DIY Project: LED Lights to Monitor Climate Control
 

Here's another little gadget I added to my car: a pair of LED lights to tell me when the a/c compressor is engaged and when the auxiliary electric fans are running in low-speed mode. My car is a 1995 E300D (W124, OM606NA) but the mod should be applicable to many other Benzes as well. This is something I did just for fun and because, as an engineer, I like keeping track of things. Should I have trouble with my climate control system not working properly, these lights might help; in the meantime they're fun to watch.

There are many ways to do this mod—you don't have to do it my way. LEDs or incandescent lamps or something else can be used to show that the a/c is working. In any case, safety first! Disconnect the battery before messing with the electrical system.

First LED: Anyone who has ever fought with a malfunctioning Klima relay knows that there is a whole list of things that must all be "right" or the a/c compressor's clutch won't engage. The first LED in this mod, a green one, is wired in parallel with the clutch. When it engages, the LED lights up. The clutch gets its power from the "87" or "output" pin of the Klima relay; in my car (Klima 003-545-52-05) this is pin 7, so the LED is connected between this pin and ground, with a dropping resistor (see next paragraph). The Klima relay in my car (and most 124s) is behind the battery; in other models it's somewhere else.

I used a 240 Ohm, 1/4 Watt resistor to keep the LED from burning out on 12 Volts. The resistor values are not critical; you should be able to pick something up at your local Radio Shack (if they still exist!). LEDs and other lights that work directly on 12 Volts are also available, of course, and don't need a resistor.

Second LED: The auxiliary electric fan(s) are powered in low-speed mode by a pressure switch in the line from the a/c compressor to the condenser. If there isn't enough air moving through the condenser (such as when stopped in traffic), the pressure in the line goes up, tripping the switch. This energizes a small relay and that turns on the electric fan(s). The second LED, a yellow one, is wired to pin 1 (also called the "87" pin) of this relay, which in my car is p/n 002-542-01-19. This relay is located in the fuse/relay box in my car; in other cars it will be a different part located somewhere else.

Operating on relay sockets to add a wire can be tricky; to save myself work I chose a simpler method (that some of you may disapprove of). Rather than cut into the socket and its wiring, I simply fashioned a loop of wire, put it over the appropriate relay pin, and plugged the relay into its socket. If you do something like this, you must remember that the wire will come loose when you pull the relay. This is especially important if you allow others to work on your car! I put a tag on the relay to remind myself of what I'd done.

http://i323.photobucket.com/albums/n...psh3vouzps.jpg

http://i323.photobucket.com/albums/n...psna5ua2j5.jpg

Fishing wires from the engine compartment through the firewall into the cabin is always tricky and very much dependent on your specific model. The only advice I can provide is to use a piece of protective insulating tubing over the wires so that you don't accidentally create a short circuit on some hidden sharp metal edge.

The LEDs or light bulbs can be mounted anywhere you can conveniently see them. Make sure they won't cause a distraction, especially at night. I didn't bother creating a circuit that would dim the lights at night but that's certainly an option.

Since I have the optional "Limo wood" lower console panel (124-683-38-37), I chose to use one of the extra holes (intended for heated seat switches—my car didn't come with heated seats). Mercedes sells filler pieces for the unused holes in the console; these are 124-683-07-10 and sell for a few dollars each.

http://i323.photobucket.com/albums/n...psll00u1tu.jpg

I drilled holes for the two LEDs and epoxied them in place. The wiring connected the LEDs to the two relays and to ground via a "Molex"-brand plug and socket. This allows me to unplug the LEDs if I have to remove the console wood. With the LEDs in place, I buttoned up the car, reconnected the battery, put in the radio code, and did the smoke test. No smoke! The LEDs worked!

http://i323.photobucket.com/albums/n...psri28ls6g.jpg

What did I learn? First, the compressor does not stay on all the time. In some models, buried in the a/c evaporator is a temperature sensor. If the evaporator's temperature gets down towards freezing, the compressor is turned off so the evaporator doesn't freeze up. This will not occur in very hot weather but is common on cool sunny days. I was quite surprised to see how frequently the compressor cycles on and off.

Second, the yellow LED gradually fades rather than blinking off instantly when the pressure switch turns off the aux fan(s). Why? This took a little longer to figure out. The answer is that a spinning fan acts as a generator, creating enough electricity to power the LED for a second as the fan spins down. Will driving the car at freeway speeds cause the fan(s) to freewheel enough so the LED will glow faintly? I'll let you know!

Jeremy