Early MCI coaches used 24 volt bulbs for all the exterior lights, including the headlamps. During the mid 1980's the 24 volt headlamps were replaced on the MC-9 coaches with 12 volt headlamps, beginning at unit 34901. By wiring the headlights in series, 12-volt lights will operate with a 24-volt system.
The problem with wiring lights in series is if one lamp fails, both lamps go out. To prevent this from happening, a "balance" wire was added. This wire is routed from the 12-volt post on the rear battery to the wire connecting the headlamps.
Depending on which headlight fails, the balance wire acts either as a power supply or ground connection for the remaining lamp. The balance wire has 12 volts of power on it, which supplies power for the headlamp connected to battery ground. For the headlamp with 24 volts attached, the voltage difference between the two wires will allow 12 volts to flow through the lamp illuminating it (24 volts on power side - 12 volts on ground side = 12 volts power flowing through light.)
Service Bulletin #1930 explains the procedure on converting your older MCI coach over to 12-volt headlamps. It also lists all the parts required for this rework. Beginning in April of 1992, the exterior lighting on MCI coaches was changed to operate on a 12-volt system. All coaches from that point on have been using 12-volt exterior lighting, so the above information does not apply to these later models.
If you need further assistance, contact the MCI Customer Support Center at (800) 241-2947.
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