Nuts and Volts - December 2008

■ Note the LEDs are lit to show the compressor is on and the fan is at low speed.

requirements — and that the hot sides of the condenser fan and compressor clutch (points Y and

Z) will be at either 0 or 12 volts — depending upon cooling and air conditioning demands — allows us to monitor the electrical end of cooling and air conditioner operation.

The first device we’ll build is a compressor/fan monitor. Its circuit is shown in Figure 2. I chose resistance values which would allow the LEDs (T1 3/4, in this application) to be visible in daylight but not distracting at night. For the same reason, I chose a blank panel fairly low on the dash for the display unit. When wiring, connect the fuses as close as possible to the relays and compressor clutch. Voltage divider R2 and R3 provide about nine volts at their junction when the fans are on, so the bi-color LED glows green when it receives six volts from the radiator fan (low speed) and red ( 12 volts) at high speed. The blue LED will be either on or off. If you hook everything up properly and get a compressor (blue) indicator light but no fan indication (red or green), stop and find out why.

I have installed this device in two cars and glance at them occasionally while driving to make sure things are as they should be. At high speed, the bi-color LEDs may faintly glow either red or green, depending upon the position of the fans in the engine compartment and their relative ability to act as generators. If the glow is very bright, that may indicate you need to slow down. In a smaller vehicle, you can use two T1 LEDs, double the values of the voltage divider resistors, and put a 15K resistor in series with the blue LED.

When I bought a 2006 model vehicle, I didn’t check the specifications to see whether it came with daytime running lights (DRL) since I assumed this had become standard by that year. It turned out I was wrong. I ordered an add-on module from an auto parts catalog which failed after a couple of weeks of use. I decided to design and build my own. Figure 3 shows the second device: a DRL module. When the ignition is turned on, the low beam headlights are fed somewhat less than full voltage. This is typical, unless the car is not in gear (the automatic transmission switch grounds relay 2 in park or neutral) or the parking brake _■_FIGURE ₃is on (relay 2 is grounded

■ FIGURE 2

■ The unused panel from the lower dash of my ‘06 Honda Odyssey; drilled to epoxy in two T1 3/4 LEDs. The wiring was braided and fed through the firewall to hook it up in the engine compartment.

through the parking brake switch). My car has separate headlights for high and low beams, so turning them on together is not a problem. If you have dual-filament bulbs, then the black wire shown grounded should go to the high beam circuit instead so that you will not have daytime running lights and high beams simultaneously. D4 is necessary so that — should you flash the high beams with the ignition off (not possible in all cars) — the low beams won’t also come on because of reverse activation of relay 1.