Truck Repair: voltage monitor/voltage, speedo sensor, shuttle bus

Question
Hello Cliff,
            I bought a 1989 Marco Polo 16 passenger shuttle bus.I am making it into a motor home.I have not been anywhere in it yet as I am not finished with it but I also do not want any problems on the road.It has a Cummins 5.9 diesel with Allison 545at trans.It runs great.When I got it there is this voltage monitor on it green light for good,red for high low.Now it is not working.I had some work done on the AC that it is when I noticed it not working.Does it matter if it works?Will it cause problems and can I fix it?Also it has a gauge in the dash for voltage.It shows good voltage until I turn the heater/ac fan on then it drops and when I run the headlights and heater/ac fan it goes all the way to 8.It will come back to 12/13 if I rev the engine pretty good.Are there a problem with my chargeing system?Can I check it someway?Does this thing have a alternator on it?If I may ask one more question.The speedo does not work either.It shows 35mph when your going 55mph any ideas?
                             Thanks

Answer
Yes it has an alternator.It should put out  min 13.8V idling.Hook volt meter on the POS & NEG of the alternator while running.On buses they usually have a larger capacity alternator.Maybe the broke a wire working on the A/C but depending on where it is mounted might have nothing to do with it going out.Try a speedo sensor.