Two of the wires large white and black supply 12v for the preheater. The other wire is the signal. I think it is green and located on the firewall. Pull back the rubber cover and attach the +terminal of your meter to it and the negative terminal to a good ground. The voltage should fluctuate from just above 0 to about .7 or .8 volts quickly--several times a second and average about .5v. If it does not fluctuate quickly it may be dead. Also try to richen the mixture by removing and plugging the vacuum line to the fuel pressure regulator and you should get an almost 1v reading. Create a vacuum leak and you should get a .1v or so reading.