Fast Idle Stopped Working

[Fredward]

Yah that is the way mine is set up Brian. Sometimes it works perfectly. But the times it doesn't work it sucks the air gauge down noticeably. Can I activate the high idle with the main switch on but the engine not running ? For testing purposes? Isn't there a relay that is activated once the engine is running that controls things like that?
Fred

[bevans6]

On my schematic, MC-5C and yours should be the same, power is fed from the master switch on the dash to the parking brake switch (I think above the spare tire) to the fast idle switch on the driver's side switch panel to the Skinner valve beside the rear control box (normally closed, opens and passes air with power on), which opens and feeds air to the fast idle cylinder.  You should be able to jump power to the skinner valve inside the remote control box (inside the box where the rear start switches are), you can trace the wire to the valve to a stud in there.  If they are labeled it might be stud 10 on the horizontal block.  You might also be able to handily jump power to it using stud 22 in the engine compartment electrical bay.  You will obviously need air pressure for anything to work.  If it's leaking it could obviously be the skinner valve or either of the two air cylinders or a hose.  If it's electrically intermittent I might start with the parking brake switch.  In your case it would be obvious to suspect the fast idle cylinder but if something else is leaking prior to it, it might just not be getting sufficient air to overcome it's return spring.

Brian

[birdarchitect26]

I wanted to reply to this because my bus hasn't had a working fast idle since I purchased it. I think this is a good description for problem solving fast idle.

Summary: As I understand it there are basically 4 potential problems for fast isle not working:
1- The switch itself is dead / malfunctioning
2- The parking brake solenoid is not working / not getting voltage
3- One or both of the skinner valves is broken / clogged / inoperable
4- The pneumatic cylinder which actuates the physical throttle lever is not working.

Please chime in if I am wrong or mislead.

Starting front front to back. I get 27V from the post to ground regardless of switch/ ign position on the #40 post (The little metal tag on the switch wire has a 40 stamped on it). Which solenoid should I be diagnosing. The one with ground to frame is first in line from front to back, then the red air line solenoid, then there are 2-3 more near the back wall of the spare tire compartment. I'm thinking its the second option but I don't know what I'm look at?

Bird

[luvrbus]

You have a picture of the fast idle cylinder on the governor housing ? if you have Jakes could be there is not a fast idle on the engine

[birdarchitect26]

Ok, this was my goal today, so I've been crawling under the spare tire comp 10+ times already. I happen to just notice the solenoid against the back wall had a wire tag. I cleaned it off and sure enough, there is was, 40. Had to be it. I started the bus, let is air up and crossed the pins with a screwdriver, BINGO, it kicked up to 1000 RPMs, or it kicked up regardless, I could see the tack.

Here is a picture for ref, I'm excited and I hope this helps someone! It is paired with another solenoid back to back and leads over to a square housing and back to the back.

J

[bevans6]

Just for reference, the fast idle air system has two separate air cylinders.  One locks the throttle lever closed, the other works on the buffer inside the governor to actually raise the idle.  Good job finding the parking brake air switch.

Brian

[birdarchitect26]

I was excited to start the bus this weekend to see if my new VDO switch would solve my problems. KINDA. I had to rewire the switch to Normally Closed (NC). This mean the switch itself isn't getting any air, I thought it was funny when I unscrewed the old switch that no air escaped, this is a whole new problem, any suggestions? Maybe the E-Brake valve stuck?

By wiring NC I was able to override the switch and get the Fast Idle to actuate. First problem was the stop cylinder leaked like crazy. I ordered a new gasket. The actual fast idle was super erratic giving RPMs all over the board. I'm assuming the way the close air cylinder is designed that the air pushes that cylinder and alows pressure to stay constant, but because it was leaking the actual govenor valve was unable to maintain a steady pressure.

I took the cylinder off, took it apart and cleaned it. I ordered a new seal for $6 - Measured 18mm ID X 25mm OD X 5mm D(maybe 5.5) - See pictures: https://www.ebay.com/itm/162011307472

I used metric because we are the only country that uses the imperial system and I ordered it online. Oh, the seal is called a U-Cup!

J