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Relay Diagnostic Techniques Have Come A Long WayPosted 7/12/2000By Jeff Bach
When I think of describing a relay to a customer that has no knowledge of such things, this simple description comes to mind: “It's a device that uses a small amount of current to switch a large amount of current.” When I started in this business they were huge metal boxes that clicked so audibly that it brought to mind the sound a musket makes when the flint strikes the steel, just before the powder discharge. The customer complaint was a simple statement, “It stopped working.” The diagnostic routine was usually simple also. You gently moved a two-finger switch, feeling for its detent as the solid copper, spring-loaded contacts slid across the brass pins, while carefully listening for the relay to click, to give up its hidden location. You then knew where to stick your 15 ohm test light, which glowed so brightly that you didn't need to shroud it with your cupped hand, creating a shadow between it and the prevailing ambient light, to see its illumination. Ahh, the good old days. I have liberated my share of electrons from their silicone and polymer cages with that old test light. There is just no way to describe the special feeling that comes over you while you observe them as they ascend in that translucent opalescent cloud, which carries them to their next destination. I can remember that pungent acrid odor from their exhaust that permeated the air after their departure. It gives you confidence knowing you're not guessing when you carry one of these former electron prison camps to the parts department and say, “I need a new ECM,” as you try to hide that “trust me” smirk. That telltale odor remains in the box long after they've gone and is a pretty good hint that either some external component or circuit has sacrificed itself to free the electrons, or that some new technician has found a neat, old, real-bright test light at a yard sale. Relays and control technologies have come a long way in the past 25 years and so have the diagnostic techniques. We now have nearly escape-proof electron prisons that will close portions of themselves off at the first sign of an electron breakout. These high-tech boxes employ corporate sounding components such as diagnostic executives and task managers. They control current limiting quad driver circuits that are self-diagnosing systems that set related codes, which limits us technicians to merely reading the information with our scan tools and replacing the component at the end of the trouble-tree branch, where we were led by the nearly flawless diagnostic procedure. Occasionally a problem will arise that slips through a crack in the new technology's logic-based diagnostics that can cause a technician to fall right out of a diagnostic trouble tree if he doesn't have a safety belt. For me, the “safety belt” includes a lab scope and a current probe. Most of the time I use these tools to make pictures for articles or simply to satisfy my curiosity about how a circuit works, while eating up the diagnostic time for which I am charging a minimum fee. On those rare occasions when the machine that the customers know you have (that's right, they're not stupid; they know you have one) that “plugs into their car and tells you exactly what part is bad” doesn't do its job, I get a chance to play detective with my tools and trace down the problem myself. The image in Figure 1 was taken with my current probe and is demonstrating the control circuit current of a simple A/C relay. This waveform was captured by setting the scope to trigger on the positive slope and clamping the current probe around the relay control feed wire at the relay control center (see Figure 2). There is such a wealth of information contained in this simple waveform that I have broken it down and attempted to explain it in an understandable logical sequence (see Figure 3). With the lab scope set to 2 mS per division and the current probe set for 1:1, you can see that it takes the relay coil 5.67 mS to reach 105 mA of current (see Figure 4). This is where the magnetic field becomes strong enough to start to pull the relay contact bar down (A to C). The drop in current from C to E is caused by the change in the magnetic field as the relay contact bar moves to its fully closed position. This is where all the mechanical movement takes place, causing the deflection in the waveform I call the “Seagull Effect.” By adding a second current probe (see Figure 5) to the circuit and displaying it on the lab scope simultaneously, you get the waveform shown in Figure 6. By setting the scope to trigger from the relay control current and monitoring the clutch current on channel two (current probe on 2nd channel is set to 100mV per amp), you can see exactly where the contacts close. This will change as the relay contacts wear. The current will begin to rise later in the Seagull and eventually get noisy. In Figure 6 you can see where the A/C clutch coil current starts to build (point F) indicating where the relay points make contact (point D). Changing the time base on the scope to 5mS per division lets you see more of the clutch coil current waveform, which shows where the clutch plate actually pulls in as evidenced by the Seagull in Figure 7. Notice that the clutch coil requires 53 mS for the magnetic field to become strong enough to pull in the clutch plate. Now compare that to the much smaller relay coil current, which only takes 8 mS to pull in the relay contact bar. The time and current level are proportional to the size of the inductor and the strength of the magnetic field required to operate it. Large field=more time and more current. The relay represents the most logical place to me for testing circuits that use them. Another way to get a quick look at a circuit is to put one current probe around the control circuit and the main current circuit together. The waveform shown in Figure 8 shows that the control current is present and that the main current takes place just as the control current starts to drop where the contacts close. Figure 9 shows the individual waveforms. The waveform in Figure 10 was taken from the A/C relay while the clutch was slipping. The drop in current was caused by the relay contacts and would have burned up another compressor. I was thinking it had a high side restriction or coolant temperature problem causing the pressure to go high, but a quick current probe check at the relay revealed the true culprit. With many of the new cars using miniature relays so small that hearing them click is all but impossible, and access to their tiny terminals with a direct contact meter is not always advisable, current probe testing with a lab scope is the logical answer to many of the diagnostic dilemmas faced by today's technician.
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Copyright (c) 1996-2008. Automotive Service Association. All rights reserved.
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