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Old 01-16-2020, 07:16 AM   #1
crocs
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Final stage unit ventilator

Hey there, can anyone tell me the purpose of this part. What exactly it controlls besides a/c blower?
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Old 01-16-2020, 07:30 AM   #2
bendoscopy
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Really not much else. From Pelican Parts:

Quote:
The blower motor resistor (or final stage) varies voltage to the blower motor to achieve the desired airflow speed. It receives the driver demand signal from the HVAC control panel and sends a high current signal to control blower motor speed. The blower final stage unit can fail resulting in a malfunctioning or inoperative blower motor. The final stage unit is a high failure item on the BMW E46 and other BMW models as well.
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Old 01-16-2020, 08:43 AM   #3
MrMCar
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A very good question indeed.

The final stage resistor was first used (IIRC) on German cars on the MB 126 chassis. It was a large unit located inside the heater-A/C plenum and about the size of a 1/2 a carton of cigarettes. (Fags for our British brethren)

Rather that use the old school method of using a resistor pack with 3 different resistors for low, med-low, medium (high was a by pass and direct batt. voltage to the blower motor, giving you 4 speeds in total) they sought to give infinitely variable blower motor speeds.
This nessicated the use of transistors inside a housing. The unit produces a good deal of heat especially on the lower blower speed settings, so they needed a way to shed the heat out of the pack by placing it into the airstream of the heater-A/C plenum.

Over the years almost every one of these (regardless of car manufacturer) has undergone revisions to the internals, size/composition of the base and potting material and the revision of the number and length of the heat sinks. (the comb things)

IIRC the latest version for the E46 has 39 heat sink rods.

It's extremely important to FULLY install these when replacing. The idea is to get as much air as possible over the heat sink rods so the unit does not overheat and fail prematurely.
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Old 01-16-2020, 06:41 PM   #4
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Originally Posted by MrMCar View Post
Rather that use the old school method of using a resistor pack with 3 different resistors for low, med-low, medium (high was a by pass and direct batt. voltage to the blower motor, giving you 4 speeds in total) they sought to give infinitely variable blower motor speeds.
This nessicated the use of transistors inside a housing. The unit produces a good deal of heat
It doesn't have to be like this and generates lots of heat. It can be designed to control fan motor speed and still stays cool as a cucumber. Imagine how big the hedgehog in the Tesla electric motor if using this design!!!! BMW could use PWM motor control method and there is just a fraction of transistor heat to cool. In fact 99% of electric motor drivers use PWM drive.
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Old 01-16-2020, 11:36 AM   #5
crocs
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Thanks guys, so it has nothing to do with valve actuators? i think theres 5.
3 of the actuators doesnt open the valve, so my guess is its the motor itself , right?
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Old 01-16-2020, 06:33 PM   #6
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Originally Posted by crocs View Post
Hey there, can anyone tell me the purpose of this part. What exactly it controlls besides a/c blower?
control the fan speed and a meat tender is its after life.
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