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:
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: http://www.crownvic.net/ubbthreads/ubbthreads.php?ubb=showflat&Number=2743054
This person found a failed solder joint at the transistor (from http://www.fordforums.com/f102/eatc-controller-holland-170397/):
This person found a failed transistor: http://lincolnforums.com/forums/threads/6966-Cheap-fix-for-EATC-blower-speed-controller?highlight=blower+motor+controller+town+car
Here's a writeup on the BMSC for a Jaguar that I found helpful: http://www.jag-lovers.org/xj-s/book/acblower.htm
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).