StanAE86

re: http://www.porsche964.co.uk/technical/noheater.htm

I don't know a lick about electronics, so hopefully someone here can tell me whether I can use a 50ohm 25watt resistor in place of the 51ohm 15watt resistor? The person at the electronics place says that resistors have a 1% variance so the 50ohm should be fine for a 51ohm specification, which is what I read somewhere else. What I'm more concerned with is the recommendation that the 25 watt one available will be okay to run in place of the 15watt the article calls for. The person told me that the 25 watter will run cooler and probably last longer, but I don't know and would rather here it's "okay" from a car guy that knows electronics before I start soldering stuff in the car.

Any help is appreciated. Thanks.

darth

yes - but it's safer to crimp or mechanically connect along with solder as that much expected power could melt a solder joint.

Hope this helps
Bill

elbeee964

^-don't know about such things but sounds like a "it can't hurt" tactic.

Power dump (P) of your resistor (watts) is purely set up by it's resistance (R), the voltage drop across it (V), and the resultant current flow (I). (This power dump is not the same as its power rating... more on this below.)
Here's the laws re DC power, volt drop, resistance, and current flow:

P= V x V / R
P= I x I x R

For the most part R stays the same across a resistor regardless the circuit it's dropped in to. So, it's easier to think of only V & I being the only 'varying' players in the above power equations for your resistor.

Power rating is just the resistor's rated power dumping capacity.

Regardless its rated capacity, the same 'heat' (P) is going to be given off for a given resistance in a circuit. IOW, put into a given circuit, two resistors of equal resistance but of different power ratings will both give off ("dump") the same power -- and not their rated power. The surface area of the resistor (ie, its physical size) will mostly determine how hot it gets.

So, yeah, all other things being equal, a bigger power-rated resistor is likely to be a physically larger resistor. So, it will run cooler proportionate to its greater size (surface area) when compared to a dinkier resistor (of dinky power rating, who will get smoking hot when dropped into the same circuit). Same resistance, same job accomplished -- but two different resistor surface temperatures.

If physical size permits, being over-capacitized (my own word!) with a higher watt-rated resistor won't hurt, and is a sound precaution regularly practiced in engineering circles. Besides, over-engineered is the Porsche way.

Sire

stan, i,m in the same proces, only i can't get the blower in the working only when i put the degrees at cold. But i think they gave me the wrong 18k ohm not 2 watt but ,5 watt. But can you let me know how it's going.

StanAE86

Thanks for all the help everyone. I appreciate it. I'm going to go with the bigger wattage resistor since that's the one I can get.

Sire, I'll let you know how it goes. I have to order the resistor then install. Right now, I have the same problem, the blower dies out when I want it hot...but I get a bad blower code when my mechanic hooks up the Bosch Hammer. So, it kind of kicked my butt into doing the mod since the bypass pipe has been sitting on my shelf for so long already.

Sire: What's your e-mail so I can keep you posted?

StanAE86

All turned out good. Thanks for the help everyone.

On a side note, I had such a hard time hunting down th 18k ohm 2 watt resistor, I bought several just in case. If anyone needs one, let me know, and I'll sell it to you for the cost of a stamp and 25 cents. =)

Thanks.

darth

Good to hear Stan.

Was the 50 ohm resistor you used, described as "wire wound" and did it look like the one in the photo? The reason I ask, is, in the write up, Bill Gregory specifically mentions using a wire wound resistor yet the one in his photo (i.e. #1) looks like a ceramic resistor. I've read that other Rennlisters have had problems getting this set up to work (i.e. I've never done it) and I was wondering how important it is for the resistor to be wire wound. The only characteristic I can see that it would be mimicing in the motor its' replacing, is the windings or inductance and I can't imagine that being a big factor since the motor runs on 12 Volts DC and inductance is an AC characteristic other than at startup. Maybe Lonnie will chime in and recognize the type of resistor in the photo.

Bill
90C2

StanAE86

Darn, I don't recall what the package said, but it was a ceramic looking one, like in the picture:

http://tech.rennlist.com/911/pdf/add...ipeInstall.pdf

I -thought- I remember the package saying it was wire wound? Is it possible, it's a wound wire covered in ceramic? (I have no idea about these things).

I couldn't find a 51 ohm 15 watt. The place I ordered the 18k ohm 2 watt resistors said I could use a 50ohm 25watt resistor in place of the 51ohm 15watt resistor, but by then, I had already bought (2) 100 ohm 10 watt resistors based Rocket Rob's response (post #6):

https://rennlist.com/forums/964-foru...lp-needed.html

and wired them in parallel.

When I pulled the blower out, I had a check light error for a seized rear blower. I haven't had a chance to get the light cleared by my mechanic yet, the only thing I've done so far is just turn the fan switch on and see that the two small pusher fans are working very softly. I assume this means the mod worked, but won't know for sure until I get a chance to clear the light and make sure it doesn't light right back up. I'm hoping all is okay though and that if the mod didn't work, I wouldn't be getting any air at all?

On a side note, the place that had the 50ohm 25watt resistor, the resistor looked like Garrett376's here (the gold one):

https://rennlist.com/forums/964-foru...resistors.html

Twood

Stan

What year is your 964?
The reason I ask is my car is a '91 model built in November '90 and i have not been able to make the heater bypass mod work even with the specified resistors. It seems the later 964's do not have a problem with the modification.
The only way I can get air is by having the temp **** on the A/C on the blue dot (lowest setting). If I attempt to raise the temp the system shuts down.
Any help offered will be appreciated.

regards Ken

mjshira

you don't need to turn the heat **** with this in place. in effect, all you are doing is allowing the front fans to work as they will not with the rear blower deleted.

StanAE86

My car is a 1993 car and when I tested it, the front pusher fans worked, regardless of the where the temp **** was. I turned it from hot to cold and the fans worked okay.

Jeff Curtis

StanAE86, I sent you a PM either due to this post or another one related to the resistors for the Cup heater bypass tube...if you have an extra resistor, I'd like one...if you have one of both, I'll gladly pay you for them.

I've been busier than a one-legged man at an ****-kicking contest here lately and that's one of the "loose ends" I have yet to tie up...1st race of the season is in TWO WEEKS!!