dim headlights 1978 MCI converted to 4 headlight system

[lvmci]

RJ, I looked up mci headlight schematic, on bno search, it didn't come up, is there another way to get to the schematic? Thanks, tom, lvmci...

[bevans6]

The later MC-9 manual section for electrics is here:  http://www.buses101.com/PDF_Files/MCI%209%20Maint%20Manual%20in%20PDF%20format/MC-9%20-%20Maint%20Manual%20Section%207.pdf

If you go through to the schematics, the first one is exterior lighting.  The headlights are on the left and bottom of the page.  What you see is pretty complicated and I would not try to duplicate it without a serious reason why.  

Here is a walk-through of the head lamp control circuitry.  It starts with the batteries, which feed a 12v connection from the center terminal of the battery pair to a headlight cut-off relay in the AC junction box (ceiling of the front luggage bay, driver's side).  That relay is the on/off relay and is controlled by the headlight switch - switch on, and it feeds 12V power to the junction box headlight relay in the front electrical bay beside the driver.  That relay is controlled by the dimmer switch.  The left hand pair of headlamps takes 12V power from the junction box headlight relay (low beam is on by default if the headlamps are switched on, and the relay switches low beam off and high beams on when it is controlled on by the dimmer switch) and goes through a pair of diodes to prevent back feeding to the 24V power stud #28 (stud #28 provides all power for the headlamps circuits).  This means that the left set of headlamps is getting 12V power between 24V and 12V on the batteries.  The junction box headlamp relay also sends 12V power to the right hand headlamps in the same fashion as to the left hand set (low beams always on by default, relay controlled on will turn low beams off and high beams on) except that the right hand headlights are grounded to complete the 12V circuit.  There are some things that I don't get, at first glance, about this circuit - it goes through a 15A breaker in the AC junction box, then a 10A breaker in the front junction box.  The feed from 24V for the left headlamps goes through a pair of diodes, which should drop the voltage to them by 0.7V and make them a tad dimmer than the right hand side headlamps.

The issues with converting with this scheme that I see (without going out to my bus to check) is that there is no headlamp relay in the AC junction box to take the 12V feed from the batteries, the wiring from the AC box relay to the junction box headlight relay would have to be added, and there is a bunch of wiring and a stud terminal strip in the front electrical bay that may have to be added.  A lot simpler to just grab the 12V from the stud in the junction bay and substitute it for the 24V feed on the existing headlamp relay, but then you need a Vanner to equalize the batteries.  Anyway, for those who were ever curious about how the later MCI system works, this is it.

Brian

[rdbishop]

Bill B/bus, Could we get a little more info on the way you wired your other buses. Did you fuse the 12v. power and if so what size fuse. Also what size 24v. relay. I have a c3 that the lts are all screwed up and like your simple system. I don't see any way you could hurt your batts. if fused right. How do you know which of the terminals are grd. or high or low on the headlights?

Thanks Richard

[bevans6]

Fusing has nothing to do with hurting batteries.  You hurt the batteries if you pull a sizable load (like headlights) from the 12V center-tap without balancing the load on the 24V tap.  Over time the lower battery gets under-charged, the upper battery gets over-charged.  The question is how bad is it - we know it's bad, but is it a beer at lunch bad, or a bottle a night bad?  If you have a Vanner or other equalizer it will accommodate the load difference and you will be perfectly fine.  If your bus is like mine there is a 12V stud on the panel in the main electrical bay, stud 55 from memory.  That is fed from a center tap with a 30 amp fuse on a 10 gauge wire.  That is ample for four standard 55 watt headlamps if you aren't using that tap for anything else.  I was using mine for trailer brakes, which are also a high amp draw, which is why I didn't use it for headlamps.

Edit:  Just went and got my MC-5C schematic for it's 24V headlamps.  The wires to the headlamps are 14 Ga which should be fused at 15 amps which would support 200 watts of lights for each of the high beam and low beam circuits.  I think my headlamps are 60 watts for the low beams and 55/55 for the high beams, so the maximum load would be 220 watts when the high beams are on, which is overloading the 14 gauge wire and the voltage drop would be significant (around 6% I think, to the RH headlamps).  The most simple way to get 12V to the headlamps with minimal changes to wiring would be to remove the wire from the common pole of the dimmer switch to the load side of the headlamp relay (leave the 24v wire from the switched side of the dimmer switch that goes to the coil side of the relay, it is what switches the relay) and connect the load side of the relay to 12V at stud 55 through a new switch.   This would get you your 12V headlamps but would be overloading the fuses and some of the stock wiring (the 14 Ga wire that feeds both the sets of high beams).  The other way to do it would be to put a 4 relays in close to the headlamps and use the stock 24V feed to switch them, and feed the load side with 12V with 10 gauge wire from the 12V stud or directly from the batteries.  The 12V stud #55 in the front junction box is fed with 10 gauge wire and a 30 amp fuse.  If you do either method I recommend removing the 12 ohm pre-heater resistor at the headlamp relay, it's not needed with modern headlamps at all.

Brian

[bevans6]

Just to explain why I am so into this, my winter project is to replace all my clearance/marker lights with 12V LED lights, so I have be designing the modifications to the stock wiring to let me do that as cleanly as possible.  As a result of this thread and the research I have done, the project now includes 12V headlights!  Nice synergy.

Brian

[farmallrick]

Thanks grumpy being a transplant from nw Iowa I was about ready to go to tractor supply and buy me a light bar to mount under the bumper or on top of the coach with about 6 field lights on it that would light up the world lol

[gumpy]

The issues with converting with this scheme that I see (without going out to my bus to check) is that there is no headlamp relay in the AC junction box to take the 12V feed from the batteries, the wiring from the AC box relay to the junction box headlight relay would have to be added, and there is a bunch of wiring and a stud terminal strip in the front electrical bay that may have to be added.  A lot simpler to just grab the 12V from the stud in the junction bay and substitute it for the 24V feed on the existing headlamp relay, but then you need a Vanner to equalize the batteries.  Anyway, for those who were ever curious about how the later MCI system works, this is it.

Brian

Yeah, that 12v stud in the front junction box is rated at 10 amp. Not gonna run a set of headlights for very long.

There are many reasons why MCI chose to wire 12v headlights up the way they did in the schematic shown.

[bevans6]

I couldn't let this go - something was bugging the heck out of me and I couldn't put my finger on it.  Then I got it - how does MCI get away with only a 10 amp breaker in the 12V feed to the headlamps?  I figured a 15 amp fuse would be minimum and I would use wiring and fuse for 20 amps.  Then I realized the MCI system was actually a 24 volt system with the headlamps in series, not in parallel like a normal 12 volt system would have.  The 12 volt tap, with all it's complex relays, four fuses, and so on - is only there so that if one headlamp fails the other one still has a path to power and can work on it's own.  So the maximum current under normal circumstances in the 12V tap to the batteries is effectively zero, and goes up to around 7.5 amps if one side has failed and the other side has high beams on.  Gumpy is dead right - there is a reason MCI did it that way!

Brian

[RJ]

Gumpy is dead right - there is a reason MCI did it that way!

Sometimes it's pretty hard to second-guess the factory, eh?

 

[Chopper Scott]

As I also have a 12 volt system installed  and such I went the 24 volt relay route and wired up the 12 volt lights. Works great.

[gumpy]

I couldn't let this go - something was bugging the heck out of me and I couldn't put my finger on it.  Then I got it - how does MCI get away with only a 10 amp breaker in the 12V feed to the headlamps?  I figured a 15 amp fuse would be minimum and I would use wiring and fuse for 20 amps.  Then I realized the MCI system was actually a 24 volt system with the headlamps in series, not in parallel like a normal 12 volt system would have.  The 12 volt tap, with all it's complex relays, four fuses, and so on - is only there so that if one headlamp fails the other one still has a path to power and can work on it's own.  So the maximum current under normal circumstances in the 12V tap to the batteries is effectively zero, and goes up to around 7.5 amps if one side has failed and the other side has high beams on.  Gumpy is dead right - there is a reason MCI did it that way!

Brian

I would say you are just now starting to understand the wiring of the MCI headlights. Study it for a few more weeks before you go trying to change your bus. It's kind of important that you fully understand all the little intricacies of this wiring schematic before you start "improving" on it or even before you decide to convert to it.