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jhrfc
Silverstone


Joined: 26 Apr 2016
Posts: 134


1994 Porsche 993 Carrera 2

PostPosted: Fri Aug 09, 2019 1:37 pm    Post subject: Rear blower eating resisitors! Reply with quote

Just noticed my rear blower is only running at full speed, indicating the resistor has gone.
Thing is, I had one of these in 2017, and then again just 5 months ago.
I also had a new temp sensor switch 5 months ago as well.
That can't be right can it?
Does it mean the fan itself is faulty on the way out? maybe sezing and or drawing too much current?
Don't know if Tore is lurking? did the self test form the fab guide on his site.
Cheers
Jon
 
  
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Luddite
Nürburgring


Joined: 18 Dec 2018
Posts: 488
Location: Scotland


PostPosted: Fri Aug 09, 2019 3:49 pm    Post subject: Reply with quote

Jon, until someone who knows what they are typing about comes along, I have time to spare and had a few thoughts re your question, though whether they help or not....hmm..?

I think Tore explained this circuit well and not too long ago... I looked but did not trip over it, and can not remember his circuit description or the address of his website... Duh!

Seems you think the resistor is the problem and site possible high current as the cause of the problem... but the fan is running at full speed.. To me that would seem to indicate that the resistor may not be in circuit, and if that is the case then perhaps it can be switched in or out of the circuit dependant on some sort of thermal switching device/sensor/thermistor placed near the heat source.. which may increase the speed of the fan relative to temperature.... and as it has been rather hot of late.. perhaps if the sensor is mechanical, it has stuck on... That being the case it seems the resistor may be OK, and that the switching device is at fault..

Usually when power resistors burn out their resistance goes high or the resistor fails to open circuit (no connection) in which case the fan would not run if the resistor was switched into circuit.. The only other guess I would have could be that the fan is designed to fail into the full speed situation to avoid overheating of that which it is designed to cool or heat.

hope I have not added confusion to an already confused situation.. Question
 
  
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Pretzel
Hockenheim


Joined: 20 Apr 2014
Posts: 700
Location: Bedfordshire


PostPosted: Fri Aug 09, 2019 10:06 pm    Post subject: Reply with quote

Possibly the thermo-switch is faulty which Tore indicates is meant to protect the resistor. (or is that the temp sensor you are referring to?)

Where did you source the replacement resistor from?



Edit, just noticed Luddite's similar reply, apologies
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993 C2 Aventura
 
  
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Luddite
Nürburgring


Joined: 18 Dec 2018
Posts: 488
Location: Scotland


PostPosted: Sat Aug 10, 2019 7:14 am    Post subject: Reply with quote

Please, no apology necessary Pretzel..

It would be handy to understand the function of the components in the circuit.... if only I remember that which Tore typed...Duh !
 
  
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Pretzel
Hockenheim


Joined: 20 Apr 2014
Posts: 700
Location: Bedfordshire


PostPosted: Sat Aug 10, 2019 8:49 am    Post subject: Reply with quote

https://www.bergvillfx.com/porschehvac2
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Pretzel
Hockenheim


Joined: 20 Apr 2014
Posts: 700
Location: Bedfordshire


PostPosted: Sat Aug 10, 2019 8:50 am    Post subject: Reply with quote

Duplicate post
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993 C2 Aventura

Last edited by Pretzel on Sat Aug 10, 2019 8:51 am; edited 1 time in total
 
  
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jhrfc
Silverstone


Joined: 26 Apr 2016
Posts: 134


1994 Porsche 993 Carrera 2

PostPosted: Sat Aug 10, 2019 12:11 pm    Post subject: Reply with quote

thanks all, as i said both the resistor and thermo swtich where replaced 5 months ago.
parts/ work was from by JZM.
i'll see if I feel bold enough to reset the switch as in tore's guide.

J
 
  
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Luddite
Nürburgring


Joined: 18 Dec 2018
Posts: 488
Location: Scotland


PostPosted: Sat Aug 10, 2019 1:57 pm    Post subject: Reply with quote

J, I have to say that Tore is some guy to create all that valuable information without him and the likes of DeMort around trying to repair and maintain Porsches would be a whole lot more difficult if not impossible in some cases.

I had a quick read at Tore`s guide though admit not to have taken all the time I need to fully understand the design concept of the system but at least I had remembered that the system provides two possible functions, as both a cabin heater if desired though far more importantly cooling for the engine.

That kind of resistor is very unlikely to burn out though it`s terminals could suffer in an overload situation or just down to crud build up over time...

The re-set you mention is likely to be no more than a mechanical trip switch that is operated by it sensing over temperature, probably made up of a bi-metal strip of metal that will bend to a certain degree and at a defined temperature.... most such devices require manual reset as they are fail safe and generally in use as a last line of defence when whatever thermostat controlling the device fails to switch off when the appropriate temperature is achieved.... Thus the trip prevents overheating of the device it is created to protect.. Generally wen resetting such a device a light click may be heard as it`s latch re-sets, to reconnect it`s electrical contacts.

From a quick read through it seems the ECU plays a part, though exactly how it is programmed I admit to not having taken the time to figure out, I am a slow learner, and get easily digitally frustrated ..(-:

I am guessing that by thermo switch, you are referring to what I know as the thermal overload protection, or the trip switch described above... That being the case then it seems possible that which should control the switching of the fan relative to it`s design function is not operating correctly.

Again with an ECU in part control, there are considerations relative to it`s inputs and outputs, though if the fan is running at full speed then the resistor and it`s thermal trip device would not seem to be the problem for if the fan is requested to run at slow speed and failed so to do it would seem unlikely to run at all of connections at or to the resistor had failed.. UNLESS as part of the design of the fan circuit it was also designed to fail safe in terms of engine cooling, thus designed to run on as opposed to shut down...

Yeah I need to go and work my way through Tores great write up to try and gain further understanding... Duh!

Give the re-set a go if you feel up to it, some tec stuff can be relatively easy, if you take TIME to try and understand what it does and what you are trying to achieve...

Let us know how you get on... Good luck. Question
 
  
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ToreB
Approved Trader


Joined: 13 May 2010
Posts: 607
Location: Oslo, Norway


PostPosted: Sat Aug 10, 2019 6:19 pm    Post subject: Reply with quote

Hi guys, thanks for all the kind words.

The resistor is used to run the fan motor in low speed. The Climate Control Unit controls and supervises the rear fan.

The resistor has an inbuilt thermoswitch that will be tripped if the fan current is too high. This has nothing to do with regulation of temperature, but it is a safety feature that will prevent overheating of the resistor if the fan seizes for some reason. There was a number of engine fires before Porsche recalled all 964's and replaced the resistor with the new design. This was also used in the 993.

The fan motor has sintered brass bearings, and if they are worn-out, they will heat up and seize the motor shaft. The fan may therefore seem OK when cold.
I would recommend to change the fan in this case, a JP Parts replacement fan it does not cost too much, about £150.

If the resistor thermoswitch "fuse" is tripped, the CCU will run the fan at full speed instead as a safety measure. The two main cabin front blower fans will in addition be shut off to signal the problem.

Cheers,
Tore
_________________
'95 993 C4

Bergvill F/X
Porsche electronics specialists, products and repair services.
 
  
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decgraham
Barcelona


Joined: 17 Sep 2008
Posts: 1362
Location: Spain


PostPosted: Sun Aug 11, 2019 7:33 am    Post subject: Reply with quote

My rear motor would squeel like a pig some days and I also had problems with the rear resistor. I changed the rear motor in 2014 and all was fine after that. Thumb

£90.88 on Ebay (No affiliation)

https://www.ebay.co.uk/itm/INTERIOR-BLOWER-OE-QUALITY-REPLACEMENT-BOSCH-0130111145-/123792983609

ATB Smile
_________________
1995 993 C2 Cab TipS in Guards Red
 



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jhrfc
Silverstone


Joined: 26 Apr 2016
Posts: 134


1994 Porsche 993 Carrera 2

PostPosted: Sun Aug 11, 2019 2:00 pm    Post subject: Reply with quote

Well after whiling away a few hours troubleshooting, i think the title of the thread is wrong!
First i removed the resistor and temp sensor, and had a look, both looked fine (as they are only 5 months old they should do).
Then i whipped the rear fan relay out and did some testing with a multimeter, and a shorting cable i made from a bit of mains wire an a couple of male spade connectors.
From the Tore guide :
short terminals 30C and 87C for low fan speed : Check, low fan starts.
short terminals 30 and 87 to start the fan at high speed : Check high fan starts
Hmm
Test the CCU control signals to the relay by measuring on terminal 85 and 85C. These will be grounded by the CCU at high and low speed accordingly.
Put on full heat and fan speed 4 connector 85 is indeed grounded.
Put full heat and fan speed 1, connector 85c not grounded, but 85 is.

To me this points to the CCU not doing what it should (or perhaps a wiring fault).

Definitely reaching the end of my expertise here, I guess the next step would be to send the CCU to someone like Tore to test / fix.
Or might an OBD check shed some light?
One for the winter i would think. Probably not really any issue for me, as I my default preference for the fan speed is 2 anyway.
Thanks
Jon
 
  
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Luddite
Nürburgring


Joined: 18 Dec 2018
Posts: 488
Location: Scotland


PostPosted: Sun Aug 11, 2019 2:48 pm    Post subject: Reply with quote

Jon, I would think the next logical check is to connect an OBD reader, which I imagine may point to an incorrect output from the CCU. The OBD test would seem the simplest option. I think you did well to get to this stage of the game Thumb
 
  
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ToreB
Approved Trader


Joined: 13 May 2010
Posts: 607
Location: Oslo, Norway


PostPosted: Sun Aug 11, 2019 8:37 pm    Post subject: Reply with quote

Maybe this is not a CCU problem. I seldom see problems in the relay driver outputs, but it does happen from time to time.

If you pull the relay, the fan motor will not run. As mentioned above, the CCU supervises the fan, and a non-op fan will be sensed as a fault.
Without a fan relay, the CCU will therefore just briefly try low speed (just half a second or so, (if at all) and default to full speed only until reset. Just as you have observed.
The CCU resets if the ignition has been off for 20 minutes, and will only then try low speed again.
You will not be succesful if you provoke rear fan fault codes in the CCU by pulling the relay, and then try to get it working on low speed within 20 minutes.

It is very difficult to determine a faulty/tripped low speed resistor thermo switch with eyesight. Use an ohmmeter on the two terminals, if you measure an open circuit, it is time for changing the resistor.
I never bother to try to clean or reset the thermo switch, I just change the resistor.

I advise to do the following:
Ohm the low speed resistor
Reinsert fan relay
Closely inspect the fan motor, check that the rotor runs freely, and that the bearings does not fail when hot.
Briefly disconnect the battery to reset the CCU
Do the rear fan test as shown in my DIY page:
https://www.bergvillfx.com/index.php/porsche-964-993-ventilation-system-test

Cheers,
Tore
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Bergvill F/X
Porsche electronics specialists, products and repair services.
 
  
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jhrfc
Silverstone


Joined: 26 Apr 2016
Posts: 134


1994 Porsche 993 Carrera 2

PostPosted: Sat Aug 17, 2019 10:22 am    Post subject: Reply with quote

got the car back and it has stopped raining for long enough to look at it!
the car has had the relay fitted back all week.
left car ignition off for half an hour after moving it out of the garage.
continuity tested the two terminals of the resistor, continuity.
turn ignition on with temp on max cold, fan switch on 0. no rear blower.
turned temp dial up to max hot, rear blower starts on fast, even with fan switch on 0. The blower does seem to pulse down to slow, and then back up to fast.
the blower speed does not alter at all changing the fan speed selector.
turn back down to max cold, blower turns off.

A bit lost, have a t-obd on order. However my garage said they cleared 'a lot' or ccu error codes when it was with them.
The rear blower does not seem to make any nasty noises as it runs, even when it runs after the hot car is stopped.
hmm.
 
  
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jhrfc
Silverstone


Joined: 26 Apr 2016
Posts: 134


1994 Porsche 993 Carrera 2

PostPosted: Fri Aug 23, 2019 4:33 pm    Post subject: Reply with quote

OK so got the t-obd now and read the codes, i had rearblower motor, temperature sensor , and speed 1 and speed 2 codes. so i cleared them all.
then i ran the tests on the fan, same results as above.
checked the codes agian and i now have just the speed 1 and sped 2 codes.
Resistor doe snot have open circuit.
hmm.
 



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