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 on August 04, 2012
Get specs and test other parts in the circuit for improper resistance. Possibly blower motor pulling to much current.
COMMENT by on 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 on 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 on 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 on August 05, 2012
I have an ohm meter, but i do not know what resistance is acceptable.
COMMENT by on August 05, 2012
I have an ohm meter, but i do not know what resistance is acceptable.
COMMENT by on 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 on 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!