ATA 24

Reverse Current Relay (RCR)

Have you experienced trouble getting the left or right generator to come on line after an engine start? Mechanics have experienced this many times and have fixed this problem by simply swapping the two generator Reverse Current Relays (RCRs). The RCRs have an internal case ground, and once the RCR is installed in the DC Contactor Box, it is then grounded to the airframe. The wiring prints do not show this case ground anywhere except in 24-30-00 (the large generic schematic). When I talked to the repair shop that overhauls the RCRs, I was told that the internal case ground is a common problem with those particular units due to corrosion and fatigue issues. So, it's not by magic that the RCRs tend to fix themselves by swapping them. In actuality, removing and installing the RCR from the DC Contactor Box more than likely cleans the corrosion from the grounding points at the mounting bolts. The first step in troubleshooting the RCR is to find out if it has power going to it. I use a simple test light made for a 28 VDC circuit with two insulated alligator clips. Clip one end of the test light to the SW terminal of the RCR and the other end of the test light to a known good airframe ground. After engine start, see if the test light is illuminated; if so, power is good, so you then have a "ground" issue with the RCR. After engine shutdown, you can check the case ground using an ohmmeter. Disconnect the SW lead from the RCR and check for continuity between the SW terminal on the RCR and a known good airframe ground. Remember that you will get resistance having to go through the coil inside the RCR, but you should have some continuity and not an open line. An open line more than likely means that the internal case ground is no longer good and the unit will need to be replaced or sent out for repairs. Compare this reading with the opposite "good side." If your resistance reading is higher on the "bad side" than the "good side," then you probably just need to clean the mounting bolts and pads by removing the RCR and cleaning all points of contact between the DC Contactor Box and the RCR, and then re-installing and re-checking. This should fix your problem, but if not, you will need to replace or repair the RCR.