Headlight Help Needed on MCI

[BC Bus]

OK Guys I am a GMC guy with a new MC 8 and am trying to save myself a lot of grief in trouble shooting this headlight problem.  I am sure that you MCI'ers will know some of these answers without me having to track all of this down.  I guess I'm lazy.


Background:
This is a 78 MC 8, Hound, that has been converted to 12v headlight system by Hound.  It has the offical stickers in the headlight cans, The bus bar and the extra relay is installed in the front electrical box. It is defintely a very professional conversion. The headlight system worked on this bus when it was a charter bus after it's hound days but the bus sat for about three years before I got it and there is some light corossion in the electrical stuff. 

Everything else electrical in the bus works perfectly.

Symptons:

I put all new NAPA 12v headlights  in and they worked great, for one day. when I noticed one low beam was out after 120 miles of driving.  I hit the high beams with the foot dimmer switch and pop, flash, and now all four lights are out.  Now I am driving in the rain at night on the Oregon Coast with my flashers on, no headlights.

After the trip I came home and checked the bulbs and all are blown except the passenger side low/high, but it won't light.   I replaced the 2 relays in the front electrical box and replaced all the lights and wow, all work again.  I play with the dimmer switch, high, low ,high, low for an hour and everything works perfect, problem solved, I'm a genius .  I'm thinking a bad relay is somehow causing 24v to go all of the lights when one bulb is burned out and I think I've solved the problem.  Not being happy with apparent success I put a blown out high/low on the drivers side, just to make sure I will have one headlight if one goes out, as the system is designed and wow, the passenger side light is still burning, dimmly but it is on. Yeaaaaaah!  Now I go in the bus and hit the dimmer switch and pop, flash, flicker and three of the four burn out again. Now this is getting expensive ! 

So the bottom line is: the system works fine when all four bulbs are good, but as soon as one goes bad, and your hit the high beams it burns all but the right front out. I have checked the circuit breaker on the 12v tap and it is not tripping.

Here are some of my questions:

I have the wiring diagram and it looks like there is a diode on the 12v tap but I can't find that diode and I don't even know what it looks like.  If there is a diode on this wire and it is bad would this cause this problem.

Are there other diodes or resitors in this system and where might I find those and what do they look like.  The diodes in my GMC are easy to spot.

Where are the low beams grounded and where are the high beams grounded, because of the corrosion I would like to make sure the grounding points are good?

Is there anything in the A/C electrical box that is a part of this system.

Any concrete ideas would be very much appreciated.

thanks,

Dark In Portland
Bill
503 789 9311

[edvanland]

I had the same problem on my MCI 7 at Lake Powell in Sept this year.  What I finally did was run a 12 from house batteries, put in a new three way switch which is center off, down low beam, up high beam.  The only thing I still have to do, darn those get around to its, is hook up a high beam indicater bulb.
Works well
ED
MCI 7

[JackConrad]

    It is difficult to say without seeing the wiring schematic.  Have you checked at headlights with a voltmeter? Sounds like you are getting 24 volts at headlights.
   We completely rewired our MC-8 headlights using all new wire, switches, circuit breakers and relays. We powered the headlights from our 12 volt house battery bank. This bank is constantly charged when driving by a 12 volt alternator we added to the engine (installed where OEM AC compressor was installed).
   10 gauge wire from house battery bank to power source on each of 2 heavy duty relays (through self re-setting circuit breakers).  Power from the relays go to respective lights (High or Low beams) .  Relays are controlled by dimmer switch. It is important to use relays that have coils that match power input to dimmer switch (12 or 24 volt).  Jack

[NJT 5573]

Bill,

 Back in the 70's I smoked the main wiring harness in an old KW Conventional at Ankeny Hill just south of Salem. It got real dark in a hurry, as well as killing the engine. I came to a stop across the exit lane in total darkness. It was just a little scarier than blowing a steer tire because you don't know where you are going to stop. It was 3 AM and OSP came by about 5:30 and wanted to arrest me for parking on the freeway!

Since that day, I have always had a drivers side spot light wired direct to a battery with a large toggle switch mounted as close as possible to where my left hand rides.

I know this won't fix your headlight problem but it can sure save your life.

[NJT5047]

First thing to check is have you miswired your 12V backup system?  The 12V lead is tapped at the center battery.  NOT on the 24V front battery.  That will blow headlights out as fast as you put them in...if your 24V system is faulty.  It is also an indication that the 24V system has failed, and you're operating on the 12V backup system.
If you have an equalizer, you have two bus chassis 10 ga leads going to the 12V center battery post.   If no equalizer, you'll only have one bus chassis 12V tap.  The 12V wire runs up the front of the battery compartment.  You may have a house 12V feed on the battery too...probably won't look like the OEM wiring. 
If you've had the batteries disconnected lately, suspect that the 12V headlight tap is on the 24V post.
Are you certain that you have a 24/12 system?  Some MCIs used 24V headlights.   
If you have a diode controlled backup, the diodes are located beneath the drivers floor approximately beneath the drivers left foot.  There's a loose fitting plastic cover about 5"X5" covering the diode block. 
The OEM ground is on the RH headlight. 
The only headlight components in the AC box is the 12V (if you have 12V backup) lead and stud. 
The headlight relays are in the electrical compartment located under the driver's window. 
Too many MCIs have had the headlight wiring butchered.   My guess is that even if you have (had) a diode system, it's been defeated.
IMHO, if the 24V system is questionable, and your's is, I'd convert it to 12V.  Wire it up like all highway trucks and cars. 
If a competent wiring job is done the bus headlights will likely be more dependable than the old 24/12 system.
Leave the marker lights and turn signals alone.  Just modidy the headlights. 
Power them with the 12V tap that normally powered the backup system. 
I'd also recommend looking at your headlight bowls and grounds.   Most are so corroded that it's amazing that they work at all. 
I'd still convert those headlights to 12V.   If mine cause any problems, they'll be converted. 
If you like to drive at night, be careful with those OEM headlights.     
HTH, JR

 
 

[John316]

With our DL3 we had a lot of headlight issues. Our solution (don't hold your breath, but I know it is funny) put a new R&M cap on the front to upgrade to the E style headlights . We can see a country mile now. Before we would always drive with our brights on, to see a safe way in front of the bus. Now if we have our brights on when somebody approaches way out in the distance, they are flashing their lights for us to dim ours.

I don't think that helps much... Sorry

God bless,

John

[buswarrior]

Hello BC Bus.

I expect your headlight wiring doesn't look beautiful. A re-wire will more than likely serve you well.

As noted, confirm that the battery related connections are correct. I too suspect the centre tap is on the 24 volt post instead of in the middle where it belongs.

The MCI style centre tap 12/24 volt headlight wiring mimics the way your house is wired. Think of the 12/24 the same as the 120/240 in your house.

Current will only flow on the centre tap if you have a blown headlight.

Unless you plan to figure out how to keep the system balanced, you can't just grab 12 volts for the headlights in a 24 volt bus. You will imbalance the batteries, which leads to $$$ premature replacements.

There's nothing wrong with returning the 12/24 volt MCI style headlight system to proper functioning. If you plan for a separate 12 volt system for other purposes, then go that way.

welcome to the contradictions!

happy coaching!
buswarrior

[BC Bus]

Thanks guys for all of the info,

My response is like this:

The 12v tap is installed correctly, and everything is wired exactly as the service update from MCI states when they changed this to 12v headlamps.  This is a VERY professional looking wiring job and according to MCI's records, which I have, it was done by Hound in 1980. It definitely is not a butcher job.
 
I think I just need to find that diode and make sure it is working right and check the grounds.  If I can just replace the diode and if it works that's fine, otherwise I'll wire it up for 12v, which I did on my GMC.

Thanks for your help.