ditto

When all the parts are healthy: Assuming the fuse is 40A and good design limits the circuit to half that or 20A in use, with no resistor in the loop the fan resistance is 14V/20A = .7 Ohms. During low speed operation there will be 1.2 ohms and the current will be 14V/1.2Ohms = 11.7 Amps. At 11.7 Amps a .5 Ohms resistor dissipates 64 Watts which still doesn't leave 100% derating margin but would probably work. If you read the fine print on the resistor it will tell you what temperature you have to keep the outside surface at to maintain its rating at 100 Watts. That temperature is likely to be 25C and which is lower than you would see in typical operation, so that is another hit against the derating margin.

There is more to electrical design than selecting parts that work when the other parts are working properly. If the fan or fan wiring develops a short, that 100 Watt resistor is going to get really, really hot before it blows. The fuse won't blow until the core of the resistor begins to melt and the resistance lowers because the .5 ohm resistor limits the current to 28A. The hot spot inside the resistor isn't the real problem, it is the surface of the resistor that could be a troublemaker. While the fuse will protect the wiring, the outside of that small resistor is likely to get hotter than the plastic parts around it are rated for. If you read the fine print on the resistor it will tell you what temperature you have to keep its outside surface (case temperature) at to maintain its rating at 100 Watts. That temperature is likely to be 25C and that is more than 100C lower than you would see if the fan shorts. The Porsche resistor is big for a reason.

I would try to save $ by sourcing a used Porsche resistor before using that resistor. It may never become a problem, but you may regret it too.

Meanwhile in my driveway, my snowblower engine has lost compression and the driveway is only half cleared and more snow is on its way.

Rubik

I just wanted to come back and drop a line about an update. After the change, things have been working pretty well - both low and high speed work great. During use, the resistor feels cooler than the stock resistor.





Used zip ties and it sits pretty firm.

silotwo

I am waiting for the stock resistors to arrive, ended up costing $180.00 for the pair. Was tempted to save a few bucks but in the end decided to continue with keeping things OEM stock with no hacks.

Glad it seems to be working out for you though.

Rubik

Also, I decided not to drill any holes for the installation because I figured once the fan dies, the new fan will come with a resistor so I'll just plug it in.

I'm wondering if Durametric is going to allow me to lower the temperature setting at which the fans kick in.

phil996cab99

Thanks for the follow up. Nice work.

Ahsai

The install looks quite professional and the resistor looks pretty robust too.

pigsypo

the stock value should be .5 ohm 50 watts?

maybe this one?

Uxcell a12050500ux0003 Aluminum Shell 100W 0.5 Ohm Chassis Mounted Wirewound Resistor

cds72911

@Rubik (and anyone else who has done this mod) is this still working for you?

I have a driver side fan that works on high speed, the fuses and relays check out, so it's probably the resistor. I ordered a resistor and will try it out.

I read some past threads where people have worried about heat and failure modes - sounds like yours were good. Any negative experiences with the mod, or would you do it again if needed?

Rubik

I'm probably too late to reply to this as I don't come to Rennlist regularly anymore. I sold the car. But the current owner has driven it for another 30K miles w/o problem. Take care.

JB001

I installed mines early March, this year. They still work fine.