Bendix Low Air pressure Switch MC9 tag Axle

neoneddy · March 02, 2020, 10:01:18 PM

Mine are leaking, pretty good so I was trying to find some bendix 285175 12PSI air switches.

https://www.anythingtruck.com/category/htp-brake-air-valves-lp.html?Per_Page=12&Sort_By=disp_order&f_brand=Bendix

I found these, but nothing at 12 psi, and at $30 each... seems a bit steep maybe? After striking out at NAPA I put plugs in just to keep it sealed up until I found replacements.

Anyone have some good sources for these?

benherman1 · March 03, 2020, 05:32:45 AM

Where does this go that getting anywhere close to 12psi is safe? As I understand it you should start seeing warnings at 60 or so PSI.

buswarrior · March 03, 2020, 06:17:04 AM

Did you try at home with Mother MCI?

This switch is to alarm when a tag axle has low air, they run at significantly less than system pressure.

Does bendix or another manufacturer have an adjustable that will go that low? Sealco is a reputable brand. There will be other places in industry that need alarms at lower pressures, don't limit the search to automotive sources or thinking.

Happy coaching!
Buswarrior

Jim Eh. · March 03, 2020, 08:14:26 AM

I thought the tag axle ran at system pressure when in the "normal" position and reduced pressure when "raised" which is not really raised but more like deflated to add additional traction (weight) to the drive axle.

buswarrior · March 03, 2020, 08:34:17 AM

Quote from: Jim Eh. on March 03, 2020, 08:14:26 AM
I thought the tag axle ran at system pressure when in the "normal" position and reduced pressure when "raised" which is not really raised but more like deflated to add additional traction (weight) to the drive axle.

Tags are supposed to have a pressure regulator feeding them, set to 35lbs, the "tag unload" switch then changes them to another regulator set at 15 lbs, IIRC.

Prevost "raise" the tag, MCI "unloads" the tag, to a certain degree.

The "main" air suspension itself does not have system pressure either. The suspension will have whatever pressure it takes to get it to ride height. All depends on how heavy the bus is/what model of bus at the moment.

Happy coaching!
Buswarrior

Jim Eh. · March 03, 2020, 05:03:45 PM

My manual is packed away for office renos so i can't look this up so how do the valves work in the RH rear compartment? Do they completely cut off the air pressure to the tag axle? I seem to remember air drain when I activated the switches (valves).

buswarrior · March 03, 2020, 08:03:42 PM

Quote from: Jim Eh. on March 03, 2020, 05:03:45 PM
My manual is packed away for office renos so i can't look this up so how do the valves work in the RH rear compartment? Do they completely cut off the air pressure to the tag axle? I seem to remember air drain when I activated the switches (valves).

The manual valves down the back will exhaust all the air from the respective tag suspension. Don't drive at any speed exhausted on dry pavement, you'll slide the tires under braking with no weight on them.

The switch on the dashboard will change the pressure applied to the tag suspension between the two regulators, so there will be some exhaust.

Put the bus on the scales and play with the various configurations, noting the changes in tag, drive and steer axle.

With a 40 foot coach, the traction conditions under which messing with the tags makes a difference are rarer than many think, and often come down to driver finesse vs equipment. The tag doesn't have a lot of load, and the transfer of weight isn't that much.

Now, they are Mission Critical in a 45 foot coach, the loads involved are VERY different on those axles, but that's a 'nuther story.

Happy coaching!
Buswarrior