92 mci 102 c , no air at rear air drain valve ?

[gene burge]

Iv'e been using bus to commute to and from jobsite for better than a month , all seems fine but last time i opened air drain valve i got nothing , gauge on dash reads 120 psi & bus seems to be operating fine , its about a 50 mile drive each way . thoughts ? 92 mci 102c 6v92 allison auto

[Jon]

Are you talking about the point at which you air up the coach from an outside source such as shop air? That will not have pressure after the dryer cycles.

[itspaidfor]

Jon is correct turn bus off let pressure fall below 90 psi drain as usual.

[bevans6]

What happens is this.  That drain is in the main air line from the compressor to the air dryer.  When the air system reaches cut-out pressure, the air governor signals the compressor to stop compressing air and signals the air dryer to purge.  When the air dryer purges, the air is released from inside of the air dryer and from the air line all the way back to the compressor, including that ping tank with the moisture drain valve.  When the air pressure drops below the cut-in pressure and the engine is running, the air governor signals the compressor to start compressing air and signals the air dryer to close its purge valve.  At that point air pressure starts to build and there is air pressure at that drain.  If you turn off the engine while the compressor is on a fill cycle, there will be air pressure at that valve until is leaks away.  If you turn off the engine after the air pressure has reached the cut-out point and the air dryer has purged, there will be no pressure at the drain.

This brings up a point of good driver practice for those who drive their buses in below freezing weather.  It is always good practice to turn off the engine while the compressor is on a fill cycle.  That ensures that the purge valve on the air dryer is closed when the engine is turned off.  When you go out the next morning to start your bus in the freezing temperatures, if the purge valve has frozen, it's at least frozen closed and you can air up the bus.  The purge valve heater will unfreeze it soon enough - if it is working properly!  On my MCI, the purge valve heater is turned on when the coach heat is turned on, and that is controlled by the discharge and blower cut in relay, which at the end of the day means the purge valve heater can't be turned on until the air pressure is up enough to turn on the alternator, turn off the low air alarm, etc.  So if the purge valve is frozen open, you can never get air pressure up so you can never get the purge valve heater on to unfreeze the purge valve.  Seems a point of potential modification to me...  

Edit:  for those of you with MCI's that have wacked out your OTR heating system, it's entirely possible that you also disconnected the purge valve heater.  If you didn't modify the circuit, the purge valve heater is only on if the big main heater fan is running.

Disclaimer - 1979 MC-5C schematic, but they didn`t change stuff very often.

Brian

[lostagain]

I installed a switch on the dash to turn the air dryer circuit on and off.

JC

[gene burge]

You guys were right , went out to bus & opened valve & got air, dash gauge showed about 60 psi, srarted it up& let air build till dryer purged & no air . never noticed it before , thanks for all the replies & the invaluable info . Happy Holidays !!