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Time To Repair An Automotive Clock
I was teaching an automotive class at the college where I work. The subject was Basic Electrical Repairs and some of my students offered up their cars for repair.
One student owned a Subaru with various electrical faults. The non-working side mirrors and cigarette lighter were easy to repair, with a faulty fuse being the cause. A common fault when an after-market CD player is installed is that the installer may leave the ignition turned on during the fitment of the CD player and the cigarette lighter being right next to the unit may get accidently shorted with the metal frame of the player, therefore blowing the fuse. After replacing the fuse, testing both the lighter and all the mirrors, I felt confident that the fault was repaired.
The next fault on the list was different. The overhead clock was not working either. Although not having a wiring diagram, I started with the basics. This was a good lesson for the students. The fuse was tested and examined visually and found to be in good condition. After removal of the automotive clock from the overhead console, I found the wiring connections were listed on the actual housing. Excellent – this would save guessing!
Battery supply is a constant supply and ensures that the clock keeps it’s memory when the ignition is turned off. Of course a ground circuit is required. LIG refers to when the dash lights are turned on and this will dim the clock for night time viewing. When the key is turned onto the ACC or accessories position, the digits will become visible.
After testing all these supplies, I determined that the digital clock itself was faulty. I told the student that I would repair it at home for him as I have better electronics equipment than the college.
After disassembling the unit, I carried out a careful visual inspection and the fault became apparent.
A 30Ω surface mount resistor had poor contact and I could even raise one end of it easily. I decided to search further. Another one, this time a 51Ω resistor was also dislodged. I tested several nearby components, but felt confident that I had found the fault.
After resoldering the resistors and testing their continuity, a final test was required. Power it up! I tested all the circuits and found that the repair was successful! The student was grateful for the free repair and I got to show the students a real life application of diagnostic techniques.
This article was prepared for you by Mark Rabone from Australia.
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Note: You can check out his previous repair article below:
and an older post by Waleed about Clock Radio Repair: