Alternator/Generator not producing power after it was rebuilt

[Dlsnow]

My generator siezed up on me a while back and I had it rebuilt.
The rebuild seems to be a quality job - this thing looks brand new now.

After reinstalling it in the bus and tripplechecking the wires are right the NOT GEN light still comes on on the dash.

Any thoughts on where to start tracking down this issue?

[gumpy]

Are you talking about your alternator?? Is it a 50DN? 

[bevans6]

Is it a 50DN that is belt drive?  Check that when running there is voltage at the field terminal, that turns the alternator on.  Field voltage is controlled by first the regulator, which outputs a control voltage to regulate the alternator output, and second by the field relay, which doesn't let the alternator turn on when air pressure is low.  You need to develop enough air pressure to turn off the low air warning switch, that switch also controls the field relay.  If you have field voltage, you should have output at the positive terminal of the alternator, around 28 volts, so check that next.  The "not gen" light is controlled separately by the relay that controls the air conditioning and heating motors, I forget it's correct name.  That relay is also controlled by the low-air switch, it won't allow the big AC motors to run until the low air switch closes AND the alternator is producing power.  It taps off the Relay output of the alternator, which produces a 12 volt output from the regulator diodes when the alternator is producing power.

[Dlsnow]

Excellent, I will be back working on the bus project next Friday and I wil get after it.

Am I right that the field connection is the smaller diameter wire?

When I tested it the bus was aired up and all the other lights (TAG AIR, LOW AIR...) were off.  Just the NOT GEN

[buswarrior]

If you have another battery charging device engaged, you can get a "no gen" light showing.

Happy coaching!
buswarrior

[Dlsnow]

Ok here is what i found so far.
Bus running and aired up, drove it yesterday.

Not Gen light is on
Battery voltage is dropping, definately not charging
No voltage at the field connection to generator.

[bevans6]

Is it a belt drive or a gear drive alternator?  If it is a belt drive, there is a field control relay in the rear electrical panel that switches the field connection between the regulator and the alternator on and off.  It is controlled by the low air pressure switch located on the firewall of the engine compartment just inboard of the electrical panel.  There must be around 80 - 90 psi of air pressure before the switch will close and tell the field control relay to turn the alternator on. 

Brian

[Dlsnow]

Its a belt drive. 
The air pressure dial shows normal...between 90 and 120 when running.  Ill see if my book shows which relay it is and see if i can track down the low pressure switch on the firewall.