My Exciting Alternator - 10/19/2018

In converting over to gauges, the car lost its ability to excite the alternator (that is, to magnetize the rotor and begin charging on its own). This is because the warning light cars used a switched +12V to the field wire through a light bulb to accomplish it. Once the gauge cluster is installed, the lamp is no longer present and the cluster doesn't even have a provision for the field wire. Gauge cars used a wire directly from the ignition switch to the field wire at the fuse block firewall junction. Not exactly an easy thing to duplicate with the harness in the car and removing the harness is a major pain in the rear. But... if a resistor was to be in parallel with the lamp circuit, it would accomplish the same thing. And more recent cars actually did this so if the bulb burns out the car continues to charge. Doing a little math on the current draw of the factory 168 bulb at 120mA, a resistor of 75-150 ohms at 12-14V, and needs to be in the 2-3W range to avoid burning it up if you leave the key on but the car not running. Being an electrical engineer, I have a stash of parts from my school days and various projects I've built over the years and located a 150 ohm 3W resistor...

Gotta remove the gauge cluster to get to the plug.

The plug to be modified. The dark brown is the field wire.

The missing field wire on the back of the gauge cluster, bottom right.

Resistor in place. Fortunately I had already made a snip to get the Fasten Seat Belt warning light to function, so it was just a matter of removing the tape covering it. 150 ohm resistor is 80mA at 12V for 960mW, so a 3W is an overkill for the application but I had it laying in a box of stuff and it was good enough for right now. It also allows me to put the lamp cluster back in the car if I want to and since the impedance is lower for the lamp, the lamp will still function.

All soldered in place and covered in electrical tape.

And like magic, it goes straight to 14.5V on the alternator as soon as the engine fires. Success.

Nice to come up with a win every now and again, especially when this one had been bugging me a while.

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Last updated October 19th, 2018