Strange electrical/charging issue

[Ryker]

When I start my 1974 4905, the alternator puts out a normal charge with the charging needle in the white (maybe 27 volts?). Once I release the parking brake and hit the brake pedal to release the brakes, the charging drops into the red (24v), and the "no charge" light comes on. It stays that way until I hit the brake pedal. So when I hit the brakes, the "no charge" light turns off and the gauge needle jumps to white (normal charge), headlights get brighter, and even the defrost fans turn on! Yeah, the defrost fans will only come on when charging is normal, in this case also when brakes are applied; and the fans turn off when I release the brake pedal. Every time I let my foot off the brake, charging is reduced, fans turn off, headlights get dimmer, etc.

As an aside, these fans are LOUD. Can they be serviced or should I just replace them? Of course, I will have to get the electrical sorted in order to keep them on, or is there added resistance with the noise? The book states that they will never have to be serviced!

[luvrbus]

I bet the books author never thought the bus would be around 45 years either 

[chessie4905]

The defroster motors can be repaired. Bearings are replaceable, as are the brushes. If enough material is left, commutators can be taken to an electric motor repair shop to be turned down if necessary. I rebuilt mine two years ago. Nice and quiet now. If you do it yourself, make note of the stack of washers and shims in each motor that control end play.
First, make sure squirrel cages on motors aren't rubbing case, creating all the noise.

[richard5933]

Just a few guesses based on my experiences...

Have you confirmed that the brake relay is functioning properly and does not have a loose/shorting connection?

Is there a short/loose wire on the brake light switch?

Other than that, the standard check for ground connections all around. Also check all the relays in the electrical panels to make sure one isn't getting hot while this is happening.

And, as chessie4905 says, the motors would very much be repairable. But, you've got to find that one shop in your town where everyone that knows takes motors. It's getting harder to find the 'real' electric motor repair shops nowadays - most shops just want to replace with an imported pos motor. If you do end up replacing, it's still possible to get the USA made motors for nearly everything, you've just got to hunt for them.

[bevans6]

It's typical to have high draw accessories controlled by the relay terminal on the alternator with blower control relay - these accessories only get power when the alternator is charging.  It's also common for the charging of the alternator to be controlled by an air pressure switch that closes at 100 psi and opens at 70 psi, if that switch is open the alternator won't charge, the low air alarm should come on, the no-charge light would come on.  Hitting the brakes shouldn't interfere with this control circuit, but all I can think of is that something is telling the alternator to stop charging.  I don't have a schematic for the charging control circuits of your bus.

[Ryker]

Thanks again for the great advice. This helps to put me on the right path to track down these problems.