RRC Daytime Running Lights module PRC 8714 fix
Posted: Sun Mar 15, 2020 10:46 am
Some years ago I set about trying to find out why the Daytime Running Lights on my '93 RRC LWB were not working . The daytime running lights are set up in a very clever fashion - the "Daytime Light Control" module PRC 8714 provides pulsed DC power to the lamp side of the high/low beam switch which produces a dimmed high beam as well as faintly illuminating the blue high beam warning lamp in the dashboard binnacle. This scheme produces effective daytime running lights without annoying other drivers and doesn't wear out the low beams.
The problem was narrowed down to the PRC 8714 module - when the plastic case is removed, a circuit board and heatsink are found to be "potted" with epoxy sealant. The component that does the current pulsing is a p50n06 (ECG2305) MOSFET that is mounted to the heatsink inside the module, but with everything "potted" and inside the plastic module case, the heatsink cannot dissipate any heat, so the MOSFET gets very hot - so hot that the solder attaching the MOSFET's 3 leads to the circuit board had melted away, hence opening the circuit and no DRL's as a result.
The MOSFET was dug out of the potting epoxy and removed from the heatsink. Three 8" wires soldered to the circuit board terminals and fed out through a hole drilled in the side at the top of the module case. The module is re-assembled and glued back together. The three wires are soldered and heatshrinked to the corresponding MOSFET leads, which is now mounted remotely on it's own heat sink which is pop rivetted to a vacant relay mounting tab. Be sure to electrically isolate the MOSFET from the heatsink with the right hardware and thermal compound. This MOSFET survived the overheating, but yours may not have - it is a common and cheap component to replace. See photo:
The problem was narrowed down to the PRC 8714 module - when the plastic case is removed, a circuit board and heatsink are found to be "potted" with epoxy sealant. The component that does the current pulsing is a p50n06 (ECG2305) MOSFET that is mounted to the heatsink inside the module, but with everything "potted" and inside the plastic module case, the heatsink cannot dissipate any heat, so the MOSFET gets very hot - so hot that the solder attaching the MOSFET's 3 leads to the circuit board had melted away, hence opening the circuit and no DRL's as a result.
The MOSFET was dug out of the potting epoxy and removed from the heatsink. Three 8" wires soldered to the circuit board terminals and fed out through a hole drilled in the side at the top of the module case. The module is re-assembled and glued back together. The three wires are soldered and heatshrinked to the corresponding MOSFET leads, which is now mounted remotely on it's own heat sink which is pop rivetted to a vacant relay mounting tab. Be sure to electrically isolate the MOSFET from the heatsink with the right hardware and thermal compound. This MOSFET survived the overheating, but yours may not have - it is a common and cheap component to replace. See photo: