MC-8 'Not Gen' Light

[DavidInWilmNC]

Reading the post about the 'not gen' light on an MC-8 got me wondering.  I posted a question a while back about where the signal to cut on the HVAC blowers comes from.  Tapping in to the 'not gen' light circuit would also suit my needs.  I want to bridge the house batteries and the start batteries when the bus is actually generating.  I have the manuals, and the electrical diagrams are a bit difficult to read.  Does anybody know off hand where the signal for 'not gen' comes from?  I'm not sure if I can tap into it in the rear junction box, the front box, or on the regulator in the rear right bay.  Any ideas or suggestions would be great!  Thanks.

David

[gumpy]

Well, I don't have an MC8 manual, but I think its didn't change in the MC9. According to the MC9 manual, Charging and Starting diagram, the discharge telltale in the front came off stud 11 in the front junction box. The rear telltale (on the remote box) comes from stud 20 in the rear junction panel. The signal originates from the discharge relay in the rear junction box, which is controlled by the R1 terminal on the alternator. It appears that when the alternator is charging, stud 2 in the rear junction box is powered, and so could very well be the source your are looking for. A simple test with a volt meter will confirm this on your particular bus.

[DavidInWilmNC]

Craig, you're exactly right!  I fired up the bus, went outside to the drivers electrical panel, and stuck the + probe of my meter on the #11 stud and the other on the ground, and waited. In a couple / few minutes, the meter read .012 volts... down from the 24.7 (I think) that it did when I first started it .  This terminal's signal will activate a NC relay, powered by the 'terminal strip' in the same panel.  In turn, this will allow the disconnect solenoid to connect the house to the start batteries and charge both sets from the engine alternator.  You saved me a lot of time dinking around with a meter in the electrical panel.  Thanks a lot for the info!


David

[gumpy]

You're welcome.

I looked a little farther, and I think you might want to look at the Heating & Air Conditioning schematic.
It turns out that the signal going through #2 in the rear junction box comes to the front junction box on stud #30.

You should consider using a NO relay, so it is off when you start up, and doesn't require the discharge
relay to be working properly. Then, when the discharge relay cuts in, it will trigger this NO relay to close and connect the two batteries. This would also assure that the batteries are not normally connected while you are boondocking or parked unless you specifically select them to be.

craig

[Tony LEE]

There are two stages to this protection circuit. The first is that air pressure must reach sufficient pressure to tension the alternator belts properly. Apparently the 4 belts have a very short life if tensioned only by the spring inside the tensioning cylinder. This air pressure switch energises a 24V relay that energises the regulator. This gets the alternator generating. A separate half-wave rectified output from the alternator energises a TWELVE VOLT coil relay (also in the rear box) and that allows the bus AC and heater blowers to fire up. (this same output from the alternator is used to supply the engine tacho.

If your alternator is direct driven then obviously the first stage of protection is not needed but the second stage should be maintained to ensure that the AC blowers are not running the battery flat if the engine is idling too slow or something else has caused the alternator output to drop.