Hello all,
We've had several discussions about the different failure modes and the reasons behind the FSR failures.
I had to swap my ventilation fan for the third time, some time ago and I had the opportunity to do few measurements at that time.
Originally we had a sudden failure of the fan unit last summer. I replaced it with a dismantled unit that superficially appeared to be in fairly good condition. However this newer fan still burned through an FSR unit after three months, which I at the time thought was a FSR quality problem, and not directly related to the fan unit. However just earlier this spring the replaced fan unit started to exhibit same symptoms as our original failed one did, few months prior to failure. Under certain conditions it started to squeak barely audibly and later after second FSR failure it was apparent that the fan unit was the actual cause of the FSR failures.
I decided to replace the fan unit with new Valeo OEM version and at that time did the following measurements.
My understanding of the issue is that since the FSR unit is a simple resistor pack in essence turning "unwanted" electricity into heat, this would mean that additional resistance in the fan unit would linearly increase the heat load for the FSR as well.
Since current used by the fan is a definite indicator of the fan unit workload, I simply measured this while I had the FAN cover open during installation of the new fan.
I measured this at fan speeds 1 to 4 because at least for me these are the most relevant settings. I also tried lubing the old unit to see what effect that had. I used light WD40 style oil for the test.
Current draws at different speeds:

Old squeaky unit:
speed 1: 5.0-5.1 A
speed 2: 7.4-7.5 A
speed 3: 10.0-10.5 A
speed 4: 13.0-13.5 A

Old squeaky unit after lubrication:
speed 1: 4.4 A
speed 2: 6.5-7.0 A
speed 3: 9.4-9.8 A
speed 4: 12.5-13.2 A

New Valeo unit:
speed 1: 4.2-4.4A
speed 2: 6.5-6.8A
speed 3: 9.0-9.4 A
speed 4: 11.9-12.3A

As you can see, there is a clear drop in the amperage drawn by the fan. It was also apparent that while lubricating the old unit did improve it significantly, it was short lived as the amperage started to creep back up, even during the short testing I did. It is possible that thicker oil or grease could be a better fix. However I do not consider this to be a really good solution since it would not give you anywhere close to same fan service life as simply replacing the unit.
I think that it is also possible that having the fan cover open lessened the load and thus the amperage numbers could've been higher after closing it up. I did not do any testing after I had finalized the installation.

As you can see, there was about 10-20% increase of amperage drawn when comparing the new and old unit.
The best case scenario for this is that the FSR thermal load increases linearly with the amperage and that the FSR will simply run 10-20% hotter. I think it is also possible that this is non linear and 10% increase could cause the FSR to run more than 10% hotter. I do not have sufficient understanding of the electrical components involved to be able to say anything definitely.

However since the FSR already runs very hot(too hot to touch) during normal operation, I think it is likely that even 10-20% increase in temperature does significantly decrease the service life of the FSR. Beyond certain temperature point, the durability of individual components plummets non-linearly and I think this is the reason why cheaper FSR units, using cheaper components, usually fail very soon in this scenario.

-
DocJones

Thanks for sharing the info

The above by docjones confirms what I found when looking into why the equivalent vw resistor pack has such a high failure rate in vehicles. It's all down to blower fan motors becoming stiff to turn which increases the motor running current which produces more heat which in turn causes the resistor pack to fail as above. At least vw had the sense to fit a replaceable thermal fuse in their units and generally fitted the blower motors in positions where they could easily be replaced.

Does anyone know the resistances at each speed? If known and combined with the above currents, it should be possible to work out the power dissipation in the FSR using P=I squared x R

P = I2 x R
P (watts) = I2 (amps) x R (Ω)

I wonder why they didn't use the blower control system like they used in the P38A, i.e., no FSR. Ron B. VK2OTC
2004 L322 V8 Auto
2007 Yamaha XJR1300
Previous: 1983, 1986 RRC; 1995, 1996 P38A; 1995 Disco1; 1984 V8 County 110; Series IIA

Your calculations using Ohm's law are good for pure resistances, However:
Not sure it is quite so easy with rotating machines (generators and motors) as the rotation causes a back emf (electro motive force) opposing the supply voltage. This results in the effective resistance increasing according to rotational speed.

An easy way to picture this would be to measure the resistance of the motor when stationary.
As the winding is simply a length of copper wire then the resistance is very low (almost a short circuit), so would draw a very high current, (and does).

In practice the current falls as soon as the motor starts to rotate. Hope this helps. Forty years of progress. 1972-2012