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Author Topic: "not gen" light on my MC9 dash  (Read 5362 times)
Angola Coach Conversion "Aesop's Tortoise"
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« Reply #15 on: July 15, 2008, 12:31:23 AM »

Bowmaga -

Another WAG is that the belt tensioner may be allowing enough slack in the belts to allow the alternator to slow down enough to turn on the "not gen" light.

As Doug says, this is something that needs to be troubleshot and fixed, or you may let the smoke out of the entire electrical system - and maybe the coach, too.

FWIW & HTH. . .


RJ Long
PD4106-2784 No More
S13406 1978 MC-5C Converted
S14947 1980 MC-5C Shell
Cheney WA
Some Assembly Required
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Slightly modified 1982 MC9


« Reply #16 on: July 15, 2008, 04:49:36 AM »

I'm inclined to go with Richard's explaination, but there is one thing that intriques me and might offer a different solution....

You said there was a click in the lower right hand corner of the bus when the light comes on.  It may be possible this is an air conditioner valve solenoid clicking, and that the light you are seeing on the dash is really the air conditioner hi/low indicator. In other words, the bulb is in the wrong hole in the dash plate. That's not uncommon.

One way to tell if it is truely the not gen is to let air out of your air system till it's down around 50 psi or below (manually pump the brakes several times). Then, when you turn on the main power, before starting, you should see the not gen light come on. I don't think the A/C light does. Then, after starting, the not gen light will stay on until you have 85 psi air pressure. You should also have a buzzer on the low air, as well as a low air telltale on the dash. When you hit 85 psi, the low air telltale and buzzer should go out, and the not gen light should also go out.

Now, that doesn't mean you are not generating power during air up. If you have a gear driven alternator, it will probably be generating power.  If it is belt driven, it probably isn't.


Craig Shepard
Located in Minnesquito

http://bus.gumpydog.com - "Some Assembly Required"
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« Reply #17 on: July 15, 2008, 05:30:05 AM »

     Could one test that by running the battery down a bit, like leave the headlights on for an hour, and see if the light stays off?  I ask because mine would be on all the time if I put the bulb back.

That is what I would do. Look for the simple things first, especially since the problem started after you had had a long run previously.


Life should NOT be a journey to the grave with the intention of arriving safely in an attractive and well preserved body. But rather to skid in sideways, chocolate in one hand, a good Reisling in the other, body thoroughly used up, totally worn out and screaming:  WOO HOO, what a ride
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« Reply #18 on: July 15, 2008, 06:18:50 AM »

A couple of things also could/would cause the light to come on...

Tag axle air pressure. although usually that just lights the TAG pressure light.

One of the air pressure switches is possibly bad. The sound that you describe might be the air pressure switch inside the spare tire compartment cutting in and out. I had a similar problem and changed the air pressure sensor switch on mine.

The switch is fed from the auxillary or accessory air tank. It could just be going bad. I was informed that the light being on does not necessarily mean that the batteries are not getting charged. It is an IDIOT light only.

Some coaches were/are wired with the/a switch that actually signals a relay to cut off charging on air pressure loss. I haven't run into that myself.

The light will come on whenever the accessory air tank is below 85 to 90 psi.

The other thing is that if the not-gen light comes on while going down the road it could be that the batteries are fully charged and will no substantial electrical load the alternator is being shut down. When the voltage drops again the charging starts back up... The blinking Not-Gen light business.

You could have a faulty voltage regulator or it may be out of adjustment. The newer electronic ones handle loading better than the old delco remy units.

This is just a guess....


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« Reply #19 on: July 15, 2008, 09:03:49 PM »

I agree with almost everyone...the belt could be slipping, or the batteries may be fully charged.   And you could have a charging problem.   
Now for my SWAG.... Wink
Operating the coach with a faulty belt-driven alternator isn't going to hurt the bus.   It could reduce you to walking however....if ignored over long periods, if the batteries wouldn't restart the bus after a shutdown.   
The not-gen light will come on if there's no load on the electrical system and the batteries are fully charged.   Turning on any voltage hungry source should shut the light off if that's your situation.  If the light goes off with a load, and the system maintains 26Vdc plus, you don't have a problem. 
if the light stays on with a load on the batteries, you may have a bad relay?   You can check the battery voltage with a dig voltmeter.  If the voltmeter reads 26-28Vdc, you don't have a problem.     
It the voltmeter reads less than 25Vdc, something's not right.  Then we get to start new SWAGS!
In a bus' designed function, it's rare that there's no load at all on the electrical system.  AC or heat is almost always on...or both.  Big loads all.   
One other item, which you haven't mentioned is that if any charging device such as a genset is running and charging your batteries, the not-gen light will light up. 
Any time the regulator senses fully charged batteries, the alternator may quit charging. 
Craig brings up an interesting point with the light locations.  He explained how to verify the air switch that controls the alt relay.
It's possible that it's air pressure related (as Dr. Dave sez).  The relay is in the rear junction box.


JR Lynch , Charlotte, NC
87 MC9, 6V92TA DDEC, HT748R ATEC

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« Reply #20 on: July 16, 2008, 09:17:28 AM »

I ran the test as suggested by DML; left the headlights and markers on for an hour or so.  The Not Gen light still remains on.  The voltage at the batteries is 27+.  I guess mine is some sort of relay issue.

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« Reply #21 on: July 16, 2008, 09:32:52 AM »

Lin, on My mc7 there is a lead from the alternator, wire #33 which provides voltage to a relay in the junction box on the driver side rear engine door.  There is a diode that provides the dc voltage from the alternator stator to the relay circuit.  There is a pressure switch that causes a not gen light and prevents the alternator from operating until there is sufficent pressure to tension the alternator belts.  The light will function even if the batteries are charged.  While loss of the light may not be a true indication of alternator function it is in My opinon very valuable on a rainy night in downtown atlanta.  Check for voltage from the alternator and if it is present at the relay coil then check the low air pressure switch by jumping it to see if that causes the light to go out.  Hope I helped.  John
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« Reply #22 on: July 16, 2008, 10:17:02 AM »

I concur......fully charge batteries have nothing to do with the Not gen light.

Betty Owner 1
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Super Betty

« Reply #23 on: July 17, 2008, 01:04:48 PM »

well.....now that i have know idea what to do or where to look or where to begin......but i did begin....I took super Betty on a nice easy 20 mile drive today, got her wound up....no lights turn on.  They all stayed off as they should have.  When we came home from our last trip, it only took about 5 miles after getting on the road for the light to come on.  NOW WHAT??

Greg Bowman
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1995 Prevost H3-41, series 60, B500 Allison

« Reply #24 on: July 17, 2008, 03:56:02 PM »

You'll have no worries until you start on a 1000 mile trip with your family...

Nothing like depending on it to have all the faults show up at the worst possible time.

Maybe use that  big rubber hammer as inidcated in another post to test it.

Lots of good ideas here when it does act up again.  Guess you'll have to keep driving it!!

Jim Stewart
El Cajon, Ca.  (San Diego area)

Travel is more than the seeing of sights, it is a change that goes on, deep  and permanent, in the ideas of living.
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