Blower Motor Speed Controller


In 2015 the blower on my friend's 2001 Lincoln Town Car was not working at any speed setting. Using sites that discuss diagnosis and repair, I found that the blower motor speed controller (BMSC) had failed. My friend and I chose to buy an aftermarket, Duralast brand BMSC at Autozone for a little under $50. The warranty is for the life of the part (with some limitations), so it seemed worth a try. I held onto the old BMSC. The BMSC is notorious for failing in many models of cars and trucks. I wanted to see what component of the BMSC failed. I used my multi-meter's continuity checker and the circuit board to figure out the circuit:
Below is detail on the connector, transistor, and relay:

A schematic of this BMSC:

Continuity checks revealed that the large inboard prong ("blower drive" on the connector drawing) was not connected to the transistor's collector. The latter meant that the blower would never be grounded. I found a solder joint had come loose and re-soldered as shown:

It took a few soldering attempts to get consistent continuity between the large inboard prong and the collector. Here are the final continuity tests, showing success:

The soldering failure I found is consistent with the reports that sometimes wiggling the BMSC connector a bit would restore the BMSC to life. This person repaired the failed solder and added another wire for extra insurance:

This person found a failed solder joint at the transistor (from

Here's a writeup on the BMSC for a Jaguar that I found helpful:

I also checked the relay and transistor. For the relay, I used a 6-volt battery charger, alligator clips, and spare wire. I connected the battery charger across the BMSC ground prong (pin 5 in the drawing above) and the small outboard pin (pin 3 in the drawing above, for high blower speed). The relay clicks and stays shut, so it works fine. I tested the transistor with my multi-meter, using guides on the net. The transistor appears to be fine. 

The intense temperatures to which the BMSC is subject, given its location right behind the engine, is perhaps a major cause of this part's failure. Also if a technician is working on one's car and has to disconnect the BMSC, then because of the difficulty of doing this, I think solder joints for the connector may break. I understand that BMSC manufacturers have revised the design of its circuit board. The new design is more expensive. The old design is still around at a lower cost. The OEM part number of the BMSC I removed from my friend's 2001 Lincoln Town Car is Yw1h-19e624-aa.

I am not planning on installing this repaired BMSC in anyone's car. The soldering job was not that great. Also I would like to see how long the aftermarket Durlast BMSC lasts. I noticed that the transistors in both the OEM BMSC and the aftermarket BMSC appear to be from the same manufacturer. I tend to think the aftermarket BMSC will last as long as the OEM one (some 14 years).