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mday
Nürburgring


Joined: 20 Mar 2008
Posts: 433
Location: Deepest Darkest West Sussex

1996 Porsche 993 Carrera 4S

PostPosted: Mon May 04, 2009 11:06 am    Post subject: AC/Oil Cooler Ballast Resistors Reply with quote

Decided to tackle the air con & oil cooler, ballast resistor replacement. I was keen to try the fix as detailed on RennList as it looks as though this will be a life-of-the-car-fix.

Briefly each side consists of:
Heatsinks made from angle aluminium: 4” x 1 ¼” x 1 ¼” x 1/8”;
2 power resistors 50w 1 ohm;
1 thermal switch, 70c 50c reset;
Heat sink compound;
6 x M3 6mm machine screws.

You’ll also need connectors; crimp-pliers; suitable wire; heat-shrink; hot air source for heat shrink; 2.5 mm & 8.5mm drill; & M3 taps.

Further details here:

http://forums.rennlist.com/rennforums/993-forum/471734-ballast-resistor-oil-and-ac-fan-low-speed-resistor-redesigned.html

http://forums.rennlist.com/rennforums/993-forum/486022-ballast-resistors-oil-and-ac-low-speed-redesign-ii.html


First thing was to determine whether the resistors were working or not & ensure the fans worked. Fuse/relay box cover off & identified relays R04 (oil cooler fan) & R14 (A/C condenser fan).

Caution: Keep in mind the relay terminals are energized 12v at terminal 3(30) even with the ignition off.

-Remove relay R04 (oil cooler blower).
-Jump terminals 3(30) & 7(87c) for slow speed. Fan should run, even with the ignition off. If the fan runs in slow speed, your ballast resistor should be okay.
-Jump terminals 3(30) & 5(87) for fast speed. Fan should run, even with the ignition off.
You can check the A/C condenser fan similarly, removing relay R14.

My fans worked on high speed OK but were dead at slow speed.

Jacked the front of the car up & removed the front section of the wheel arch liners. Starting with the A/C (LHS) side I discovered how bad that resister was. The leads fell out (lucky it hadn’t shorted out!) & the ceramic crumbled away. The date said 96/01 so maybe this is the original? The bolt was well rusted in so I left the remnants behind, as it didn’t get in the way.

Made up the resistor package & bolted to the frame. The cut off fly-lead from the dead resistor was used to connect up to the plug. Tested at R14 & the fan now works in high & low speed.

Similar on the other side, but because the resistor is mounted well forward it’s impossible to access without removing the front bumper cover. Identified the wire & gave it a pull to disconnect the plug. The plug was cut off & the resistor package bolted to the frame & connected to the wiring with spade connectors. Tested at R04 & this fan also works in high & low speed.

The resistors do warm up in use but as they are wired in parallel the two 1 ohm resistors give an overall 0.5 ohm resistance but with double the power capacity of the OE. I tested them for 5 minutes on slow speed. I could touch the resistors for a few moments but the heat sinks were doing their job well as they were only warm to the touch. If they did overheat, or there was some internal failure, the thermal switch would cut the power at 70c. The resistors are also mounted in an air stream so should be sufficiently cooled whilst the car is moving.
 
  
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ballcock
Sepang


Joined: 21 Jul 2008
Posts: 2997
Location: Dublin

1996 Porsche 993 Carrera 2

PostPosted: Mon May 04, 2009 3:55 pm    Post subject: Reply with quote

Hats off to you Mday , that's a great job well done.
I wouldn't have the electrical knowhow to do what you did (nor the tools for the job) , so I fitted the standard resistors .. They should last another 5-10 years Sad
I noticed that they're made in Ireland .. That explains a lot Grin
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'96 Iris C2 vario manual coupe, Koni FZD's/M033's RSR's and HID's.
‘98 Ocean blue 996 C2 Tip pressed into daily service.

Ex '83 W460 LWB G Wagen updated with OM602 engine and 6 speed 'box
Ex. '99 996 C2 manual coupe.
 
  
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993C2S
Zolder


Joined: 05 Aug 2005
Posts: 5996
Location: Berkshire

1997 Porsche 993 Carrera 2S

PostPosted: Tue May 05, 2009 10:07 pm    Post subject: Reply with quote

Nice one mday Thumb

I'm surprised the OCD'ers on here haven't commented that your inner arches need scrubbing down! Grin
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Asim
 
  
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mday
Nürburgring


Joined: 20 Mar 2008
Posts: 433
Location: Deepest Darkest West Sussex

1996 Porsche 993 Carrera 4S

PostPosted: Tue May 05, 2009 10:20 pm    Post subject: Reply with quote

My inner arches have been scrubbed.....................



...............just not as well as some of the others.


Grin
 
  
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Porsche
Donnington
Donnington


Joined: 31 Dec 2002
Posts: 12733
Location: Nürburgring Nordschleife

2003 Porsche 996 GT3 Mk2

PostPosted: Tue May 05, 2009 10:35 pm    Post subject: Reply with quote

mday, very very impressive

time to open your own garage me thinks !

Oil Cooler Ballast Resistors on my old 993 was the only think to ever go wrong and was a real pain to fix...
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964 Project Car
996 MK2 GT3 CS Manthey
993 Targa
 
  
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Drew
Nürburgring


Joined: 28 May 2004
Posts: 433



PostPosted: Wed May 06, 2009 7:25 am    Post subject: Reply with quote

tested them both last night and my oil cooler ballast is fine but the aircon one has no low speed function, great....

could this be why my aircon never really gets going? I had assumed it was running out of gas. I'm onto this mod, nice looking pair..... of resistors

Cool
 
  
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mday
Nürburgring


Joined: 20 Mar 2008
Posts: 433
Location: Deepest Darkest West Sussex

1996 Porsche 993 Carrera 4S

PostPosted: Wed May 06, 2009 11:22 am    Post subject: Reply with quote

Drew

My A/C works better now that the low speed fan is working. Think about it: if the heat can't be properly dissipated, through the condenser, then you won't get good cold air. I also suspect that the low speed fan is on 90% of the time the A/C is in operation.

It would be nice to have some dashboard warning of fan operation. I was thinking yellow warning lights, maybe placed in the spare holes in the dash: dim for low speed & bright for high speed.

Anyone want to design the circuit?

Sundeep

Half the fun is creating these bits oneself. It's actually an easy excercise, although fiddly & time consuming.

Mick
 
  
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Zuffenhausen :O)
Sepang


Joined: 15 Mar 2009
Posts: 2848
Location: Hampshire


PostPosted: Wed May 06, 2009 4:49 pm    Post subject: Reply with quote

Big respect Mick, a job well done, I hate electricals with a vengeance, nothing but the spawn of Satan, so I doff my cap to you; erm are you thinking of putting these resistors into production? If so what's the 'charge', not too expensive I hope, don't want the missus to wont blow a 'fuse'. Very Happy Very Happy
 
  
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Achtung
Kyalami


Joined: 22 Apr 2009
Posts: 1995



PostPosted: Wed May 06, 2009 6:32 pm    Post subject: Reply with quote

Hi mday and all other forum members and 993 owners. Smile

Lurker since January and first post today. Embarassed I bought my 993 Targa late December and am thrilled with it! Will post some photos when I get a chance. Thumb

Anyway, great article mday and seems much cheaper to manufacture than the original ballast resistor!

I have found this power resistor on the RS site.
Would two of these do the trick if they are connected in parallel?

http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0158301#header

Can you please post a link to the thermal cut out and the heat sink?

Also, did you attach the thermal cutout to the resistor bodies or onto the heat sink?

Many thanks,

Achtung Smile
 
  
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Drew
Nürburgring


Joined: 28 May 2004
Posts: 433



PostPosted: Wed May 06, 2009 8:02 pm    Post subject: Reply with quote

got stuck in tonight and took the front wheel arch lining out by the a/c condensor. One of the wires had already fallen off the resistor so the problem is quite clear!! Getting the old resistor is quite a job, came out in several parts in the end.

So, as to putting a new resistor in the circuit.... what is the advantage of going for two 1 ohm resistors in parallel? Why not go for a 0.47 ohm resistor. I found a 0.47 ohm resistor with a 100 watt power rating at RS, why not just use that?

I thought I might bolt a single resistor to a heat sink (= chunk of steel) and then bolt that in the original location on the condensor shroud
Question
 
  
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Achtung
Kyalami


Joined: 22 Apr 2009
Posts: 1995



PostPosted: Wed May 06, 2009 8:07 pm    Post subject: Reply with quote

Drew wrote:
got stuck in tonight and took the front wheel arch lining out by the a/c condensor. One of the wires had already fallen off the resistor so the problem is quite clear!! Getting the old resistor is quite a job, came out in several parts in the end.

So, as to putting a new resistor in the circuit.... what is the advantage of going for two 1 ohm resistors in parallel? Why not go for a 0.47 ohm resistor. I found a 0.47 ohm resistor with a 100 watt power rating at RS, why not just use that?

I thought I might bolt a single resistor to a heat sink (= chunk of steel) and then bolt that in the original location on the condensor shroud
Question


Tell me, does your old resistor say 4.5 ohms?

Also could you post a link to the o.45 ohm , 100W resistor?
 
  
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Drew
Nürburgring


Joined: 28 May 2004
Posts: 433



PostPosted: Wed May 06, 2009 9:19 pm    Post subject: Reply with quote

the old resistor somewhat bought the farm as it was being oh so carefully removed Floor

so no idea as to the resistance, although one wire had become 100% unstuck so I think the question is a bit academic

this is the 100W resistor
http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0188065

they also do one at 150W for a couple of quid more
http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=3091712
 
  
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Achtung
Kyalami


Joined: 22 Apr 2009
Posts: 1995



PostPosted: Wed May 06, 2009 9:45 pm    Post subject: Reply with quote

Drew wrote:
the old resistor somewhat bought the farm as it was being oh so carefully removed Floor

so no idea as to the resistance, although one wire had become 100% unstuck so I think the question is a bit academic

this is the 100W resistor
http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0188065

they also do one at 150W for a couple of quid more
http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=3091712


The reristor that I removed has 4.5 ohm , 52.5W written on it yet the circuit diagram says 0.45ohm. This puzzles me! Dont know

I think I will go for the 100W resistor. That gives a factor of safety of almost 2 if used with a heat sink as the specification of the original resistor is 52.5W.
The good thing about your resistor is that even if you do not use a heat sink, it still will tolerate 50 W of power.
 
  
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Achtung
Kyalami


Joined: 22 Apr 2009
Posts: 1995



PostPosted: Wed May 06, 2009 10:07 pm    Post subject: Reply with quote

Drew wrote:
tested them both last night and my oil cooler ballast is fine but the aircon one has no low speed function, great....

could this be why my aircon never really gets going? I had assumed it was running out of gas. I'm onto this mod, nice looking pair..... of resistors

Cool


Hi Drew,

Even if the aircon balast resistor is open circuit the aircon should work because the high speed fan setting is still available.

Can you turn the outer pulley on the compressor?

Does the clutch on your compressor engage when the engine is running and the aircon switched on?
 
  
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mday
Nürburgring


Joined: 20 Mar 2008
Posts: 433
Location: Deepest Darkest West Sussex

1996 Porsche 993 Carrera 4S

PostPosted: Thu May 07, 2009 8:29 am    Post subject: Reply with quote

The OEM Porsche resistor is 0.45 ohms & 52.5 watts.

Going for a 100 watt resistor gives twice the power dissipation & therefore a good safety margin.

The thermal switch is connected to the heatsink.

It would be possible to use a 100watt 0.47 resistor but check the overall dimension. It'll be quite large & won't fit on the heatsink as designed. Power resisters get quite hot in operation so need to be on a good heatsink.

This is where one can get all the bits:

Resistors: http://cpc.farnell.com/1/1/23704-resistor-ww-50w-5-1r-wh50-1r-ji-welwyn.html

Thermal Switches: http://cpc.farnell.com/honeywell-s-c/2455r-01000074/thermal-switch-n-c-70c-55c-reset/dp/SN35992?_requestid=1044612

Heatsink Paste: http://cpc.farnell.com/ambersil/200001035/heat-sink-compound-35ml-syringe/dp/SA01504

Heatsink: http://www.metals4u.co.uk/detail.asp?Cat_ID=12&Prd_ID=26 Or find a local supplier for this.

From my understanding of the A/C operation I think the low speed fan is on for 90% of the time the A/C is in operation. The high speed fan only comes on when things get very hot. If the low speed fan doesn't work then the cooling effect is reduced, especially when the car is at a standstill.
 
  
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993C2S
Zolder


Joined: 05 Aug 2005
Posts: 5996
Location: Berkshire

1997 Porsche 993 Carrera 2S

PostPosted: Thu May 07, 2009 10:34 am    Post subject: Reply with quote

mday wrote:
From my understanding of the A/C operation I think the low speed fan is on for 90% of the time the A/C is in operation. The high speed fan only comes on when things get very hot. If the low speed fan doesn't work then the cooling effect is reduced, especially when the car is at a standstill.


+1
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Achtung
Kyalami


Joined: 22 Apr 2009
Posts: 1995



PostPosted: Thu May 07, 2009 11:17 am    Post subject: Reply with quote

mday wrote:
The OEM Porsche resistor is 0.45 ohms & 52.5 watts.

Going for a 100 watt resistor gives twice the power dissipation & therefore a good safety margin.

The thermal switch is connected to the heatsink.

It would be possible to use a 100watt 0.47 resistor but check the overall dimension. It'll be quite large & won't fit on the heatsink as designed. Power resisters get quite hot in operation so need to be on a good heatsink.

This is where one can get all the bits:

Resistors: http://cpc.farnell.com/1/1/23704-resistor-ww-50w-5-1r-wh50-1r-ji-welwyn.html

Thermal Switches: http://cpc.farnell.com/honeywell-s-c/2455r-01000074/thermal-switch-n-c-70c-55c-reset/dp/SN35992?_requestid=1044612

Heatsink Paste: http://cpc.farnell.com/ambersil/200001035/heat-sink-compound-35ml-syringe/dp/SA01504

Heatsink: http://www.metals4u.co.uk/detail.asp?Cat_ID=12&Prd_ID=26 Or find a local supplier for this.

From my understanding of the A/C operation I think the low speed fan is on for 90% of the time the A/C is in operation. The high speed fan only comes on when things get very hot. If the low speed fan doesn't work then the cooling effect is reduced, especially when the car is at a standstill.


So ... Best to establish if the aircon is working, but at slightly reduced efficiency or not at all!
 
  
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Drew
Nürburgring


Joined: 28 May 2004
Posts: 433



PostPosted: Thu May 07, 2009 12:43 pm    Post subject: Reply with quote

very off topic, but yes everything does seem to work ok for the aircon, but cold air is a bit rare. I'll fix the resistor and report back

cheers all
 
  
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993C2S
Zolder


Joined: 05 Aug 2005
Posts: 5996
Location: Berkshire

1997 Porsche 993 Carrera 2S

PostPosted: Thu May 07, 2009 2:17 pm    Post subject: Reply with quote

Drew, while the resistors will definitely need doing, I suspect you have very little gas in the system of your car, which suggests a leak. It's quite easy to diagnose for a specialist/air con company.

Air con condensors are usually shot by this age and may well be the culprit, but a proper diagnosis will tell you for sure.
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jonnyoutram
Trainee


Joined: 29 Oct 2008
Posts: 52
Location: GUILDFORD, SURREY


PostPosted: Wed May 13, 2009 4:22 am    Post subject: Reply with quote

could any of these resistor things be causing my aux heater fan in the engine bay to stay on? i get out the car lock the door walk down the road and the fan is still blowing!!!!!!!!!! dead battery soon to follow! Confused
 
  
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