head lights flash when

[busshawg]

Good morning once again, since you guys give such good advice I thought I'd put this out there to. My lights seem to work fine but when I switch the OTR fan on for the heat the lights shut off for a second or 2  and  then work fine. Is this normal?

Grant

[robertglines1]

Had a 78 Mci 8 and didn't have problem...mabe a weak breaker triping and reseting...should not be on same circuit though..

[robertglines1]

after thought fan should be 24volt circuit and headlights 12 volt..just an observation  Bob

[RichardEntrekin]

You might want to see where your headlights and OTR fan are grounded. If you find they are grounded on the same terminal then I would use individual grounds. Just a guess. Grounding is always the first place I look with a 12V ghost.

[bevans6]

When you switch on the OTR fan, does it not work while the headlights are not working, or does it work instantly but the lights go out while it is working?

Are the headlights on a voltage converter 24v to 12v?  Is it possible the  24v drops when the fan motor is starting up, and the converter drops below a threshold of some sort, stops producing 12 volt?

I think your bus is an MCI but you don't specify.  On my MCI the feed for the headlights goes through stud 14 in the front wiring panel.  The feed to control the OTR heater motor also goes through stud 14, so there is a common point.

Brian

[JohnEd]

The fan draws maybe twice as many amps at start up but that tapers off as the motor comes up to speed.  Your circuit can't handle that start load and the voltage ACROSS the lights is dropping at motor start.  You can't measure the volts at the headlight y measuring from the hot lead to ground.  You will miss a bad round that way and even if it is good you might be missing a volt and you should know if that is true.  Pierce the wire insulation at the bulb for both the _ and + leads and measure.  Should be within a volt of the bat voltage.  Then measure the voltage at the groundlead at the bulb to frame.   That should be less than a volt.  If the ground circuit volts jump way up at motor start you have located a bad ground problem.  If not try the volts on the plus when you start the motor.

That advice to install separate grounds for each device is superb.  Also, install a heavy gauge + line from ? and use that to feed two new relays for the lights and the motor.  Use the existing + lines for each circuit to "signal"/control the relays.  I know, I know.....the original system worked and you could "fix" that but you will have to replace every stinking connector and splice ad terminal to get there ad the original system probably toleate a couple volts drop as acceptable.  My advice is oh so common practice here.

Be well and lit,

John

[Lee Bradley]

If your lights are going out completely not just dimming, you may have a relay in the circuit that is dropping out because of low voltage.

[gus]

I go along with Richard, headlight problems are almost always bad  grounding.

[buswarrior]

"pierce the wire insulation..." ?

And wonder where the electrical problems are in the years ahead?

Find somewhere to put your test leads that doesn't involve introducing corrosion to your wiring.

happy coaching!
buswarrior

[busshawg]

Thanks once again, Yes I do have a MC9, and the light only go out for that brief moment when the fan is starting up. Brain thanks for bring up the post # , I will definitly look at that.

Grant

[bevans6]

Stud 14 is a main distribution point for an awful lot of things, running directly from the master switch in the dash.  The connections to it are spread between several pages of schematics in my manual (I am working from a 1980 MC-5C manual).   what I would suggest as a place to start is to totally isolate the headlight feed from where it is now, and see what happens.  I suspect at the end of the day it will turn out to be an ageing relay dropping out.  Do you have the schematics?  Stud 14 is connected to the Discharge and Blower cut in relay ( the relay that connects the charging system after the auxiliary air system airs up past 80 psi), the coach heat relay, the drivers blower relay, the drivers AC switch, the lavatory blower, the reverse switch and solenoid, the starter motor relay, the hi/lo beam dimmer switch and hence to the headlights, the parking brake and stop light switch, the turn signals, and lord only knows what else.  all through auto-reset breakers.  Try taking the wires off of stud 28, which on my bus is the headlight feed point, and connecting the feed wire directly to a buss bar temporarily and see if the headlights still go out when the heater  motor is switched on.  On my bus the headlamp breaker is the left-most of three on a sub-buss directly to the  left of the big stud panel.  The headlamp relay is the middle of three in the panel below the main electric bay.  Actually, the very first thing I would do is replace that relay, if only because it's really easy to do...

Brian

Brian

[busshawg]

Thanks Brian, Lots of info, I'll print and take home. I am not familair with any of the relays as of yet but I'm sure I'll learn, haha.