Diagnosing Air Throttle?

freds · February 02, 2022, 11:42:12 AM

Sitting out in desert in Quartzsite and fired up the bus to head into town after sitting for a few days and I am getting absolutely no response from the air throttle.

Looked through my manuals and I am missing the air diagram page.

The owners manual refers to a manual cable while the parts manual shows the air throttle since I have a turbo.

Where do I start?

Thanks

PS. Air horn also doesn't work which is kind of normal after leak down until its been driven a bit.

richard5933 · February 02, 2022, 12:06:29 PM

Air throttle is typically supplied with air from the same place as the horn. On my bus they're both fed from the auxiliary tank which gets no air until there is 90 psi. If you've been having problems with the horn, perhaps it has totally crapped out - as in the pressure switch which opens the air to the auxiliary tank at 90 psi (or wherever it's set on your bus.)

tr206 · February 02, 2022, 08:11:41 PM

Disconnect airline at air throttle control unit on the engine then have someone press on the throttle to see if air is coming out first if you get good air at full throttle hook it back up and make sure the control unit isn't leaking if it is you can rebuild it. if not getting air to the control unit it could be a foot valve issue you can do the same make sure you have air going to the throttle and/or foot valve (supply line) and air coming out the control valve. It's a start.

freds · February 03, 2022, 09:01:01 AM

Thanks guys, verified this morning there is no air reaching the engine actuator.

Tackling the foot throttle next; luckily mine is style where the connections are not below the floor.

Does anyone know if there is typically an electrically activated value between the throttle and the upstream air source?

chessie4905 · February 03, 2022, 10:11:05 AM

I'm sure you'll find an air supply issue. Those throttles don't just suddenly go bad. Probably a solenoid or air protection valve under driver.

richard5933 · February 03, 2022, 10:41:39 AM

The air needs to reach the foot control from somewhere - follow the line supplying it from the tank. Then check your manual to see what opens the air flow to your auxiliary tank. That may have an answer for you. Even better is if you find a air plumbing diagram.

freds · February 03, 2022, 01:48:31 PM

Quote from: richard5933 on February 03, 2022, 10:41:39 AM
Even better is if you find a air plumbing diagram.

Got a couple versions of the diagram and it doesn't show the air throttle that is on buses with Turbo's.

Kind of hard to trace the lines as it immediately disappears into foam as soon as it goes below the floor.

Prevost support is trying to help but thinking they need to start calling their retired engineers since it is a 1980 bus.

I have verified that it is upstream of the throttle pedal...

richard5933 · February 03, 2022, 01:58:24 PM

Is your air horn still not working? What about your wipers (if they are air operated)?

I'm guessing that you have a failed pressure switch which is not letting air get into the tank feeding the foot pedal. Any way you can check your tanks to see if they all have air pressure? Maybe pulling the drains on them?

freds · February 03, 2022, 04:09:01 PM

Quote from: richard5933 on February 03, 2022, 01:58:24 PM
Is your air horn still not working? What about your wipers (if they are air operated)?

I'm guessing that you have a failed pressure switch which is not letting air get into the tank feeding the foot pedal. Any way you can check your tanks to see if they all have air pressure? Maybe pulling the drains on them?

Hey Richard

Great catch!!! Your right the windshield wipers are also not working...

I will try attacking that tomorrow and see what happens.

Fred
PS. The tank that is supposed to service them does have pressure...

richard5933 · February 03, 2022, 04:35:31 PM

Sounds like you're on the right path at least. Just have to figure out what's keeping the air from getting out of that tank. My money's on a stuck valve.

6805eagleguy · February 03, 2022, 06:43:26 PM

I would also think there would be check valves that could stick?

chessie4905 · February 04, 2022, 05:12:10 AM

There should be a pressure protection valve that holds off air to aux. systems and immediately routes air to brakes till certain pressure 60 or 80 psi or so. Then valve opens and fills aux. system.

Bendix pp2 pressure protection valve.
https://www.jittruckparts.com/bendix-service-valve-pressure-protectio-277148rx

dtcerrato · February 04, 2022, 06:48:06 AM

What Chessie said. We replaced our PR-2 valve recently. Not saying that is the problem but should be on your list of suspects!

oltrunt · February 04, 2022, 07:57:33 AM

It might be fun to bipass the bus air system and hook up a small compressor to the gas pedal. Would that get you down the road? Jack

luvrbus · February 04, 2022, 08:08:19 AM

That is a dual air system both fill before the auxiliary do have 120 llbs on the other tanks that valve to the auxiliary doesn't open if under 90 lbs