Master switch, head lights and marker lights switches interconnected?

lostagain · August 17, 2011, 07:32:40 AM

1977 MC5C, no 7401.

I ran a new wire yesterday from stud no 16 in front panel to the dash lights to correct a short to ground.

So now, while I am in there, I'd like to know if this condition is normal: master switch off, if I turn the head lights and the markers switches on together, the bus lights up as if the master was on and is ready to start.

In other words, the bus can be turned on by either the keyed master switch, or by flipping both the head lights and marker lights switches on at once.

Is this a normal thing from the factory, or is there something screwed up? It's been like this since I got the bus a year ago.

Thanks,

JC

bevans6 · August 17, 2011, 07:58:12 AM

Not supposed to work that way. The way I read the schematic the clearance light switch is connected directly to the front buss independent of the master switch, and the clearance lights are also fed via one of the terminals on the master switch through the headlight switch. According to the schematic if the master is on and the headlights are on the clearance lights are supposed to be on regardless of the position of the clearance light switch, but if the master switch is off, the clearance lights only can be on if the clearance light switch is ON. Clear as mud, right? But there is no path to the other three terminals of the master switch that feed the rest of the bus.

If your master switch wiring was ever played with, it would be very simple to put a connector on the wrong terminal and back-feed the bus via the light switches. If the headlight switch was replaced with an incorrect replacement you could feed power back into stud 14 and power up the bus. If you ran the bus that way, the bus would be running via a 15 amp breaker rather than the correct 30 amp breaker that feeds the master switch. I will try a photo of the relevant bit of the schematic.

Edit: the main distribution point to the bus for main switch power is stud 14. Are you sure that it is getting energised when the light switches are on and the master is off? Does the engine start?

lostagain · August 17, 2011, 09:46:19 AM

Thanks Brian, I have to go shoe a horse now. I'll look into it more this pm.

JC

lostagain · August 17, 2011, 05:24:17 PM

Well, I've fixed it. I don't really know how, but it is all good now.

The PO converted the head lights to 12V, and installed a keyed ignition, so the wiring is not like the manual shows.

I took wire 15A out. That is what connected the head light switch with the clearance lights switch. So now, I can turn on the clearance lts, or the head lts independently of each other, and neither of them turn the ign on.

JC, the shade tree electrician.

Van · August 17, 2011, 06:02:14 PM

Now that was a really GOOD SHOE!

lostagain · August 17, 2011, 06:36:50 PM

Van, I knew you would appreciate, LOL.

Van is a horseshoer too (or used to be like me). We found out we had that in common, besides buses when we visited last March. I still do my wife's and a few of my friend's.

JC

bevans6 · August 17, 2011, 07:07:50 PM

My neighbour is a farrier, and a harness racer. Has between 20 and 30 horses usually.

bevans6 · August 17, 2011, 07:09:24 PM

BTW wire 15A is actually a 15 amp fuse, but if the wire is gone the fuse really doesn't matter much any more does it?

Glad you got it working.