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AC/Oil Cooler Ballast Resistors

mday

Well-known member
Joined
20 Mar 2008
Messages
444
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/rennforu...and-ac-fan-low-speed-resistor-redesigned.html

http://forums.rennlist.com/rennforu...sistors-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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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 :(
I noticed that they're made in Ireland .. That explains a lot :grin:
 
Nice one mday :thumb:

I'm surprised the OCD'ers on here haven't commented that your inner arches need scrubbing down! :grin:
 
My inner arches have been scrubbed.....................



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


:grin:
 
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...
 
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

8)
 
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
 
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'. :D :D
 
Hi mday and all other forum members and 993 owners. :)

Lurker since January and first post today. :oops: 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 :)
 
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
:?:
 
Drew said:
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
:?:

Tell me, does your old resistor say 4.5 ohms?

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

8)

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?
 
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-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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