Return trip from Arcadia & our 24vdc generator.

[JerryH]

Is the device you are asking about not a pressure regulator to control the amount of tension on the belts?

Stan:
I don't "think" so as this device is on the unloading line of the cylinder.  Pressure regulation I would think would have been on the supply end to the cylinder.
Jerry H.

From the foggy recesses of my brain this AM...and a schematic...  No air pressure on the belts, the system won't allow the alternator to be loaded, and the low air light will be on.  Somewhat goof proof, eh?   On my '75 MC8, the "Gen Control and Low Air switch" is plumbed into the loop to the alternator belt tensioner.   So, this is where the air pressure is sampled for the low air warning, as well as allowing the alternator to come online once sufficient belt tension (air pressure) is achieved to prevent throwing them off or slipping under load.   Good engineering? Two birds, one stone.
happy coaching!
buswarrior

Buswarrior:

I think I'll look at this intermediate device.
I can't believe there aren't others out there with the same configuration.
Somewhat striking out here.

Jerry H.

[JerryH]

Feel likes I am (almost) talking to myself in this thread.
But ... the jury is back ...

It's the air cylinder.
I checked the valve ... checked the diaphram above the valve ... nadda.
disconnected the hoses and bypassed the valve to see what the cylinder would do ... it don't DO what it's supposed to do.

So the thought...

When we left Florida and engaged the AC, I figure the cylinder pretty much failed then.  It could no longer maintain adequate tension on the four belts causing them to spin, heat and blow apart.

So that's all I have to offer on this subject.  Now just need to find a replacement.  Anyone know whether you can rebuild them.  I haven't tried to dissect it yet.

JerryH

[Brian Diehl]

Jerry,
I don't know what the air tensioners were like on the older buses, but the air tensioner on my 96A3 is simple to pull apart.  You can buy a rebuild kit from Luke and easily rebuild in about 1 hour.

[mikelutestanski]

Hello
    My first post   I have a 72 7 with the 4 belt setup. Before I bought the bus someone replaced the air tensioner system with a solid turnbuckle; not knowing any better I have put 40000 miles on the engine and it has been fine. (the bus air did not work and I removed it;  R12 system )  Having thought about the original bus wiring diagrams system I can offer several reasons why it was setup the way it was. The coach air system use high amperage motors and when the coach is first started if the air is selected on this would put a large load on the alternator before spinup occurs. So the air tensioners needed to be tight before a large load is presented to the generator. THe penn switch in the gencircuit does not let the alternator field energize until about 75 or 80 lbs air is present. This lets the tensioners work as I believe the regulators are set for 65 lbs or so.  Then the alternator can make amps and run the AC system. If you think about a bus in service sitting at the terminal the bus wants to be cooled asap when passengers start to load. THe door is open and the driver wants it cool now.  So the circuit was designed to save the alternator from this high loading before it is ready.  If you do not use the original bus air system then the 3 or 4 large amperage motors are not in use and a solid turnbuckle tensioner works ok especially if the penn switch is working and I wonder if the penn switch is really necessary.  If you extend your thinking the 275 amp alternator is not really necessary without the AC unless you use it to charge your house batteries etc etc.  THe AC compressor had a tensioner on the belts also . If you take that tensioner away remember to seal the air lines. I had a 5a that would not stop because someone removed the ac compressor tensioner and did not cap the lines and that air circuit fed the shutdown cylinder. The bus did not stop and we had to stall it until  the lines were capped...    My opinions are my own and are subject to discussion .  I may not be right and .... i can stand correction....  I am presently working on an L10 mechanical B400Relectronic combo to retrofit into the 7  but that is another topic....     cheers   mike





if 

[JerryH]

...I can offer several reasons why it was setup the way it was. The coach air system use high amperage motors and when the coach is first started if the air is selected on this would put a large load on the alternator before spinup occurs. So the air tensioners needed to be tight before a large load is presented to the generator.     cheers   mike

Hey Mike,
Welcome to the board!!
You know ... I never thought of that.  You are right.  If the air/heat switch was on and the coach was started, it won't engage until the air is up and tensioning properly. 

And Brian ... thanks, I will call Luke tomorrow.  Looking forward to heat!!

Jerry H.

[buswarrior]

Hello.

The air tensioner is there so a competent professional driver may replace their own belts at the side of the road without any tools.

Can't do that with a fixed rod to hold belt tension. Also defends against the wrong tension being put to the fixed rod by mechanics or others.

Teenage daughter and I swapped a set in under 6 minutes start to finish, belts out of hold, hatches open, hatches closed, hands washed. Works great to have the assistant down through the floor holding the belts against the engine pulley, while you are outside feeding them onto the alternator. They like to jump one over the other if you do it alone from outside.

The electrical draw reason only works if there is a loss of air prior to start-up. All of our coaches were capable of holding their air for long periods when new, and could again, if we made it our mission to tighten them up. Back in the day, it would all turn on as the engine started.

A good old bus company would instruct its drivers to shut down the AC before shutting down the coach, for this very reason. Any who ignored it, would eventually have to put a set of belts back on...

My vote would be to maintain the air tensioner if for no other reason than ease of roadside maintenance.

happy coaching!
buswarrior

[mikelutestanski]

Hello:
    I have to say that Buswarriers comments are all on the money.  My experience has been with 4 or 5 buses that are all antiques and suffer from the fallen arches syndrome of leaky beams , failed maintenance, and northeast salt. Never having been a professional driver I do not have the benefit of that experience and training.   I agree that the changing belts with no tools makes alot of sense especially if you are sitting about two feet from the edge of 70mph traffic. 
        I do want to say that every bit of engineering has a purpose even if some of it is forced by regulation . No one does anything for nothing so if you think about it good engineering takes thought and a reasoned process to check beforehand that all the ways the circuits work are what was intended .  Thanks for the info ...   cheers   mike