How long do FSR last?

[OptimusPriM5]

My original part crapped out at 49k. Put OEM part back in and that also solved my parasitic draw that I had that would never let the battery tender keep up. Failed in mid July of course when I needed AC and was on the way safety inspection too

[Fifty150hs]

original FSR and blower. 195000 miles.

[dkl]

My 2004's FSR died at around '09. The OE replacement is still running. My blower is always running even when I don't have the A/C on.

[crystalworks]

The results seem to be fleshing out my experience as stated. It can be a pretty random failure. I know people who keep a new porcupine in the cabinet for the day theirs up and dies. I might consider doing the same as I am up to 113,000 and don't know if I am on the factory one or a replacement.

[oldskewel]

Originally Posted by upallnight

If you run the blower at a low speed I think you will use the FSR more since more of that energy is converted to heat, whereas if you run the blower at a high speed less of the current is converted to heat by the FSR, but since the blower motor is spinning at a faster speed, the blower motor will expire quicker.

You choose what you want to replace. Blower motor of FSR. If I don't need to run the blower motor I just keep it off.

Neither the FSR nor the blower motor are pure resistors, but they can be considered close enough for this analysis. Maximum heat dissipation in the FSR will actually occur when the voltage drop across the blower equals that across the resistor.

So when the blower is set for low power, the FSR is set to a high effective resistance and the current is low - so the blower spins slow and the FSR does not dissipate much power due to low current.

When the blower is set for high power, the FSR acts as if it has low resistance, so the current is high - the blower spins fast and the FSR does not dissipate much power due to low resistance.

A little circuit analysis and basic calculus shows the max power dissipation when the FSR resistance is in the middle. If they were pure resistances in series, it would be when R_FSR = R_blower, which would also be when V_FSR = V_blower.

So no need to worry about your fan idling at low speed. Or at full speed.

Here's a theory about why some FSRs fail quickly while others last a long time ... a weak/failing blower motor will stress the FSR. If the blower motor spins slower than it should due to mechanical problems or increased internal resistance, then the back EMF will be smaller than it should be. So the current will be higher than it should be. So the FSR will have more energy to dissipate.

So if you keep blowing FSRs, there could be a problem with the blower, either electrical or mechanical.

[AV8R4AA]

I installed my new porcupine. I am still getting a fluctuating airflow.
The fan speed indicator remains constant but the volume of air changes.
The temperature stays the same, just the flow varies.
Could a dirty temp sensor bulb be causing the airflow problems?

I removed the panel today. My air sampler fan was dirty and dusty.
The temperature bulb was gross.
Not to be satisfied with just a blast of air, I disassembled the unit.
Removed the fan and washed it with a toothbrush and soap.
I used wetted Q-tips to clean off the circuit board.
Re oiled the fan axle and put it back together.

Ops check tonight for a Frozen yoghurt run, works perfect.

[wpoll]

Originally Posted by oldskewel

Neither the FSR nor the blower motor are pure resistors, but they can be considered close enough for this analysis. Maximum heat dissipation in the FSR will actually occur when the voltage drop across the blower equals that across the resistor.

So when the blower is set for low power, the FSR is set to a high effective resistance and the current is low - so the blower spins slow and the FSR does not dissipate much power due to low current.

When the blower is set for high power, the FSR acts as if it has low resistance, so the current is high - the blower spins fast and the FSR does not dissipate much power due to low resistance.

A little circuit analysis and basic calculus shows the max power dissipation when the FSR resistance is in the middle. If they were pure resistances in series, it would be when R_FSR = R_blower, which would also be when V_FSR = V_blower.

So no need to worry about your fan idling at low speed. Or at full speed.

Here's a theory about why some FSRs fail quickly while others last a long time ... a weak/failing blower motor will stress the FSR. If the blower motor spins slower than it should due to mechanical problems or increased internal resistance, then the back EMF will be smaller than it should be. So the current will be higher than it should be. So the FSR will have more energy to dissipate.

So if you keep blowing FSRs, there could be a problem with the blower, either electrical or mechanical.

Nice description. Clearly the FSR is solid state (and not just resistive) but like you I suspect that is has significant dissipation when running mid-load. If the motor was purely resistive and since P=VI, then the dissipation should remain the same over the whole speed range but the impedance of the motor is somewhat more complex than a fixed R value!

It's a pity that BMW have couldn't run the blower control like the aux. fan and tail lights (on the LCI models) and use PWM control - Class D if you like - but that would have made the blower motor more complex. And expensive. Like the aux. fan.

Such fun machines to own....