Fast Idle Stopped Working

Debo · September 29, 2014, 05:54:42 PM

Hi all,

Like the title says, the fast idle just stopped working on my 1981 MCI MC9 (8V-71N). I have the Spicer 4-speed manual transmission. I was moving it in the driveway jockeying it around, and when I went to park it, the fast idle wouldn't kick in. It's never done this, so I'm seeking advice for the first places to look. Here's my plan...

Start with the electrical and see if the Skinner valve is seeing voltage. If it's not, then I'll trace that down. Looking at the schematic, it looks like it could be the parking brake switch, or the neutral relay. (Is there even a neutral indicator on a manual transmission or is that something only for an auto transmission?)

If the Skinner valve is seeing voltage, then I'm thinking it is probably an air problem. Anyone here know of an obvious place I should start looking that I'm not thinking about?

Thanks in advance for any assistance. I'll post what I find out when I start digging. It stopped working so suddenly I'm thinking its got to be electrical.

LuckyChow · September 29, 2014, 06:35:43 PM

Mine stopped working back in the summer. It was the parking brake switch.

challenger440 · September 29, 2014, 06:51:57 PM

I could be wrong but make sure the parking brake is set. I don't think fast idle will work otherwise. j

LuckyChow · September 29, 2014, 09:24:56 PM

Quote from: challenger440 on September 29, 2014, 06:51:57 PM
I could be wrong but make sure the parking brake is set. I don't think fast idle will work otherwise. j

That is correct.

Newbob · September 30, 2014, 09:28:54 AM

Ahhhh, Good to know! My fast idle hasn't worked since I bought it - and the park brake switch was unhooked also. Guess Ill get a new switch and see if that fixes it. While we are on the subject, what do you all use your fast idle switches for?

Debo · September 30, 2014, 10:40:50 AM

Thanks for the replies guys. I did have the parking brake engaged, so it very possibly could be the switch. This gives me a good place to start, which is exactly what I was looking for. I haven't had a chance to take a look yet to see if the Skinner valve is seeing voltage.

Newbob - I've got a 4000 watt inverter that I'm running two roof airs off of when I'm going down the road. If I stop for a minute or two I flip the switch just to keep the RPM's up a little so the A/C isn't drawing as much from the batteries. I also use it if I'm going to be idling for a few minutes to keep the oil pressure up. From what I understand, none of this is really necessary in the case of a bus used as an RV, but it can't hurt I guess. It may burn a little more fuel, but I don't run mine long enough to make much of a difference. If I'm idling longer than a few minutes, I just turn it off. The real reason for my desire to fix it is that if I'm going to keep it, I'd just like for it to work. Thanks again. When I get to it later this week I'll let you know what I find.

Beesme · September 30, 2014, 04:39:23 PM

I still have 2 figure out my fast idle . Only thing really wrong with my bus. I can turn it on while driving and cruise 65 on freeway . Of course I dont ! It was while troubleshooting .. I'm glad now I know it should only work while parked ... But at park it will runaway at ungodly Rpms.... I am going to address in future .. Thanks for the additional info ... Lvrbus gave me some ideas as to problem ... Just need to get back at it but using bus every wknd ....good luck!

Debo · October 01, 2014, 02:55:20 PM

OK, so here's the update. Since LuckyChow stated that his switch had gone bad, I decided to start there. The emergency brake air switch is located in the spare tire compartment above the removable fiberglass panel. I clipped a jumper across the switch closest to the front of the bus, flipped the fast idle switch inside, and voila, it worked. Bad switch. Here's where I need a little help...

I can always call Luke and get the right thing, but I wanted to see if it's available locally first. Does anyone have a part number for the emergency brake air switch? Mine has the following stamped on it, and nobody is able to cross reference it with anything. "60 BREAK 5653700351"

Just as a data point, there are three air switches located in the spare tire compartment of my 1981 MC9. One is for the brake lights as applied by the treadle valve. One is the switch that activates the brake lights when the parking/emergency brake is on, and one is for the fast idle.

luvrbus · October 01, 2014, 04:46:46 PM

Those are just S3 or S4 stop light switches should be easy to find locally, they may look different but any air switch will work if it closes under pressure and has 2 terminals I think I bought a Velvac for under 10 bucks at CarQuest

Debo · October 02, 2014, 03:37:13 AM

Ah, thank you Clifford. Exactly what I was looking for. I'll check that out.

I thought of something else that may be helpful to some one who owns a 9. While I was troubleshooting, I put a jumper across the terminals of the bad air switch in question. It enabled me to flip the switch and power up the fast idle, but because it wasn't being influenced by pressure, it also enabled fast idle to run with the parking brake off. If you're one of those folks who says their fast idle will operate even with the parking brake off, your switch might be stuck in the closed position. Just a thought.

In hindsight this all makes perfect senses, and everything is operating like it's supposed to, but I hadn't thought it all the way through that if the switch is stuck closed, fast idle will operate with the parking brake off - which on my bus is a disaster waiting to happen. When my fast idle is engaged, you lose all throttle input. This is why they wired it to the parking brake in the first place - so the bus has to be stopped so you can use it.

I hope this is helpful to someone. I like to add stuff like this to the board so folks can find something in the future that might be useful. I'll let you know what switch I go with.

Debo · October 09, 2014, 05:54:30 AM

So I thought I'd post what finally happened with my fast idle for posterity...

After troubleshooting with my handy alligator-clip jumper wire, I was able to figure out that the problem with my fast idle was the pressure switch (located in the spare tire compartment.) Despite some good information from the board here, my local parts guys weren't able to find one. I live in a pretty remote location, and they basically looked at me like I was from Mars for even asking - even though I had the part with intact part number in my hand. Oh well, no worries. Living here has taught me well how to source things online.

I found what I was looking for at eGuages.com (not a plug - just information.) It's a VDO Pressure switch, part number 230-660, that can be wired normally open, or normally closed. Since I wasn't exactly sure which I needed, I ordered one (about $35), thinking that I wouldn't have to wait another 3-4 days if I ordered the wrong thing. Wired it up last night in the normally closed position, and my fast idle now works perfectly. Hope this helps.

Newbob · October 16, 2014, 06:43:53 AM

GReat info - thank you for sharing!!

Fredward · October 22, 2014, 01:30:14 PM

Adding to the thread; I have an intermitent high idle on my 871. Flip the switch and the piston pops out that keeps you from operating the throttle pedal. And the air pressure gauge starts to drop but the engine doesn't always hop up on high idle.

Is there an air cylinder reaching into the governor body that actually speeds up the idle? Maybe it's worn out and leaking and causing my intermitent symptom.

Fred

bevans6 · October 22, 2014, 02:42:23 PM

The Skinner valve that actuates the fast idle operates two air cylinders - one locks the speed control lever and the other is on the left hand side of the governor (as you are facing it). It is also the buffer screw. When the air cylinder gets air it pushes a rod with a spring into play that pushes on the idle speed control inside the governor (the same one that the buffer spring very lightly assists to stabilize the idle) and adds the strength of the fast idle spring to the idle spring. That is what gives you the fast idle. You should not be able to see the air pressure gauge change when you click on fast idle, virtually no air is actually used (call it about a cubic inch of air to actuate both cylinders). Trace the air line that goes to the air cylinder that locks the speed control lever - it should have a tee connection and one line will go to the skinner valve and one line will go to the fast idle cylinder on the governor.

This is all if you have the same fast idle setup on your MCI that I have on my MCI, of course...

If you go poking around with the fast idle cylinder remember that it is also the buffer screw and as such must be adjusted precisely and carefully. Too far out, and your idle will not be stabilized. Too far in and the engine can run away.

Brian

Debo · October 23, 2014, 03:23:44 AM

Hey Fred,

Given your setup is the same as Brian's and mine, I think you're on the right track. The cylinder that controls the governor (or the air line) must be leaking like crazy to make the air gauge drop. Like Brian stated, it hardly uses any air at all. Good luck. Intermittent stuff makes me crazy.