What is this electrical component?

Bobbo wrote:
How critical is the 1/2 watt part of this? My local radio shack had 1 ko and 1.5 ko resistors, but no 1.2 ko. They were also much smaller in size.

I have found online a source for a 1.2 ko, 12w resistor. Would that work?

The actual resistance is likely not critical. A 1 k? or 1.5 k? resistor should be OK. You might also be able to get a couple to put in series that add up to 1.2 k? if you feel better with keeping the circuit exactly as it is. The resistor is there mainly to limit the current that the zener has to drop to keep the voltage reasonable; apparently the input for the control module doesn't need any significant amount of current. (The zener diode is basically just a cheap voltage regulator; it conducts when there's a certain voltage across its leads, in this case it would be wired to shunt the excess to ground. Polarity is important for the zener, but you don't need to worry about that at present. The zener would presumably be on the step side of the resistor, not on the engine power side.)

The power dissipation rating of the resistor is not too important in this case, either. The maximum possible power dissipation would be determined by the current through it, which is directly related to the voltage across the resistor and should be the difference between the engine side voltage and the zener voltage. Suffice to say that it would need to be dropping a little over 24 volts to be dissipating 1/2 watt, which is not at all realistic for a 12V system. A 1/4 watt resistor should be perfectly adequate.

Do note that you can't judge resistor power ratings purely by physical size; there are a number of different ways of constructing them, and they vary a good bit in bulk. In some circuits the exact type can make some difference (particularly when high frequencies are involved, as some have much more inductance than others), but in this case it makes absolutely no difference whatsoever.