However, if a sensor or one of the wires becomes shorted to ground, then that wire will have a small but measurable amount of amperage. That’s basically true, under normal conditions. Some people will argue that the amperage on a 5V reference circuit is too low to measure. How can you easily isolate which area has the problem? One basic step is to check the amperage draw on each of the 5V reference wires at the computer. But you need to keep in mind that the computer or computer’s power supplies could also be at fault. The problem could very well be a sensor or the wiring. Some manufacturers use two separate 5V reference circuits however, only one is needed for the computer to communicate with the scan tool. The fact that the circuit’s wiring color codes and circuit names are different may lead you to think that each of these circuits has its own 5V supply, when actually the system is probably using only one. The wiring schematic identification labels are sometimes different as well.įor example, the labels and color codes for the 5V reference circuit wiring in a 2012 Ford Mustang’s engine performance schematic are: FTPREF (brown/blue), APPREF1(green/orange) and C-REF(green/violet), etc. The wire color codes may change for different 5V circuits.
Where do you start looking? As with any electrical problem, you need to review the vehicle’s wiring schematic. The reference circuit may have as many as five to nine wires from the computer to different sensors on the vehicle. When you confirm that the 5V reference circuit is dead, you need to locate the source of the problem. The easiest place to start testing is at one of the engine sensors (MAP, TPS, etc.). Once you determine that all the fuses are okay, check for voltage on the 5V reference circuit. If you find that your scan tool will not communicate with the computer, check for a blown ECM/PCM fuse. Second, check the computer system for codes. An accurate repair history often can be very helpful. When a vehicle comes in with a no-start condition, ask the customer if any type of repairs had been performed on the vehicle recently. The answer was so simple and very basic.įirst you need to gather the usual information. So I needed to devise an approach that would consistently locate the problem. The 5V reference circuit can be very challenging to diagnose at times. One of the technicians I work with asked me how I would approach and troubleshoot this type of problem.
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The article I wrote on how to locate a short-to-ground using a lab scope (November 2015) did not cover how to diagnose a computer’s 5V reference circuit that’s shorted to ground.
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