Garth S

" Redesigned" may be a bit pretentious ... but here is a DIY to eliminate the miserable OEM ballast resistors that are doomed to fail..... oh, and they work too!

This project started recently when I pulled the nose, and found the AC resistor a crumbled mass of ceramic bits, described here. Thinking that that approach ( in the linked post) was the cure, cold reality struck when the good looking oil cooler resistor was subsquently tested - and found to be a useless 300 ohms ( vs. 0.45 ohms spec) ie. - effectively an open circuit, meaning no low speed operation.

So, what to do??? I searched for contemporary power resistors, and Paul902 shared PM's with me as we compared notes as how best to work around this questionable item: the OEM ceramic resistor is rated at 0.45 ohms with an unspecified precision (%), and a power dissipation of 52.5W.
Conventional power resistors are encased in finned aluminium channel, and are made to be heat sinked:looking in this direction, I selected pairs of 1.0 ohm(1%) 50W units for each ballast resistor module, for when wired in parallel, that provides a precise 0.5 ohm with 100W dissipation capability .... ie., a pair can easily 'drop' the voltage for low speed fan operation - and never break into a sweat!
Using some scrap 1/8" aluminium plate, two heat sinks of ~1x1x4" were formed, and drilled and tapped for short M3x 0.5mm screws . With a thin smear of heat sink paste ( kindly provided by Paul ), the units were screwed together, and wired up to the recovered connectors from the ceramic units. With a 8mm hole punched in the plate, they are mounted on the lower bracket stud indicated, and sit within a seperately double walled chamber of the front fender liner [ apparently, this is a separate fresh air duct, or exhaust, for the round nozzle shown above the mounting point].

Relays R04 and R14 were pulled to test low & high speed operation on both Oil & AC coolers - perfect, and absolutely cool operation of the paired resistors in low speed mode.

The pictures tell the story .... but before anyone chips in with a , " why, you cheap SOB - why non flip the $80-$100 and buy real Porsche parts?? - they're the best, you #@$& moron !! " .... in my defense, I suggest that even new OEM ceramics will soon fail - why?? Take a close look at the middle ceramic in the first pic. The resistor is a short coil of nichrome wire crimped to a fusible link ( silver capsule shown), with dual leads crimped at either end - all in steel crimps: the whole assembly is potted into the ceramic cup with a porous, plaster like compound - unsealed to moisture. The telltale rust marks showing on the 'good' looking resistor on the right indicate its demise> Enough said - they are a piece of crap, dying to suck up moisture to rust out at the core.

Excuse the wiring job - this is a development project after all: that will be tidied up later.

epj993

Wow Garth - great work! What was the net cost for this mod?

dcdude

To the contrary, Garth. This is bad-***! I don't think any OEM has 100% infallable parts, and this is a known weakness.

Do you have links to where you bought the resistors? I assume you recycled your original leads, right? Anything else besides the bracket stock to add to the shopping list?

(Uh, oh...sounds like I'm about to "fabricate" )

tj90

Garth. I love the ingenuity in putting those together. I will go this route next time I replace the resistors.

gonzilla

Good job man!!!

brucec59

Edited again: This is 1 ohm 50 watt. Jeez, what's happened to my brain?!?
http://www.mouser.com/Search/Product...zEddl4Ow%3d%3d

I replaced both of my resistors not long ago, and I went with stock parts in the stock locations.

AOW162435

Garth,
You're a bloody genius!


Andreas

nile13

I've been thinking about doing this for a while (well, need to use that EE degree somewhere).

Garth, you could and probably shoud make a cople dozen sets of these already attached to the aluminum bracket and wired in parallel. I'm sure Rennlisters would happily pay $20-25 a pop instead of sourcing their own resistor, bracket, paste, soldering, etc.

Great implementation!

rufstuf

Nice solution to replace a flawed design (dare I say) from Porsche.

nile13

It's not just Porsche. BMW had a very similar ceramic A/C resistor on their E30s, for example. It wasn't round and was easier to get to, but it would crumble just the same and cost $40+.

murfysflaw

Bloody brilliant!

Garth S

Hahaha - yes, the thought of mass production has crossed my mind in a Walter Mitty fantasy

The reality is that a single 0.5 ohm/50W resistor mounted on a heat sink should be several times better than the stock ceramic: I went for overkill .... 100W dissipation via some available 1.0 ohm resistors; however, that may be the better 'life of the car' type of a solution. Any "EE" thoughts on that???

Either way, resistors are the primary cost element. I would then turn to proper heat sink extrusions, or formed aluminium channel stock. The other items are readily available .... with the exception of the connectors. The latter is an ITT Cannon-G SS2P (male) end for 16g wire, likely available commercially in boxes of 5000 .... so I was happy to reuse the old ones.
Assembly is pretty obvious, and the only PITA step is tapping the M3 threads: if one were to assemble several units, I would set up a layout jig for the drill press, because PITA #2 was center punching the screw holes.

So, for you experts, is a single 50Watt dropping resistor fixed to a modest heat sink set in an ambient air stream adequate for the job? - or would you go for twice the wattage rating?


BTW - appreciate all the comments in the posts above!

perlfather

careful because this part number is a 100 Ohm resistor not 1 ohm

99three

I will be replacing the resistor in a few weeks. I'd be happy to fit the redesigned unit if you're building them Garth. Awesome work!

perlfather

As for the power dissipation safety factor - what is the total current? One has to look at the circuit to figure it out. Let's assume it is 10a than the voltage drop is 5V and the power is 5*10 or 50W. I assume the current cannot exceed 20a due to fuse size (what size is the fuse size?) so 100W would be OK (barely) even with 20A.

I think using this type of resistor is far superior to a ceramic one, however:

1. great care should be taken to insure that the Wattage is not exceeded as one has to insure that the heat dissipation is not excessive. The wirewound precision resistors can get very, very hot. A fire can be easily caused by such a resistor even when it functions correctly and especially if it fails.

2. One should check on the failure mechanisms to insure that under no circumstances will this heat up excessively (when it fails) and if it fails it will just fail as the ceramic ones do with an open circuit!

This is a great idea, however, keep in mind that the Porsche engineers were not stupid. Keep thinking of failure not just steady state!