No Gen tell tale light

[cross express]

 I have a 78 MCI 8, and No gen tell tale light came on. What could be the problem, never happen before. noticed coach heat fan doesn't come on either.

[Scott & Heather]

Does it stay on? Mine lights up while bus is airing up and at some point it goes out. Do you have 24 volts being generated? What is you volts gauge reading?


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[pvcces]

Scott, I think that you will find that the HVAC system will be inoperative anytime that the engine generator is not generating power.

Fixing the charging system should fix the HVAC.


Good luck.

Tom Caffrey

[gumpy]

No gen light means your alternator is likely not putting out anything. On some buses, though, it could just be a bad air pressure switch. You need to put a volt meter across your
batteries and see what it's reading.

I had this happen once when my inverter started my generator while I was driving. The inverter then tried to charge the batteries, and the resulting higher voltage on the system
caused the regulator to shut down the engine driven alternator, causing the no-gen tell tale to light up.

The other time I had it happen, the alternator was toast!

[thomasinnv]

The low air relay switch engages the coach air/heat and the alternator. You will find it on top of the rear electric box inside the left engine access door. if the problem was the alternator then the air/heat fans would still operate from the batteries once the air pressure is up.

[thomasinnv]

The relay serves multiple purposes. during low air situations it disengages the heat/air motors and provides power to the gen dash lite and low air alarm. When pressure is normal it disconnects power to the low air alarm and gen lite, and engages the fans. I  would start there.

[gumpy]

The low air relay switch engages the coach air/heat and the alternator. You will find it on top of the rear electric box inside the left engine access door. if the problem was the alternator then the air/heat fans would still operate from the batteries once the air pressure is up.

Not quite. The discharge and blower cut-in relay is controlled by both the air switch and the alternator 12v output terminal. If the alternator is not putting out, the relay is
not engaged, and the blowers will not function.

Get the meter out and check the voltage on the batteries when the bus is running. Then report back here.

[thomasinnv]

Not quite. The discharge and blower cut-in relay is controlled by both the air switch and the alternator 12v output terminal. If the alternator is not putting out, the relay is
not engaged, and the blowers will not function.

Get the meter out and check the voltage on the batteries when the bus is running. Then report back here.

If that's the case then mine must be wired wrong.

[gumpy]

If that's the case then mine must be wired wrong.

Certainly wouldn't be the first one to be "fixed".

Check out the schematics in your manual. You'll see what I'm talking about.