sel common rail injector pre-charging. A coil's self-inductive kick is typically referred to as a voltage spike or fly-back voltage. Coil winding fly-back voltage occurs when the coil is switched off and can measure anywhere from 30 to 300 or more volts. While the current is very low (won't injure you) the damage that occurs to relay contacts (remember points and condensers?) and the electronic modules that control relays can be a problem. level (zener diodes) the blocking diodes used inside relays nearly block all the flow of our power source so the current has to go through the relay's windings and not through the diode. When a diode is wired with its anode (negative side with the vertical line) towards the positive side of a DC circuit it mainly blocks current flow. (Figure 5) This is called reverse bias. Creative Commons Spike Suppression The solution for many years was a simple diode to give the fly-back voltage a safe path to travel for the brief period of time it exists. Flyback voltages (a.k.a. counter EMF) are the reverse polarity to the coil windings vehicle connections. (Figure 4) A diode (basically a one way check valve for electrons) is ideal to allowing the fly-back charge to take a path back through the relay winding's coil until it has safely dissipated. You're most likely familiar with diodes from the years of compressor clutch diodes taped up in the clutch connector. Most modern vehicles have moved spike suppression diodes into their B.E.C.s (Bussed Electrical Centers). While some diodes will allow current to flow in both directions if the voltage reaches a certain General Motors and Creative Commons Figure 5: Diodes allow the flow of current in one direction and block the flow in the other direction much like a mechanical check valve. Diodes will cause a slight voltage drop as they forward bias (allow current flow) which can be read by a multimeter that features a dedicated diode test. A simple ohmmeter can be used to look for continuity with the leads connected one way and an open circuit with the leads reversed. Figure 4: Quick review: Why Diodes on a relay winding? Think of the example of a compressor clutch turning off, causing a voltage spike to occur as the lines of magnetic flux that had been built up when the coil was turned on (Left) collapsing across the clutch coil's winding. (Right) The spike, often as high as 100 volts, is the reverse polarity of the positive and negative circuits that are connected to the clutch. The current (amps) is very small but the higher voltage can wreck electronics and cause relay contacts to arc and fail. The diodes that traditionally were mounted at the clutch coil have been moved into BECs (Bussed Electrical Centers) but still exist for the same purpose. Relays themselves have similar voltage transient issues as clutch coils and therefore require similar protection. An open / missing diode will allow an unwanted high voltage spike to take place. (Center) January 2017 3 MACS Service Reports