AC/Heater Control Unit LCD Backlight Repair

Recently the backlight for the liquid crystal display for my dash-mounted AC/Heater Control Unit stopped working. I disassembled the unit to inspect the innards and discovered that the backlight had been provided by three incandescent lamps soldered on a PCB whose filaments had expired.

These tiny light bulbs were sheathed by translucent green rubber “condoms”. Before I disconnected the PCB from the unit, I measured the open-circuit voltage (~9.7 VDC) across the leads of the bulbs.

I also noted the polarity of the leads using a marker on the PCB. This is because I was planning to replace the bulbs with LEDs. LED is a polarized device, with cathode and anode leads, unlike an incandescent bulb. The polarity of the supply voltage across an LED matters, just like with any diode. I do not know why Toyota never designed this backlight using LED lights in the first place, but being that it is a solid-state device, it is much more reliable and efficient than a filament bulb.

I purchased four 5mm white LEDs from Radio Shack that were selling for $1.99 for a pack of two. These LEDs were rated for a luminosity of 7000mcd. I also bought 270 Ω resistors to be connected in series with the LEDs as current limiters to provide a forward current of 25 mA and a forward voltage of 3.3 VDC.   This link has a useful current limiting resistor calculator for simple LED applications.

The following photo shows one of the bulbs (bottom) extracted from the PCB next to the LED (top).

I soldered the LEDs and the resistors onto the PCB. One lead of the resistor was soldered into the via marked positive. The anode of the LED was soldered into the via marked negative. The cathode lead was fed through the lamp base mount hole and soldered onto the other lead of the resistor for a series connection. I covered the  LED/resistor pair on the right with heat shrink tube to prevent possible short circuiting with an adjacent IC.

This view shows the LEDs on the other side of the PCB after the soldering was completed.

I took the PCB to the car to connect it back to the AC/Heater Control Unit. I turned the ignition half-way to provide power. The LEDs turned on. I could tell they were much brighter than the previous bulbs.

I reassembled the unit and covered the LEDs with the green condoms. This is how the display appears now.

Total cost of this repair job: $5.32

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4 Responses to “AC/Heater Control Unit LCD Backlight Repair”

  1. light bulbs these days are getting replaced by compact fluorescents and LED based ones, original incandescent bulbs are power h “

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