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Mark w

Joined: 08 Sep 2017
Posts: 12

PostPosted: Thu Oct 12, 2017 1:32 pm    Post subject: Heater problems ~(now with relay pictures) Reply with quote

Hi All

I'm new to this forum as I only recently acquired a 1994 C2 993 (non air conditioned)
After a short while of ownership I found that the heating system (which I hadn't checked during the purchase in the summer!) was not working.

it took me a week or two to get to the bottom of the problem with many thanks to the amazing Tore (Bergvillfx.com) who guided me to the solution. So I though I would post my findings to help anyone who may encounter this same issue.

The Fault:
1. with ignition on front blower fans would run when i turn the blower through the different speed on the CCU but when I try to turn the heater temperature knob up on the CCU the front blower fans cut out.
2. press the demist button on the CCU and the front blower fans run for a second then cut out.

what did next:
1. In the engine bay I removed the resistor from the heater duct and checked if this was ok (it read 0.5 Ohms across the terminals, so all
ok there!)
2. I removed the double relay outer case for the rear blower fan and then put it back in place. I then pressed each one manually, the high speed worked easily the lower speed required me to push the contact very hard but when it did eventually make contact the rear blower fan did run. on closer inspection of the relay once removed it was blackened and burnt out with no contact points left (photo of this I'll add tonight).
3. Then I removed the temperature sensor in the rear heater tube, visually it looked damaged and when applying a meter no resistance was found! so definitley trashed.

4. I replaced the relay and temperature sensors with new ones thinking this would fix everything. How wrong I was!

Next Fault:
so now I have a new problem, when I turn on the ignition I find the rear blower fan goes into full speed, when i turn the ignition off the rear fan stops and have no controls at all on the CCU!

What I did next:
1. check the continuity between the 2 temperature sensor wires in the engine bay to the pin connector on the rear of the CCU (G10, G18) to see if they were no longer joined, these were ok so definitely no breaks in the wires.
2. so the problem looked like the CCU itself, so I removed the CCU and slowly took it apart to look at PCB's inside.
3. Close inspection of the 2 main circuit boards show a track that had blown/lifted and could be traced back to pin G18 sensor ground.
4. I soldered a bridging wire to bypass this damaged track and then checked the connection to pin G18 and hey presto all good again.
5. Cleaned up the CCU while it was apart.(good opportunity to get the dirt from behind the slide controls)
6. Reconnected the 2 large connectors to the rear of the CCU (leaving the CCU out of the slot in the dash as I was not sure if it would have to come out again).
7 Ignition on, great news all functions of the heater now work perfectly and rear blower does not come on unexpectedly.
8. took the car for a run to get it hot and checked the heater works and directs the heat to the right places, all good so put the CCU back in the dash.

I assume the combination of temperature sensor and relay failing contributed to the failure of the PCB in CCU but I'm sure Tore will have a definitive answer to this.

I am now a very happy chap as previously my Porsche specialist diagnosed the problem as the CCU but wanted £1300 +vat for a new CCU plus fitting and to be honest that was not the only problem so I question if they would have ever discovered it?

Sorry if my description of the issue and resolution is basic but I hope it can help anyone else who encounters this issue

Many thanks again to Tore for his help! Thumb [/list]

Last edited by Mark w on Fri Oct 13, 2017 8:36 am; edited 1 time in total

close up of relay burnt out contacts
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Relay failure
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bridging/link wire
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larger view of broken track
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broken track
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Temperature sensor
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kerb scraper

Joined: 25 May 2012
Posts: 394
Location: Hertfordshire

1997 Porsche 993 Carrera 4

PostPosted: Thu Oct 12, 2017 4:57 pm    Post subject: Reply with quote

Great post Mark!

I absolutely love a comprehensive DIY fix at low cost that would have cost a fortune if left to a professional.

Credit of course to Tore - he's a legend: (repaired my cabriolet control module earlier this year).
1997 C4 Cabriolet, Arctic Silver, black interior, 18" hollow spokes, Bilstein PSS10s, RSRs, 200 cell cats, Mobil 1 sticker, LED to interior. Bergville t-lights.
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Watkins Glen

Joined: 24 Oct 2009
Posts: 2175
Location: Kent

PostPosted: Thu Oct 12, 2017 11:12 pm    Post subject: Reply with quote

Hi There

That is indeed very impressive.

I had the demist not working combined with front fan speeds falling when turning up the temperature just this evening. From reading the guides I understood that it was most likely a fault with the rear blower and indeed it was.

In my case we had moved the blower aside in order to access the CHT plug underneath and in doing so had dislodged a wire at the motor. Next time I will unplug and remove it.

It's all back to normal now. The symptoms made no sense at all until I read how the system works and how the rear blower interacts with the front fans.

Well done again with fixing yours.

993 C2 Tip 1994 187k miles. Black and Grey. M029 & Bilstein HD's, Elephant Racing RS bushes F&R, RS mounts, strengthened engine carrier, 100 cell cat inserts, HID's. Solid steering arms mod, top of the engine rebuilt, mapped by Wayne, suspension set up by Chris.
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Joined: 21 Sep 2015
Posts: 533
Location: Somewhere Nr Tunbridge Wells, Kent

PostPosted: Thu Oct 12, 2017 11:21 pm    Post subject: Reply with quote

Super post and congratulations on the purchase. I envy anyone with electrical skills!
1994 Aventura Grün C2 993
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Joined: 25 Sep 2004
Posts: 332
Location: Cornwall UK

PostPosted: Fri Oct 13, 2017 3:17 am    Post subject: Reply with quote

Thank you very much for taking the time to photograph and post
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Mark w

Joined: 08 Sep 2017
Posts: 12

PostPosted: Fri Oct 13, 2017 8:27 am    Post subject: Reply with quote

thanks everyone!, I hope it will be of use to others in the future
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Approved Trader

Joined: 13 May 2010
Posts: 561
Location: Oslo, Norway

PostPosted: Fri Oct 13, 2017 10:38 am    Post subject: Reply with quote

Congrats on a successful repair. Well done!
Thanks for the references, I am glad to help.

The reason for the CCU PCB fault is a short circuit in the car wiring somewhere. +12V has been connected to the Sensor GND circuit, causing the PCB track to blow like a fuse. This probably happened before you had your engine wiring loom changed under warranty.

Fan relay faults are commonly seen, this is normal wear and tear due to the large motor currents involved.

'95 993 C4

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