Dave Hobbs General Motors to continue thereby creating a reverse polarity voltage spike. Relays with built in diodes usually have the diode symbol and pin identifications (with polarity shown) printed on the relay body. (Figure 3) Diodes tend to break down over time, are more finicky about temperatures and other automotive concerns compared to other, simpler methods of fly-back voltage suppression like resistors. Resistors tend to 'absorb' the transient spikes of the fly-back voltage and therefore do in many cases almost as good of a job as diodes. Besides, diodes cost more. Relays w/o the diode symbol printed on them usually have a high ohmage spike suppression resistor in parallel across the relay winding. Since the resistor doesn't block current, it has to be large enough in value to not factor into the resistance Figure 6: Relays are not all created equal. Some relays suppress voltage of the coil winding which it is wired in parallel spikes with a diode in parallel to the relay's coil winding. (Fig. 2) Others use a higher value resistor in parallel to the coil winding like shown with. Adding a 300 ohm suppression resistor above. Which circuits connect to the various numbered terminals can in parallel across a relay coil winding that has vary slightly too. This is very important to have correct when a relay equipped with a diode is used. by itself 120 ohms of resistance brings the toWhen the coil winding (saturated with a mag- tal relay winding resistance down to 85 ohms. netic field) is switched off, the current wants (Figure 6) Figure 7: Connect your ohmmeter between terminals 85 and 86 and you will be measuring 2 resistances in parallel to one another. (Left) Ohms' Law for calculating the total resistance of multiple resistances in series is to simply add them up. When the resistances (a.k.a. loads) are in parallel the formula is a bit more complex. (Right) Called the 'inverse of the sum of the inverses' you can do it on paper, with a calculator that has the "1/X" button. Put in a value for each resistor and press the "1/X" button, then press + and repeat for each resistor. Hit the "1/X" one last time. January 2017 4 MACS Service Reports