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Author Topic: MCI over the road AC question? UPDATE  (Read 4253 times)
jmblake
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« on: April 22, 2009, 05:58:37 PM »

My bus ac quit working on our last trip, the blower runs and the condenser fan works it seams to me that the compressor is not kicking in to cool the bus. The breakers in the bay are not tripped is there a relay somewhere that controls the compressor that could be bad. I think it has to be something electrical because it just quit all of a sudden. a\Any help would be appreciated thanks in advance Jason.
« Last Edit: May 20, 2009, 06:57:34 PM by jmblake » Logged

buswarrior
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« Reply #1 on: April 22, 2009, 07:35:43 PM »

Do you have any refrigerant left?

Check the sight glass on the little tank in the condenser compartment.

Better, put a set of AC gauges on it to see what's what.

Belts are still on? Any loose wires hanging around?

Check to see if you have any voltage at the various electrical connections on the compressor.

A set of AC gauges and some basic AC maintenance knowledge is critical to a busnut to maintain the coach AC at a reasonable cost.

happy coaching!
buswarrior

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junkman42
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« Reply #2 on: April 22, 2009, 08:19:08 PM »

My mc7 1971 vintage OTR decided to quit one fine day!  What happened is that the controller which is mounted under the floor between My driver and co pilots seat decided to give it up.  I managed to add a switch to bypass the controller and basically control the temperature by turning a toggle switch on and off.  If You need a copy of the mod if it is applicable to Your coach I will copy and send it to You.  Regards, John
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« Reply #3 on: April 22, 2009, 09:05:41 PM »

You need the gauges to do most any trouble shooting.  You can't check the freon level with the sight glass if you can't get the compressor to engage.  You can't use the gauges either.  You need to determin if you have freon in the system.  there is a little pressure switch somewhere that detects a low freon condition.  If you developed a serious leak and all the freon ran out you would see that the compressor clutch wouldn't actuate when the system was turned on.  Find the low pressure switch, see if the contacts are closed.  You can jumper that switch and see if any pressure develops but you can only test that way for a short period of time because with no freon the compressor will burn up if allowed to run without oil.  You should see freon gushing past the sight glass in just a matter of moments.  Breaking a fitting open is also a test but there will remain some small amt of freon under pressure that might deceive you.

No freon would be my first guess.  Get a basics AC systems book....it ain't rocket science but that experience is critical.  Nick!!!

John
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« Reply #4 on: April 23, 2009, 03:37:34 AM »

This happen to me on my OTR, it just stopped one day in really hot weather. My compressor has a low pressure and high pressure switch located right on the compressor. You can jumper them out just to see if the compressor clutch engages which is what i did. My clutch was fine, it turned out that I had one loose fitting and a leaking hose connection where it is crimped. Tightened one and replace the other, now we are putting in new compressor head plates. Hopefully this will get my system back up and running again.
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jmblake
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« Reply #5 on: April 23, 2009, 04:06:20 AM »

Thanks guys, I just had the freon toped off before our trip that is why I thought it might be electrical or a relay some where. I'll keep diging. Thanks Jason
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« Reply #6 on: April 23, 2009, 12:08:42 PM »

 Embarrassed I know I don't qualify to answer this post, but just an outside shot: The MCI 9 that I was looking at today had a tag that stated very clearly that air pressure should be at 100PSI or it won't stay on. Funny, I was going to post this question as part of a new thread anyway.

So what does air pressure hae to do with OTR AC?

Doug
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« Reply #7 on: April 23, 2009, 01:07:59 PM »

I don'k know if this the reason, but air pressure keeps the belt tensioner cylinder extended.
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jmblake
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« Reply #8 on: April 23, 2009, 01:52:09 PM »

Yes the air is over 100psi and it does keep the tension on the belt. I called MCI Tech today and they don't think its electrical, they said it may be air locked at the compressor for some unknown reason, they sudjested a AC specialist. I'll have to contact my local AC guy and see what he thinks. Jason
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Nick Badame Refrig/ACC
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« Reply #9 on: April 23, 2009, 04:10:41 PM »

Hi Jason,

Your system could have a High PSI cut out safty switch that has a manual reset.

Your system could have been overcharged causing it to trip. Look through your manual for the location of this safty.

Let us know
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« Reply #10 on: April 23, 2009, 04:52:45 PM »

Hi Nick
 I don't have a manual yet, Where would i look for a reset switch? MCI Tech wasn't the friendlyest since I don't have a fleet of buses. Any help is appreciated. Thanks Jason
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« Reply #11 on: April 23, 2009, 05:33:38 PM »

Hi Nick
 I don't have a manual yet, Where would i look for a reset switch? MCI Tech wasn't the friendlyest since I don't have a fleet of buses. Any help is appreciated. Thanks Jason

MCI tech support has always been quite helpful to me.  They even employ some of the engineers who designed the Dina and I have talked to them.  Maybe I get better service because my bus is a 1995. 
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« Reply #12 on: April 23, 2009, 05:54:56 PM »

I have a copy of the Greyhound factory a/c training and installation/service manual for the MC9 hvac system that I am going to sell when I remove my chassis air. It is about 400 pages of accurate information, and it is THE manual for these systems. Email me at bottomacher@hotmail.com if interested, and I will post it on the classified section of this board.
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Nick Badame Refrig/ACC
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« Reply #13 on: April 23, 2009, 07:46:38 PM »

Hi Nick
 I don't have a manual yet, Where would i look for a reset switch? MCI Tech wasn't the friendlyest since I don't have a fleet of buses. Any help is appreciated. Thanks Jason

Hi Jason,

Well, it would only be between the compressor and the condencer coil and it would be the smaller diameter tube.
Look for some kind of "TEE" with a device that has wires connected to it.
When in doubt, jump it out!
Just remember, that psi switch tripped to save your compressor from failure.

Good Luck
Nick-
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« Reply #14 on: April 23, 2009, 08:11:03 PM »

Jason,

Whatever is wrong with your bus must be contagious. When we first bought our bus a year and a half ago the air was really, really cold and slowly last summer it started to be less and less cold. I pulled the filter this spring and it was covered in dog hair, so I figured when i cleaned it the air would work fine. No such luck. On a test drive yesterday it blew warm, almost hot air. I checked the sight glass in the condensor compartment and it shows halfway up the sight glass but that is with the engine off and on a healthy driveway incline with the bus nose downward.

Do you check the sight glass on flat ground with the engine running and the ac on???

My next question is, do we ever stop spending money on these things we call buses??

I need a good old healthy gambling addiction...  just kidding. I gripe but I love my bus and the freedom and joy it brings my family.

Good luck Jason and if you find the problem keep me informed and I will do so for you as well.

Rick
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Nick Badame Refrig/ACC
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« Reply #15 on: April 24, 2009, 06:08:32 AM »

Jason,

Whatever is wrong with your bus must be contagious. When we first bought our bus a year and a half ago the air was really, really cold and slowly last summer it started to be less and less cold. I pulled the filter this spring and it was covered in dog hair, so I figured when i cleaned it the air would work fine. No such luck. On a test drive yesterday it blew warm, almost hot air. I checked the sight glass in the condensor compartment and it shows halfway up the sight glass but that is with the engine off and on a healthy driveway incline with the bus nose downward.

Do you check the sight glass on flat ground with the engine running and the ac on???

My next question is, do we ever stop spending money on these things we call buses??

I need a good old healthy gambling addiction...  just kidding. I gripe but I love my bus and the freedom and joy it brings my family.

Good luck Jason and if you find the problem keep me informed and I will do so for you as well.

Rick

Hi Rick,
Only check the sight glass when the system is running.  You should have flow, with or without bubbles..

Nick-
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« Reply #16 on: April 24, 2009, 06:24:16 AM »

A quick look at the book shows no reset switch. There is the 100# low air switch, and the high and low pressure switch, both located on the top of the compressor. FWIW
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« Reply #17 on: April 24, 2009, 07:40:03 AM »

A quick look at the book shows no reset switch. There is the 100# low air switch, and the high and low pressure switch, both located on the top of the compressor. FWIW

The low air switch is in the Penn switch box that is on the outside of the rear electrical panel ()at least that is where it is at on our MC-8). This switch prevents the AC from working until the you have air pressure. This same switch also prevents the alternator from charging until you have air pressure.  If the alternator is working properly, I doubt if this switch is the problem.  Jack
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« Reply #18 on: April 24, 2009, 10:00:12 AM »

The alternator is working fine, The high & low preasure switch would that be in one of the two junction box's mounted on the side of the compresor? Jason
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« Reply #19 on: April 24, 2009, 11:23:04 AM »

The book says the pressure switches are mounted directly on the compressor body.
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« Reply #20 on: April 24, 2009, 07:16:24 PM »

Jason, I looked at My notes and here is what fixed My OTR air.  If You have the mci prints for the bus air, there is a relay that is controlled by a device they call a sensing unit, in reality it is a thermistor that controls relay cr1.  I am forcing cr1 on by bypassing the thermistor with a toggle switch.  I am to cheap to buy the sensor.  I once had a problem with  the compressor solenoid valve which controls how many cylinders of the compressor gets used.  Jumping the thermistor will not hurt anything, the air will start making cold air or not.  Not much help I know but might help.  Regards, john
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« Reply #21 on: April 25, 2009, 06:08:43 AM »

Well Jason I told you i would let you know if I made any progress and yesterday my AC started working again. IU'm not really sure why, the only thing I did was turn the temp control dial to full cold but I have doubted whether that thing is even hooked up. Sorry I can;t be more specific all I know is it is working and I am grateful
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jmblake
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« Reply #22 on: April 25, 2009, 08:08:00 AM »

Rick Happy yours is working, Mine still will not work, I took my temp control out of the return air duct and it looks very clean, I heat light on the dash works normal when you turn the heat up it comes on and turn it down it goes off so I think it is working, My AC guy must be away for the weekend so Ill try him again the first of the week. Until the Iam going to keep checking things out. Jason
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jmblake
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« Reply #23 on: April 25, 2009, 01:49:31 PM »

Don't know if this may be part of problem but this is the 2 junction boxs mounted to the compresor in the first pic the 2 terminals on the left have voltage to both terminals and they are for the Lb presure cut in & cut out, now the 2 terminals on the right only the bottom terminal has voltage the top has none and it is marked High pres cut out and is set at 250, I turned it up to 300 and down to 200 and still no conection. Question should there be power to both terminals for the compresor to work? and how far can you turn them up or down or can I jump the two to see if it works? then the second pic is the other junction box and both terminals have power also. Thanks Jason
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« Reply #24 on: April 25, 2009, 03:39:41 PM »

Hi Jason,

Think of the psi devices as switches. They mearly complete a curcuit. If you remove the wires from the terminals,

you should have continunity through the switches [from one terminal to the other]. If not, it may be faulty or open for a reason.

Good Luck
Nick-
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« Reply #25 on: April 25, 2009, 05:11:01 PM »

Well I just got back home and decided to try and jump the switch and still nothing, Ill have to wait and get the freon checked next week and see what happens. I don't know anything else to try. Thanks a bunch guys for all your help. Jason
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« Reply #26 on: April 26, 2009, 04:38:30 AM »

Jason,

This may sound rudimentary but have you cleaned the filter lately?

It is in the front start battery compartment. Big ol' thing. Just a thought...

Good luck,

Rick
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« Reply #27 on: April 26, 2009, 05:58:56 AM »

Rick
Yes, I cleaned the filter, The compresor just isn't kicking in. Thanks Jason
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« Reply #28 on: May 20, 2009, 06:16:15 PM »

Well finally got my AC guy over It's a busy time and we found a leak in one of the pressure valves on the compressor so he is going to order a new one and put it in next week, But after he left I went to shut the condenser door and heard a hiss coming from there to, I'll have to get a hold of him tomorrow again, it is coming out of the can where the sight glass's are there is a piece coming out of the can that has a slotted end in the end like for adjusting something and there is 2 small hole's on the side of it where it is leaking from. Any body know what this is? Thanks Jason
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« Reply #29 on: May 20, 2009, 07:48:48 PM »

Hi Jason,

The valve on your compressor that is leaking is an unloader valve and this valve may leak if the system is overcharged.

Sometimes they go bad but, just make sure you have the proper amount of refrigerant in the system.

The piece comming out of the canistor that you discribe in the condencer comp. is also a safty.. It's a "fuseable link" It will also

purge refrigerant if overcharged..

Jason, if you need to call me tomorrow, 609-263-2296 office

Good Luck
Nick-
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« Reply #30 on: May 21, 2009, 04:07:22 AM »

Thanks Nick
If its allright I will give your # to my Ac guy because I don't know what I'am looking or talking about, The bus is a black area for him he is mostly commercial and heavy equipment and I don't have any specs on the bus so it is mostly trial and error. Thanks Jason
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« Reply #31 on: May 21, 2009, 05:35:28 AM »

Sure, I'll talk to him.

Nick-
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