Air Brake Problems

PNWorBUST72 · April 07, 2018, 05:46:51 AM

So my MC8 was leaking air, alot of air. It couldn't get to 90 psi at idle but worked ok on the road.

Turns out I had a brake chamber, the brake air release valve and the D2 governor all leaking. We replaced those and boom! Airs up to 90 at idle and keeps air.

But first we realized that the Bendix governor wasn't adjustable and is set for 105-130 psi so it kept building pressure past 120. My mechanic says thats too much air for this system and shut it down.

Then we realized that there is some safety switch that isn't allowing us to start the engine after it gains pressure. Empty the tanks to 65-85 psi and it can start again.

Any thoughts?

bevans6 · April 07, 2018, 06:07:18 AM

D2 governors can be adjustable, you need to take the black top cap off to do it though. 130 is a little high. You probably have a switch set to off to make the Skinner valve operate the engine stop air cylinder. That Skinner valve is just behind the rear control panel - make sure your switches are set to "run". The Skinnder valve must have +24 volts applied to it, if there is no voltage it opens and allows air pressure through to operate the engine stop air cylinder. The air supply to that Skinner valve comes from the auxiliary air system, which is after the pressure protection valve. What's happening is the air pressure drops below 65 psi, the PPV acts to separate the brake tank from the aux system so pressure in the aux system can drop to zero, and no air pressure to operate the engine stop air cylinder.

Geoff · April 07, 2018, 06:10:34 AM

Could be just a weak Skinner shut down valve.

PNWorBUST72 · April 07, 2018, 07:10:34 AM

Quote from: bevans6 on April 07, 2018, 06:07:18 AM
D2 governors can be adjustable, you need to take the black top cap off to do it though. 130 is a little high. You probably have a switch set to off to make the Skinner valve operate the engine stop air cylinder. That Skinner valve is just behind the rear control panel - make sure your switches are set to "run". The Skinnder valve must have +24 volts applied to it, if there is no voltage it opens and allows air pressure through to operate the engine stop air cylinder. The air supply to that Skinner valve comes from the auxiliary air system, which is after the pressure protection valve. What's happening is the air pressure drops below 65 psi, the PPV acts to separate the brake tank from the aux system so pressure in the aux system can drop to zero, and no air pressure to operate the engine stop air cylinder.

The mechanic told me the D2 I got was non adjustable...not sure. I am gonna look up the Bendix number and check it out.

Where is this Skinner valve switch? A manual reference is fine too...

Thanks!

luvrbus · April 07, 2018, 07:48:05 AM

If the D2 has a blue cover it is not adjustable, the black cover D2 governors are adjustable

PNWorBUST72 · April 07, 2018, 09:35:00 AM

Yes, thats it. Mine had a black cap but the dealer gave me a blue cap one. I hope they take it back...

Still cannot find that Skinner Valve Switch...

buswarrior · April 07, 2018, 10:14:46 AM

Start at the air plunger on the top of the governor.

Work back along the airline to the electric solenoid that triggers the air plunger.

trace wires back to somewhere that you can put your multimeter.

Before all that, do some diagnostics, watch the air plunger against the fuel shut off, see what it does under different air scenarios to prove out these theories.

The smart busnut is the lazy busnut. Lots of looking before you get dirty or have to go through contortions?

Happy coaching!
Buswarrior

bevans6 · April 07, 2018, 10:47:04 AM

If it's in the stock location it's about three inches past the rear control box, kind of up along the top of the engine bay. It has wires going into a plug on the bottom and two air connections, probably one hard line (supply) and the other goes to the air cylinder on top of the governor that activates the no-fuel lever. If you have fast idle there may be a second skinner valve beside it that operates the fast idle air cylinder and the throttle lock out air cylinder.

PNWorBUST72 · April 07, 2018, 01:25:28 PM

Just to be clear, its not a no fuel thing its a wont even turn over thing...if that matters.

The key does nothing until you drop the psi again...doesnt turn over at all.

Are you thinking its a bad valve/switch or is there a setting I need to adjust? I might be misunderstanding here...

Any idea on part number?

Dave5Cs · April 07, 2018, 02:17:49 PM

sorry but I have to ask.
Is it in Neutral?

PNWorBUST72 · April 07, 2018, 02:33:55 PM

No worries, yes.

Once I get it down to 65ish PSI, I can start it again. But if I let it run for 5 mins, air up to 120ish PSI, turn it off and then try to start it right back up, it wont do anything. No cranking nada.

I let it bleed down and it cranks right over and starts like normal.

Now I will say we did remove the shifter to replace the Brake release valve mounted in the same housing. Could it be something with the position of the shifter?

PNWorBUST72 · April 08, 2018, 06:40:44 AM

So is there an "on/off" switch somewhere that I can check on the Skinner Valve?

richard5933 · April 08, 2018, 06:48:31 AM

I'm having a similar issue on mine. Problem I'm having is the low-oil shutdown system is kicking in due to a faulty relay/sensor/ground. Haven't entirely figured it out yet, but I can get it started if I unplug the low-oil shutdown wire on the shutdown relay.

Do you have an air cylinder that controls the fuel shut-down? If so, is it being triggered to shut down? If you have a wiring diagram you can start to trace the problem backwards.

buswarrior · April 08, 2018, 07:04:28 AM

This is mysterious.

No cranking when air pressure is up?

What system crosses over to the starter that air pressure would be involved?

I'm wondering about one of the lesser used, obscure "safety systems" screwed up?

Happy coaching!
Buswarrior

gumpy · April 08, 2018, 08:14:40 AM

Check the switch on the rear control panel to make sure it's in the RUN position.