MC8 Headlight Wiring ?

[Sam 4106]

Our MC8 headlights had been converted to 4"X6" 12 volt bulbs by the previous owner when we got it, but they don't work right. There is a slight glow in the high beam bulbs when the low beams are on and the low beams are dimmer than they should be. I think the sockets for the low beam bulbs are mis-wired. The low beams are 2A1 bulbs and the high beams are 1A1 bulbs. The headlights are powered from the center tap of the house batteries. There are two 50 amp battery equalizers hooked in parallel to the 24 volt house system. Can anyone provide a picture or diagram showing which of the three prongs on the 2A1 bulbs are supposed the be connected to which wires (low, high, and ground)? I hope I can get this fixed before I have to drive at night again.
Thanks for any help, Sam MC8

[Just Dallas]

Removed

[Sam 4106]

Thanks for the reply Dallas,
I agree that the low beam is grounding through the high beam filament. I think the reason is that the low beam socket is mis-wired but I will try your suggestion and see what the result is. The PO wired the sockets, he didn't use a factory wiring harness. There is a separate ground on each side of the bus but both high and low bulbs' ground wires are screwed to a common frame ground.

[Chopper Scott]

This has to be something like the third time I have read about this problem. My 7 had the same thing going on and I have some diagram that the po kept in the bus's papers that he copied off of the internet. It seems others also did and it is wrong. Some funky stuff. I already added a 12 volt system also so I used relays to run the lights off of the original controls and went with a conversion kit that changed the lights to the h4 type bulbs. Somewhere along the line someone set up a diagram that was wrong and it seems a lot of guys followed the bad advise. I was only getting 8 volts at the headlights and they had wired the dim and bright in series. When they were on bright the dims also lit up somewhat and visa versa when the dims were on. Check your voltage at the lights and see what you are getting. The 8 volts I got at the lights were enough to run 12 volt relays so I could keep the original controls coupled with running the 12 volt system to run the lights. Hope I'm not as clear as mud!

[Just Dallas]

Removed

[gumpy]

Sam,

Look at this pdf. There's a diagram about half way down that shows the sockets.

http://files.thehoffmangroup.com/instructions/kickz.pdf


Here's another one.

http://www.hotrodders.com/forum/attachment.php?attachmentid=44154


craig

[NewbeeMC9]

I'm not sure when MCI started it, But the original on the MC9 had 12v head lights and a setup with diodes where one side was tapped between the 24v & 12v and one side would tap between 12v and ground.


HTH

[Sam 4106]

Thanks Gumpy,
The sockets are wired wrong. I will redo them and see what the result is.
It Will probably be tomorrow before I get to it because  my semiretired electrical contractor neighbor is here today helping me with some other projects. He just has to be doing something all the time.
Thanks everyone for the help, Sam MC8

[Sam 4106]

I got the headlight sockets rewired and I seem to have brighter low beams now but there is still a slight glow in the high beams when the low beams are on. Voltage at the low beams was 12.7 and at the high beams was about 2.0. I tried disconnecting the high beams with the low beams on and the low beams got real dim. I have checked all the screws on the relays and tightened them. There is no noticeable corrosion on any of the connections. I tried a separate ground from each socket with no effect. I checked continuity on all the wires to ground to see if there was two a short, none found. I checked each of the power leads to the lights to see if there was continuity between them, there wasn't. At this point I don't know what to check next. Any suggestions?
Thanks for all the help, Sam MC8

[robertglines1]

I hate electrical gremlins...OK dis connect all plugs.turn on low beams and see which socket feeds them..make sure that feed doesn't go to the high beams any way..same feed both sides..same headlight spade..sounds like ground mixed up on low beam...could be bad sockets or to low of amperage sockets (possible if using 65 watt Halegon)..Just some things to think about..mine was a spade connection problem....(wrong spade) Good luck.Do you have a car or truck with same headlights??ck to see which is ground on them

[bevans6]

My MC5C has a kit from Universal Coach Parts, which is or was an MCI part, that converts the 24 volt to 12 volt for the headlights.  The diagram shows a diode that reduces the voltage for each of the filaments, high and low beam.  Also, ground for the headlamp sis switched through this module, not directly tied to ground.  So if you have this module system, it could be cross-feeding internally, but I honestly doubt it.

Now, reading the instructions for this thing I came across a funny thing.  MC5, 7, 8, and early 9 coaches had a 12 ohm 100 watt resistor between the low and high filament feeds for "pre-heat".  It obviously pre-heated the high beam filament for quicker turn-on.  It's entirely possible your system still has that preheat resistor in it.  That would certainly feed a low voltage to the high beams all the time that the low beams are on, on purpose.  Remember that these coaches came with 24 volt lamps, so the schematic I am looking at is for that vintage of coach, and I don't have a schematic for the funny split 12 volt system that late MC9's went to.

I don't see any reason why having a pre-heat on the high-beam is a bad thing for incandescent bulbs.  I am not so sure what would happen with high powered quartz halogen bulbs though.  It might drop the voltage on the low beams, it creates a voltage divider putting the low beam filament in parallel with the high beam filament plus 12 ohms.  The instructions for my module says to remove it and that it is located in the left hand front tool compartment.



Brian

[gumpy]

Sam,

First, note that the diagrams I posted above are shown as mirror images of each other with respect to the headlight socket. I think the second one is easier to understand
and basically represents looking at the back of the bulb (at least that's how I interpret the notes on it, and it appears to match the wiring on some headlights I just pulled
from my old rusted Ranger). So, from your description, I think you may have the high beam and ground wires crossed.  The high beam filament glows when on low beam
because the circuit has to go through that filament to get to ground. Try swapping the two side pin wires.

Is the high beam filament on both bulbs glowing?  You may not have a good ground on one side and it's having to go through the high beam to the other side to find ground.

Unplug one side at a time and see if the glow goes away on the opposite side. If so, you have a bad ground on the side you unplugged.

Replace, rework, etc the grounds on both sides. Make sure they have good clean terminals and go directly to bus chassis.

craig

[Sam 4106]

Hi Guys,
I finally figured out my headlight problem. I rewired the sockets yesterday according to the diagram on the second link that gumpy posted and I still had the same problem. So today I started at one hi/low bulb with the socket off the bulb. Starting with the two prongs that are oriented in the same direction, I hooked the ground and 12 volt wires as shown in the diagram. The light was dim. Then I reversed the two wires and the light was bright. That told me that the diagram was wrong for my bus. Then I moved the 12 volt wire to the other prong and the light was bright. Now we're getting somewhere. Then I figured out which prong was hi and which was low which was easy with the light shining on the garage door. Turns out that the crossways prong is high and the prong opposite the ground is low which contradicts the diagram as well. So I rewired the socket to my new diagram and plugged in both the hi and low bulbs and the lights work as they should. What a relief! This wiring method may not work on all MC8s but I'm happy it works on mine.
I took one of the bulbs out to see if it was marked TOP on the side that was up, it wasn't. I thought possibly the bulbs were installed upside down but they are oriented so the lettering reads correctly so I doubted that was the case.
When I had the bulb out I noticed that the buckets were installed wrong so I had to switch them from side to side. There are protrusions on the four corners of the bulb and one is larger than the others. That larger protrusion has to go in the larger recess in the bucket. Now the bulbs seat where they belong and once I get the headlights adjusted I should have better lights. If not I will order the E-code bulbs.
This has been an ordeal I don't want to repeat, but it is rewarding to get the problem solved.
Thanks for all the help, Sam MC8

[gumpy]

Well, that's good news, and I guess, whatever finally works. I was amazed that there was no real information on this.

So, as you look at the back of the bulb, what does each prong do?  Left, right and center?  Low, High, and Ground (respectively)?

craig

[Sam 4106]

Hi gumpy,
With the prongs oriented the same as in both diagrams (center prong on top) low is left, hi is center, and ground is right. These bulbs are 4''X6" rectangular 2A1 Sylvania Halogen with prong orientation of center prong on the bottom (opposite of the diagrams prong orientation) with the prong assembly slightly tilted to the left.
I had a 7" round headlight bulb on the shelf and prongs on it are oriented the same as the diagrams (center prong on top). I wonder if there is a different wiring system used on different type bulbs? I burned out the round bulb trying to test it so I don't know how it is supposed to be wired. My thought is that there has to be a standard for each of the different types of bulbs or you wouldn't be able to go to a parts store and get a replacement.
In my attempts to find the wiring problem I noticed that there are diodes parallel to the coils on both headlight relays. Do you know what purpose they serve? Also, can you tell me how to test a diode to tell if it is good?
Thanks for your interest in this topic and the help you have offered, Sam MC8