MCI 8 air problem

buylow8 · February 14, 2012, 09:24:14 PM

I have a 76 mci8. my problem is my air bags and emerg brake sw will not work. my low air light never goes out, the air gauge keeps going up past 130, if i step on the brakes the pressure reduces with each push. Does any one have an idea and a air system drawing?
kurt

buswarrior · February 15, 2012, 12:06:46 AM

I anticipate you may have more than one problem happening at the same time.

The low air switch, attached to the side of the rear electrical junction panel might be sticking/is broken/needs adjusting. It has an airline leading to it.

Is it below freezing where you are? You may have frozen water moisture disabling the proper function of your air system.

We need your unit number off the builder plate, to choose the correct schematic.

The MC8 are the most complicated of them all, there are 8 different schematics in my book for a 6 year model run 1973 to 1979.

Operators are waiting to take your call...

happy coaching!
buswarrior

Busted Knuckle · February 15, 2012, 03:34:23 AM

"Operators are waiting to take your call...

happy coaching!
buswarrior"

bevans6 · February 15, 2012, 05:52:46 AM

"Operator, Operator, make me a call..."

It strikes me that if the suspension isn't filling, the low air warning isn't going off, the governor pressure sensing line isn't switching the compressor off, the brakes work but the emerge brake switch doesn't, and the air pressure gauge works - a common point is the check valve and line coming out of the dry tank. They all tee off that line either just before or just after the E-filter and the pressure protection valve.

Edit - on my MCI the governor pressure sense line, the rear low air switch, and all of the accessories in the engine compartment are all fed off of one line that tee's from just after the pressure protection valve. The suspension tees off there as well, and the emergency brake tank is fed from there too.

Brian

RickB · February 15, 2012, 06:27:05 AM

I can't speak to the other issues you are experiencing but the gauge should lose a bit of pressure every time you engage the brakes as you are releasing air to apply the brakes. The cutout point for your governor is easily adjustable with a set screw on the top of it. Although I can't remember what the factory cutout pressure is I seem to remember somewhere between 115 and 125 lbs. Can you hear a leak when the bus is running? if not, do you have an access point to your system to hook up a compressor? If you are in an area cold enough to hard freeze you may need to idle the bus for quite awhile during the warmest part of the day to get it to thaw and I'm sure some here know how to add alcohol to your air system to thaw it.

Hope you get it figured out.

RB

robertglines1 · February 15, 2012, 06:37:12 AM

Need more info: see your new to board. need location so we know your general weather. and bus nut maybe close by. been there in you problem. so need a little more info. might be as simple as getting all the air pressure to bleed out of system by shutting engine off or letting engine run to heat up engine compartment.or placing heater inside compartment driverside outside lower compartment where overrule/acessorie tank is. does engine shut off work? reason for asking dumb question. Bob

buylow8 · February 15, 2012, 08:29:00 AM

The bus is located just north of Little Rock. I think Brian is on the right track. i also think one part is causing the problems. I let the bus idle for atleast 30 min yesterday and sun. sun was cold so i thought it might be somethig frozen, so i went out yesterday at the end of the day and it still had the problem. I will go back today and get the unit #.
one more thing, i have a gauge on the back of the compressor and it read 0 psi
kurt

buylow8 · February 15, 2012, 08:36:42 AM

Bob
i have never been able to shut off the bus from up front in the 5 years i have had it. i have to put the sw upfront to off then go to the back and turn a lever on the engine behind the belt to get it to stop. the emerg protected shut off sw upfront doesnt work either
kurt

bevans6 · February 15, 2012, 08:57:18 AM

I would start by draining the wet tank and the dry tank to zero and see what comes out. The dry tank is behind the wall of the drivers side rear luggage bay, you get at the valve by lifting up a little access door immediately on the right with the bay door open.

Brian

thomasinnv · February 15, 2012, 11:32:17 AM

stuck pressure protection valve.

buylow8 · February 15, 2012, 07:45:40 PM

ok i got the #'s

unit # T57-579
ser # 1590
sept 1976
kurt

buylow8 · February 17, 2012, 09:51:52 AM

Thomas
thanks for the air system drawings. All of them are very similar to each other. I think your right, the pressure protection valve is stuck or even the filter before it is clogged. would you have a part # for them?
Also does anyone know where the low press light gets it info from. Where does the press gauge get its info from? The drawings dont show this info.
Kurt

bevans6 · February 17, 2012, 09:58:46 AM

On my similar vintage MCI the pressure gauge reads from the dry tank, and the low pressure light reads from accessory system. actually the same line that feeds the governor sense port and the accessories in the engine bay.

Brian

buswarrior · February 17, 2012, 02:15:17 PM

great news, you're getting there!

That's a TMC coach, not an MCI, the unit numbers are a different scheme.

You need to be very watchful as to which way your coach is plumbed, depending on the job you are trying to do. There were big changes in the 1975/76 years, which were then reversed, and then somewhat reinstated, over the course of approximately 2 years. Air gauge plumbing is one of the things that wandered around, back and forth between accessory and service tanks

The confusion has only gotten worse as the years pass, not better, as various owners have changed things.

Maybe I drew it up once and posted the points of division by MCI unit number? Manual is hiding at the moment.

The low air switch is a "Penn Air" switch, housed in a metal box attached to the rear electrical junction box, all the way around the corner from where you look in. It served multiple purposes, show the low air light to the driver, and engage the alternator and coach HVAC once sufficient air pressure was present for a belt drive alternator to not throw the belts on loading. It is adjustable and like all moving things that are old, sticks with disuse. There will be a small airline going to it, from a junction block on that side of the engine, if all that hasn't been stripped out by a previous owner.

happy coaching!
buswarrior

buylow8 · February 17, 2012, 08:48:33 PM

I am going to look under and around the bus and see if i can find that bad part in the morning. Why couldnt they build the bus so it sits more than 3" off the ground when the air bags are flat. guess i will bring my shovel and dig a trench under it.
kurt
Also, this bus has the 2 tanks above the front axle. and i assume there is no access panel from the top?