Saturday, July 14, 2018

Corrosion


Here is again my 2006 Ford Taurus (~161000 miles) with new problem.  Cabin fan stopped to  work at any speed apart of  maximal. For me it looked like very strange malfunctioning. While it is not that crucial  but still annoying. I shared my frustration with  co-worker, who owns and maintains himself Ford Taurus 1994 and he (without even looking) provided me with diagnostic: there is need to change motor blower resistor.


As you can see online estimate was not really encouraging. Price of repair is comparable with KBB trade-in value of my car. No way I am going to pay it. Some more online, search gave me this video:

Immediately after watching I ordered replacement  part (~$20 with delivery) and made the fix. Works like a charm. After that I decided to look at the broken part (photo of it you can see at the very beginning of this post). Why it stopped working? On the photo you can see how corroded  it is. But was it the actual reason? To answer the question one needs first to understand how device works. The schematic is fairly simple: three resistors and thermal fuse connected sequentially . Control switch selects how many resistors stays in the circuit, limiting the motor electrical current. Thermal fuse has to prevent overheating. It should blow-up when temperature exceeds allowed value (~ 121 degree of Fahrenheit). At the highest speed switch connects motor directly to the ground bypassing the resistors and fuse. That why motor works at maximal speed even if resistor circuit interrupted.

Visual inspection of resistors did not show any damage, so my guess was that indeed blown fuse caused the malfunctioning. I cleaned from rust some spots on contact plates and tested resistance of the circuit. If fuse is blown resistance should be infinite. But to my surprise I have found that it is not infinite, but rather small (several Ohms). That what you expect from the normal working device. The only reasonable conclusion would be: because of corrosion there was no electrical connectivity between device and socket contacts. As a result blower motor stopped working at any speed below maximal.



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