Front AC/Heater Fan speed resistor keeps failing every 8-9 months. on 2000 Mercury Villager

The fan speed resistor for my front A/C - Heater keeps failing. Part only seems to last about 8-9 months before it burns out (then only high speed works). Any ideas why?

by in Hamden, CT on August 04, 2012
1 answer 7 comments
ANSWER by , August 04, 2012
Get specs and test other parts in the circuit for improper resistance. Possibly blower motor pulling to much current.
COMMENT by , August 05, 2012
Agree with 1 resistance is also a bad connection anywhere in the circuit. Check connector's for signs of high heat ie melted conn.plastic,blackened metal terminals. Repair connector kit avail.
COMMENT by , August 05, 2012
Thanks all! Good advice, although where can I find the specs for the Fan blower motor circuit? My repair manual only provides a wiring diagram and fuse rating. BTW, I did check wiring for evidence of over heating. All looks OK, but the failed resistor boards do show signs of overheating (excessive current draw?). Would a low voltage condition cause the problem? My alternator is orignal (156K miles), but I do not have a starting problem or low battery issue.
COMMENT by , August 05, 2012
Check all grounds carefully & if in doubt clean/repair as necessary.Are you using an ohmmeter or just visual inspection?
COMMENT by , August 05, 2012
I have an ohm meter, but i do not know what resistance is acceptable.
COMMENT by , August 05, 2012
I have an ohm meter, but i do not know what resistance is acceptable.
COMMENT by , August 06, 2012
For any ground circuit -as close to zero as possible. Also just thought of something else-- is there any damage to the heater box that would restrict or block airflow past the resistor?
COMMENT by , August 07, 2012
No damage or blockage that I can see. I'll check the wiring when I replace the resistor board. Thanks for your help!