Not Gen light

ceieio · March 16, 2008, 06:55:00 PM

Greeting fellow bus nuts.

I woke the bus up yesterday from her long winters nap.

All is well except that the not gen light is on. Everything was working just fine when I parked her last October.

I have been able to trouble shoot it down to the air pressure switch that enables the field relay that powers up the regulator.

The air pressure switch closes a connection from the field relay to ground. That is not happening and I assume a connection needs to be cleaned or the switch is gummed up. If I ground this terminal the "not gen" light goes out and the voltage on the batteries jumps up to charging level.

Here is my problem... I don't know where the air pressure switch is, nor do I know what it looks like. Can someone give me a hint on where to find the air pressure switch (and what it looks like)?

Bus is a 1973 MC 7 8v71/auto.

Thanks!
Craig
73 MC7 Oregon

NJT5047 · March 16, 2008, 07:32:41 PM

The field relay is located in the lower RH corner of the rear junction box on an MC9. I'd assume (cautiously, of course) that an MC7 relay would be located in the same place.
Pull the cover off the rear junction box and look for an air line going to a relay. That will ID the relay.
Are you giving the bus time to air up? Since you operated the coach last year, you probably are familiar with its idiosyncrasies, but the 'not-gen' light will stay on until the bus reaches about 80 or 90 lbs of pressure. If your low air buzzer is going off, the fast idle kicks in, and the bus airs up normally, you're probably on the right path with the relay. Have you checked the wiring at the alternator? Vibration may have damaged the field conductor, but the damage isn't visible. The insulation looks normal.
I've read that the contacts in field relays can be cleaned? Not sure about this...seems plausible.
HTH, JR

ceieio · March 16, 2008, 08:40:00 PM

JR - thanks for that. The field relay on the 7 is in the upper right corner of the rear junction box, so it sounds a little different than a 9. The regulator is on the rear bulkhead of the back baggage bay on the curbside (this is like an 8 and a 9 that I have seen so that is similar at least).

All components are working fine except the air pressure switch is not grounding the field relay.

The bus will air up, fast idle will come on, and so one, but the field relay does not get grounded to energize the alternator.

When I ground the terminal on the field relay that is connected to the air pressure switch, everything works fine, soI need to figure out where the air pressure switch for the alternator is located. My schematic indicates that it should come on with 60 lbs of air. I hoped it would "get over it" when I drove it the 100 yards or so to get it up by the house, but no deal.

Thanks,
Craig - MC7 Oregon

Sean · March 16, 2008, 09:41:28 PM

Craig,

I also posted this answer on the other board:

If you no longer have the A/C blowers or other major electrical loads, you can probably just bypass the switch and forget about it.

I'm open to being corrected by someone who is more familiar with MCI's, but I believe the intent of this switch is to keep the electrical load off the alternator (and engine) until air pressure has built to brake-release levels, thus assuring full engine power is available to the air compressor.

The way most conversions are designed, it's hard to imagine that air pressure is going to build significantly more slowly with the alternator connected. FWIW, my Neoplan has no such air pressure switch. The alternator starts charging even before the starter is done cranking. If my tanks are empty, it will take two or three minutes for the master Low Air warning alarm to shut up, but we'll be charging that whole time.

HTH,

-Sean
http://ourodyssey.blogspot.com

dparker · March 16, 2008, 10:11:06 PM

Craig,

Sean is on the right path. I also have a MC7 -- the purpose for this interlock is to allow the air to build up so that there is adequate air pressure to properly tension the belts on the generator...the mc7 uses air cylinders to tension the belts and by keeping the generator from not generating the unit will not cause the belts to slip until the air pressure is adequate to properly tension these belts -- i have seen buses that do not have a air cylinder on the generator and as Sean suggest you could simply bypass this feature ...

FWIW --

David

Sean · March 16, 2008, 10:20:19 PM

Quote from: dparker on March 16, 2008, 10:11:06 PM
... the purpose for this interlock is to allow the air to build up so that there is adequate air pressure to properly tension the belts on the generator...the mc7 uses air cylinders to tension the belts and by keeping the generator from not generating the unit will not cause the belts to slip until the air pressure is adequate to properly tension these belts -- i have seen buses that do not have a air cylinder on the generator ...

I'm glad someone with more direct knowledge has stepped in here -- I did not know there was an air tensioner on those belts. If it's still there, you might want to fix the switch.

Our belts are mechanically tensioned.

-Sean
http://ourodyssey.blogspot.com

JackConrad · March 17, 2008, 04:35:20 AM

Craig,
Look on the outside of the Rear Electrical panel (side closest to the engine/transmission). You should see a "box" about 4"x4". This is the Penn Switch and will have an air line coming to it. It will have wires from it to the relay located inside the rear panel. If I remember correctly, this Penn switch also controls the low air buzzer. When low air buzzer is on-No Gen, when low air buzzer is off, gen is on. Jack
PS: this is based on our MC-8, your MC-7 may be different.

Stan · March 17, 2008, 05:45:54 AM

The MC-7 air system drawing should show the relative location of the switch. The drawing is sort of a top view of the bus looking at all the air components.

ceieio · March 17, 2008, 07:34:48 AM

Thank you all for the pointers!

Jack - I looked square at that box on the side of the rear electrical panel and didn't know what it was! Now I know where to poke around. I have found that the late production 7's (which mine is) have very few layout differences from an 8 under the skin.

Stan - I didn't think to look at the air diagrams. That would have been a big time saver! I spent quite a bit of time reading repair and the abbreviated theory of operations in the manual, searching for that side comment that would reveal the location of the box. Talk about doing it the hard way! I'll take a look at it after work sometime this week (in between rain showers... it is the Pacific Northwest after all!

) I guess reading more of the book and learning new things about the bus is not an all bad thing...

Craig - MC7 Oregon

Tony LEE · March 17, 2008, 02:02:30 PM

That air pressure switch will have two contacts - one normally closed to operate the air warning light and buzzer and the other is a normally open to energise the field relay. It will be safe to bridge the contacts to check whether that is the problem.

If your alternator is belt driven, you do need the switch if you are likely to draw full output from the alternator. Same if you have the bus air-con in operation and the compressor is belt driven.
Even if not, if the alternator is oil cooled it is probably a good idea to have the time delay to allow time for the oil to get to the alternator.