywhere in it`s circuit could likely equate to a variable HIGH resistance and a fan running intermittently or at an even slower speed, if running at all.. Whereas if the resistor had shorted out internally there could be LOW resistance equating to the fan running at close to full speed... With a dead short across a faulty resistor the circuit would operate as if the resistor was not in circuit, thus the fan would be running at full speed.

Hope this makes some sort of sense... Question
 
  
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H0ME
Newbie


Joined: 21 Feb 2018
Posts: 38
Location: Peak District - South Yorkshire


PostPosted: Fri May 03, 2019 8:45 am    Post subject: Reply with quote

Luddite wrote:
HOME, I know nothing much about modern cars but I would be surprised if your HEATER resistor would discharge your battery when parked up with neither the heater switched on or more significantly your ign switched off, as I would not expect the heater circuit to be live in that situation ...?

Were the resistor part of a COOLING circuit in a turbo which I believe can be live after the ign is switched off and the fan(s) can run on dependant on engine temperature.... then perhaps, IF the engine temp sensor or the relay it controls or it`s wiring had an intermittent fault causing the fan to come on sporadically when the engine was cool...??

As for kn*ckered.... in terms of a resistor, it would either be open circuit (broken internal connection) or shorted out (perhaps physically damaged or burned out) ...

9xxnick a poor connection at the resistor or anywhere in it`s circuit could likely equate to a variable HIGH resistance and a fan running intermittently or at an even slower speed, if running at all.. Whereas if the resistor had shorted out internally there could be LOW resistance equating to the fan running at close to full speed... With a dead short across a faulty resistor the circuit would operate as if the resistor was not in circuit, thus the fan would be running at full speed.

Hope this makes some sort of sense... Question


As I understand this is part of the COOLING of the engine circuitry and would run with ignition off. Blows the heated air out under the engine.
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ToreB
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Joined: 13 May 2010
Posts: 583
Location: Oslo, Norway


PostPosted: Fri May 03, 2019 9:20 am    Post subject: Reply with quote

The rear fan resistor stops to work when the inbuilt thermoswitch fails and disconnect the circuit. There is no way problem with this resistor would cause a abnormal current drain.
However, the rear fan can run in cooling mode as long as the Climate Control Unit is active, for up to 20 minutes after ignition off. This depends on the temp reading in the rear air duct.
If low speed fails due to a non-working resistor, the CCU will default to high speed in order to maintain cooling and feed of heated air to the cabin.

Here's a web page that explains the most common current drain problems and how to fix them:
https://www.bergvillfx.com/porsche-964-993-battery-current-measurements/

Cheers,
Tore
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Luddite
Spa-Francorchamps


Joined: 18 Dec 2018
Posts: 328
Location: Scotland


PostPosted: Fri May 03, 2019 1:03 pm    Post subject: Reply with quote

HOME...ToreB is the undoubted expert here, of course my simplistic thinking had not known until now of a car HEATER circuit being designed to also to cool an engine, though have done that myself in my old and very basic sports car when it was running HOT while driving in Italy..... We were both over heated and my dear lady wife was not too happy when I prioritised adding heat to the passenger compartment in a bid to achieve a measure of engine temperature reduction..!!!

Ever in search of simplification..... It seems "climate control" is the way of it these days involving a ever more complex aspects of heating and cooling systems for the cabin, aspects of which have seemingly been PROGRAMMED to perform engine cooling after shut down...?

Not knowing the construction of the circuit, I imagine there will be a sensor which ToreB identifies as the "thermoswitch" it will probably be this device that controls whether the fan is switched on or not.... with a resistor in circuit, logic seems to suggest that the speed of the fan is desired to be controlled for cabin heating AND/OR engine cooling... are we talking the same fan...? If so, I guess there may also be auto control of flaps to divert the airflow from cabin to the car exterior when in engine cooling mode...?

The thermoswitch may be other than an off on switching device for the fan, and may have the capability to also control the fan speed, the thermoswitch having internal contacts that can switch in or out sections of the resistor dependant on temperature, and in so doing raise or drop the current available for the fan... which affects it`s speed..

While ToreB determines there is no way a problem with the resistor can create an abnormal current drain in this case, I would not be confident enough to make such a statement having discovered burned out resistors causing shorts and higher than expected current drains... BUT I have ZERO experience in Porsche terms in this regard thus would automatically give ToreB`s statement the credibility it deserves..

If there is indeed no way that the resistor could cause your discharge problem, then the thermoswitch becomes the next in line suspect HOWEVER if there are any relays in the circuit they too can hold the fan ON even after the thermoswitch has operated properly disconnecting the supply to the relay(s).... How you might ask..? Well the contacts in the relays can burn over time after many switching operations... The effect can then be to create an INTERMITTENT fault...as the relay contacts can stick on or not after each switching operation, the cause of which can be a real pain to determine... more so when relays these days can not be opened and contacts cleaned... Thus this where replacement takes over from attempts at repair.... Replacing the relay might be cheaper than the thermoswitch, though I might expect a garage to replace both rather than have the customer have to come back when the same fault occurred... Of course if the thermoswitch requires an engine out or any such nonsense, then economics is ever bound to play a part in such decision making... Question

Making any sense.. Question

More than happy to be corrected on any and all that I type, I come here to learn.

ToreB, if you read this far... Yours is a CRACKING web site... THANKS.
 
  
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ToreB
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Joined: 13 May 2010
Posts: 583
Location: Oslo, Norway


PostPosted: Fri May 03, 2019 3:23 pm    Post subject: Reply with quote

The Climate Control Unit use a thermistor (temperature dependant resistor to measure the rear air duct temperature. The CCU then runs the rear fan in low or high speed dependent on the temperature.

The thermoswitch on the rear fan low speed resistor was implemented after several engine fires. The resistor gets overheated if the fan motor fails mechanically, and the thermoswitch prevents overheating of the resistor.
All early 964 cars were called in to replace it with the safety resistor version.


You are right, the rear fan relay wears due to the large motor current, and sometimes stick on due to this. As the fan relay teminals are powered at ignition off, it could cause the rear fan to run until the battery goes flat.

Cheers,
Tore
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Bergvill F/X
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Luddite
Spa-Francorchamps


Joined: 18 Dec 2018
Posts: 328
Location: Scotland


PostPosted: Fri May 03, 2019 5:13 pm    Post subject: Reply with quote

ToreB, many thanks for taking the time to respond to my ramble on some possibilities relative to the operation of circuitry I have had ZERO hands on experience with. It is just that over the years I have had to repair many items of kit without the benefit of training, diagrams or manuals and using little more than a basic understanding of the kits operation when it would have been operational..... I did not always get it right first time..!!! Since digital electronics came on the scene my old fashioned and basic modus operandi became next to useless..

From that which you type ToreB the thermistor logically alters it`s resistance relative to the changing temperature around it in the duct, the thermistor being connected to further circuitry/components in the CCU which dependant on the resistance sensed in the thermistor then operates a relay or relays, to achieve an appropriate fan speed relative to the temperature sensed...?

If this as just a two speed situation then it would seem the resistor is a unit with two connections, unlike the three connection type used to achieve three speeds, whereby a direct supply to the fan bypassing any speed controlling resistor would provide max speed, the first slow speed or second slow sped option switched on or off via a relay for each speed...?

Again from your description it seems the thermoswitch is a... safety device.... that is not used to control the fan in any way relative to temperature in the duct... but to cut the supply voltage to the resistor if the resistor it`s self overheats due to a rise in current in the fan circuit caused by a seized fan bearing or similar not allowing the fan to run freely.

The next question would be... Given the thermoswitch is a safety device designed in as a result of experience gained due to engine fires... once switched.. is it possible to re-set it manually, or does it have to be replaced..?

Of course some temperature operated switches can re-set themselves, though perhaps less than ideal if they do so when they are used as safety devices..?

It is my understanding that the burning of relay contacts occurs when the contacts are switched to BREAK the circuit, if the unit being controlled has coils in it`s make-up (inductive) such as a DC electric motor. Perhaps replacement of such relays after a given amount of miles should be carried out as a general long term maintenance issue for those capable of such a task.... assuming the relays are plug in type and not soldered in to a circuit board within the CCU.... Hmm.. Question

As ever just thinking in type happy to be corrected if and where I may have got it wrong.. Thanks for reading this far.. Thumb
 
  
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