2G HVAC Fan Motor Control

The cabin ventilation fan recently stopped working on my 2004 (219K miles). It just stopped one day as I was driving home, and would not restart, regardless of fan speed selected on the MFD.

A few other owners reported the fan motor needed to be replaced, so I started by replacing the blower fan with a new TYC aftermarket fan manufactured in Taiwan, costing $40. (The correct Denso part has an MSRP of $151, $104 plus shipping at parts.com.)
New 2001 2009 Toyota Prius TYC 700153 Heater AC Fan Blower Motor Assembly | eBay

While waiting for the new fan to arrive, I tested the original fan using a 12V lab power supply limited to 7A current output. The voltage was around 10V, which shows the fan required more than 7A current to operate at 12V. The fan motor was spinning fine, which made me wonder whether it really needed to be replaced.

When I received the new fan, I tried it on the lab power supply, and it operated fine at 7A current and the voltage at 12V, so it required less current than the original equipment fan. However, when I installed the fan into the 2004, it did not work.

My next step was to order a used blower motor controller (BMC) on eBay, which should arrive by Wednesday of this coming week. The price range for a used part is $20 - $30 or so including shipping.
04 05 06 07 08 09 Toyota Prius Blower Motor Control Module 87165 47020 | eBay

Compare the price of a used part to the $213 MSRP of a new part. Parts.com sells the new part for $147 plus shipping.

Yesterday I removed the original BMC, and opened the cover for inspection. I tested the BMC and found it to be defective, so I am hopeful that installation of the replacement BMC will solve the fan issue.

My method to test the BMC was to apply 12VDC from a portable jumpstart pack (containing a 7Ah 12V battery) to the power input terminals. I used a halogen headlight bulb to serve as a load, connected to the BMC output terminals, Then I hooked up a test lead to the sense terminal which is supposed to receive pulsed 12V from the AC amplifier. The duty cycle of the pulses controls the output. I tapped that lead to the +12V input and nothing happened, hence I assume the BMC is defective.

12V can be measured at the BMC output terminals when no load is connected, but once a load like a light bulb is connected, the voltage measurement becomes zero.

When viewing the photos, note that one connector has three terminals (the input) while the other has two terminals (the output to the fan motor). Two of the input terminals are larger, those are for the input voltage. The smaller center terminal is the terminal that receives the pulse width modulated signal from the AC amplifier. One of the photos shows the labeling of the terminals. The M+ output terminal is directly connected to the +12V input terminal, hence the M- output terminal is controlled.

Note the potting substance on the board was flexible and clear. It was very soft, like rubber cement. I had not seen such a substance used for that purpose before.

It was not obvious how to detach the circuit board from the heat sink (maybe the mounting hardware is hiding under the large electrolytic capacitor) so I did not disassemble the unit beyond removing the cover, which was held in place with one screw after the two mounting screws are removed.

There are five sheet metal screws used to attach the white plastic cover below the blower motor fan, and two screws used to hold the BMC to the blower enclosure. I lost one screw behind the carpet. The replacement size is #8, 5/8" length.

 

I have not needed to mess around with the traction battery cooling fan. In general, the current required will depend upon how fast you want the fan to run.

The least costly power source for your purposes probably would be for you to purchase a 12V SLA battery with 7Ah rating or so. This is the kind of battery used as backup power supply for residential burglar alarms, portable electronics devices, etc. That should deliver adequate current to run the fan for testing purposes, then you can recharge the battery using your 2A power supply.

 

12.5A should be more than adequate as that will easily drive the cabin blower fan and the traction battery fan is smaller.

 

I received the used blower motor controller today and installed it this evening. It works great and the air conditioning is back to full operation. Now the 2004 can return to its role as my daily commute vehicle.

 

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