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Looks like an elegantly engineered solution.
Could one simply make a jumper to eliminate (bypass) the low-speed resistor and then the cooling fan would just have one speed, high, that would come on when the low or high-speed connection is initiated? Seems like a simple solution. Does anyone see a drawback in this approach?
Andy
A bunch of Porsche engineers clearly saw a problem with that soloution!
Good comments. A better approach would be to use a PWM (pulse width modulated) output inverter verse a linear DC to drive the motor.
That approach could eliminate the output diode, which needs to handle the motor currents.
Yes, we did initially consider a PWM-based solution. However, that would have required designing and testing a custom circuit. We did look at existing speed controllers for model RC cars but didn't find anything suitable (but maybe didn't look hard enough). The advantage of a DC converter is that those are readily available for a few dollars in compact pre-packaged form such that the whole solution can be put together without requiring soldering etc. If I would do a custom PWM solution, I would add a micro-controller with bluetooth support so that the whole thing can be controlled from a phone app ;-)
As I indicated in my original post, one can get rid of both the diode and the relay (which is also prone to failure and comes with its own power/heat losses) by modifying the DC converter to allow setting of 3 output operating points (off, low and high output voltage) as a function of the original relay control signals. We just don't have the time and capacity to realize and test such a solution.(including an integrated solution that covers both oil and AC cooler fans) right now. Obviously, the same (replacing the relay) could also be done with a PWM-based solution.
P.S.: My ideal would be a plug-and-play solution that fits into the case/space of an existing relay, i.e. can just replace the relay. If somebody has the time and energy to realize that.
Last edited by agerstla; 07-15-2020 at 06:35 PM.
Reason: Added P.S.
Interesting thread. Seeing that our cars are 25 years old we should be testing the resisters and fans for operation by jumpering the relay socket from time to time just as we periodically test the dual distributor belt.
Andy
Interesting thread. Seeing that our cars are 25 years old we should be testing the resisters and fans for operation by jumpering the relay socket from time to time just as we periodically test the dual distributor belt.
Well, you will notice (as I did) when it fails if the oil temperature gets too hot. The low fan speed setting should normally kick in around the 9 o'clock position on the oil temperature gauge. If it routinely (e.g. just standing in traffic) goes above that, you may have a problem with the fan.
The driver’s side can be accessed by pulling the wheel and liner, but the passenger one requires the removal of the front bumper.
There's no need to remove the bumper for the oil blower resistor. Can be accessed from the wheel well after removing the liner and shifting the cooler. New resistors come with a metal disk to enhance heat transfer. Apply high quality thermal paste and it should it last for a long time.
Looks like a nuclear option to fix a simple problem ....
Change the ballast resistor and you are good to go for another 20-year. During this progress, relocate the resistor so it will even be easier to replace next time.
07-13-2020, 04:59 PM
