Wiring 12 volt head lights in a 24 volt system

[Danny Holder]

I was reading on the MCI site about wiring 12 volt head lights in series on the existing 24 volt system.  Has anybody tried this?  Results?    Thanks

[sledhead]

is this what you are looking 4

http://www.mcicoach.com/service-support/serinfo/serinfo07G.htm 

dave

[gumpy]

This was standard from the factory after about 1978.  Obtain a schematic for an MC9.  You need the 12v relay and diodes for it to work properly, but if wired correctly, it works quite well and does not discharge your batteries unequally, as wiring directly to the 12v take off will.

BTW, here's the service bulletin they talk about in Dave's link:  http://www.mcicoach.com/service-support/serinfo/assets/1930.pdf

It has a slightly different schematic than the MC9 Maintenance Manual, and doesn't use diodes.

[Danny Holder]

Thanks for the information

[bevans6]

In the simplest way to explain it, you are running the right hand light and the left hand light in series, so they each get 12v shared from the 24 volt supply.  The relay adds a switched center tap connection to the 12 volt tap on the battery pair so that if one light fails the other side still has it's 12 volts and works just fine till you replace the bulb.

Brian

[gumpy]

In the simplest way to explain it, you are running the right hand light and the left hand light in series, so they each get 12v shared from the 24 volt supply.  The relay adds a switched center tap connection to the 12 volt tap on the battery pair so that if one light fails the other side still has it's 12 volts and works just fine till you replace the bulb.

Brian

Brian, take a look at the link above. It's kind of interesting. They eliminated the diodes, and they're running one side off the 12v tap, and the other side off the 24v side feed with 12v tap through the master switch.  Haven't seen that one before.

[bevans6]

The original MC-9 circuit still had the three-gang master switch, and 24v power to the 12V relay went from the switched side of the master switch to the headlamp switch to the control side of the 12v relay.  Output from the 12v relay went to the headlamps about the same in both , except for the diodes that feed 24 volts to the left headlamps.  The modification changed the master switch to a two gang, and used the freed up terminals to directly switch the 12v from the 12v relay before it goes to the headlamp relay.  Power source to the 12v relay is directly via a fuse from the main 24v bus (in this case the bus in the AC box).   The difference must be to improve reliability.  I like not having the voltage drop from the relays in the left headlamp circuit.  The master switch still controls the headlamps.  Nice catch, I go crosseyed trying to read those schematics.  My MC9 schematic is 1989, so I wonder if production buses ever had the conversion circuit layout.  The conversion is dated 1980, and it calls out a change to the buss bar on the master switch.