[Archived]

AC or the pneumatic system?

Lots to check on both, but in the case of the pneumatic system it will take more than a walk around. The AC can be operated and temp differentials checked with a thermometer or IR gun.

[Larry]

to make it clear, asking about A/C system. All of my other buses have not had OTR ( Bus Air ) so this would be a new system to me.

[DON]

Run the coach and check the differential temps between the supply and return air.

You don’t say the age of the coach, but on some multiplex coaches you can do a system check by initiating it at the message center and then following it around the coach and hearing all the various clicking and fan operatig noises.

[Larry]

The bus is a 2000, and what range should the diff. tempatures be?

Thanks Jon.

[Archived]

Here is the shop manual for the Carrier 05g compressor.

http://prevostcommunity.com/PDF/Carrier%20AC%20compressor%20manual%2062-02756.pdf

Pressures are normal automotive R134 #s just like your car. I always get at least a 30+ air temp differential. Another tuning tool is an IR temp gun at very close range at the suction & discharge lines on the compressor when tuning the system.

In the curbside OTR air condensor bay there is a large black accumilator with 2 sight glasses w/a red ball in ea. With the bus at high idle, the OTR air running, quickly open the bay door and you should see the following if your system is healthy: the bottom red ball should be floating in the middle of the bottom sight glass when properly charged.

The compressor has 6 cylinders. If pressures get to their happy place, the system unloads some of the cylinders. You can touch each head and feel this happening when its happening, and you can see the gauge pressures change.

The compressor has 6 cylinders. If pressures get to their happy place, the system unloads some of the cylinders. You can touch each head and feel this happening when its happening, and you can see the gauge pressures change.

[Tim]

That is very helpful, thanks Lonestarace.

[Archived]

Helping is what forums like this are all about. Its a two way street.

To get a normal R134 gauge set functional so you can add and/or subtract freon and see what’s what on the gauges, the suction & discharge fittings hook up at what’s called “King Valves”. The valves must be opened. To open them you screw them “IN”(clockwise) 2 or 3 turns. Yes screwing them “IN” opens them. Then the work can be accomplished. When done the king valves both need to be closed(counter clockwise). This is called back seating. Then put the caps back on with good o-rings.

[Archived]

I hope everyone pays attention to what Lonestarace is offering as advice because it is good advice and make good sense.

The quick and dirty way to get a quick read on the system is look for the red ball centered in the lower sight glass on the receiver when the system temps have stabilized and it is running, make sure the fans are running and get some temperature differential readings for both the driver and coach air areas. Your only tool is a thermometer.

[Dick]

in my h3 2000 liberty I don’t see a fan speed for the passenger side, only the driver. Is that normal? My system has the electronic display in the dash below the radio.

I downloaded the docs available from Prevost and I don’t see anything to show where to change this.

Yesterday i I went by to check out my latest addition – the rv dash on my iPad. Very cool and good info.

But as the coach was 88 when I first arrived I expected after taking a 30 minute drive for it to be less than 84

air seems cool, but low amount of fan speed .

I will take the above info from lonstarance woth me next time – as well as my ir gun.

Thanks for any assistance 🙂

[Mike]

I use a digital meat thermometer and set it in the vent by the driver. I couldn’t see using a digital infrared thermometer to check temp because you have consistently “hit” the same point to get an accurate reading. I do the same thing on the outlets for the Cruise-Airs.

When I switch the OTR/bus air on I watch the temp on the digital count down. Lowest temp I’ve ever seen is 53 or 54 degrees as I recall (I should write this down in my log book) when we need the OTR to be comfortable. I don’t think I’ve ever seen anything less than that.

When we bought our coach and Jeff Rowe “zero’d it out”, we knew it been to Liberty in North Chicago in the previous three years for OTR/bus air service. I recall Jeff changing a valve in the system and probably putting some freon in (don’t all A/C system need freon?). It was quite a while before we ran it in really hot weather and needed the OTR A/C. But when we finally did, it just didn’t seem to cool down … especially when I read about Prevost owners with OTR taking about really getting cold temps in the coach. We weren’t uncomfortable, but I couldn’t brag about hanging meat or aging fine cheddar in the coach on a hot day with rest of the OTR crowd.

So I mentioned it to Jeff during a service. He worked on it, deemed it fixed. When we went to leave I left it sitting on his driveway for a while with the OTR running, came out and was ready to go, and it wasn’t as cold as I thought it should get. Jeff came out and went to work. Agreed it should be colder.

I have come to understand from the forum that there is a solenoid valve I think is up front (under the dash?) that opens and closes to provide hot engine coolant to the OTR heat (and maybe temper the A/C) and I wondered if maybe this valve wasn’t closing all the way. But that wasn’t the problem.

This is where Jeff showed me that the Carrier compressor is a six cylinder compressor in a V-configuration. The system is designed to “drop” pairs of cylinders … apparently being able to run on 2 or 4 or all 6 cylinders depending on the A/C load (a “smart” idea I thought, helps us get great fuel mileage in our coaches). Using a DMM Jeff checked to see if the control for the cylinders was working and apparently it was. (I thought “wouldn’t it be nice of it was only running on one pair of cylinders! Bad wire, loose connection, bad controller.” NO such luck! That kind of luck is for you other guys.)

After about 20 to 30 minutes of fairly deep diagnostics, he declared the compressor “shot”. He put in a rebuilt one. At some point he pulled the heads off the original compressor and discovered that some (more than just one!) of the reed valves (in two or three cylinders I think, can’t find the pictures he sent me) were broken off and laying on top of the pistons. Keep in mind that another forum member who is in the HVAC business says that the same Carrier compressor we have in our coaches is the same one Carrier delivers for commercial A/C applications and those things run for many decades and are very reliable … so they must be pretty stout.

So … IMHO the Prevost OTR/bus air system works on the relatively simple physics of A/C systems found in our homes and passengers cars and trucks … but because of it size to cool a 45′ steel box in very high temperatures with 42 or so passengers throwing off lots of BTUs, the system is much more involved than what most of us have encountered in much smaller vehicles. So there are lots of “things” that could go a miss in the OTR system, I happened to “buy” the “rebuild compressor” lottery ticket.

Reflecting back, it would be pretty hard to figure out if you had broken reed valves in the compressor in a pre-purcahse inspecting. If you’re inspecting on a cool, cloudy day its hard to build heat in the coach to try to test the effectiveness of the OTR. You could fire up the Webasto and put on full blast, but you’re dumping the coach BTUs out into cool air and I don’t think that would be a thorough, comprehensive test in my mind. 100 degree day in the hot sun … that’s a better test.

Maybe Jeff will offer some more color on what I’ve said, or even correct what I don’t have just right. I’m sure Jon will have some insightful observations to add. Did you guys and gals know that Jeff has been to Carrier school and is certified to repair/rebuild these compressors?

With the rebuilt compressor it really works nice.

[William]

Mike,

You have dash air don’t you, not OTR?

[Archived]

dick. Thanks for the insight. I was told the unit was good to go and recently serviced at Olympia when I bought. It was December and cold, so there was not a good opportunity to check it properly. It blew cool and of course it would because of the cold outdoor air.

I agree on your point of the webasto. That thing will run you out of the coach in no time. Honestly the most comfortable I’ve been in chilly / cold weather. My plastic coaches were always drafty and cold.

Yes my ac system is a otr – it has a passanger and driver controls. The fan only controls the driver side from the main display – I was just wondering if there is some other type of fan speed control.

Im going to have a hvac friend put some gauges on it and see if the pressures are correct.before the season really heats up.

thanks all

[Deleted User]

Mike,

The passenger side on the OTR air does not have an adjustable fan speed. You can only adjust the temp, not the fan speed for pass side.

Don, what should the low side read on the guages when working properly?

[William]

thanks Jerry!

I had seen other mention a fan speed knob before, but I think it was for models prior to the digital controls on the dash.

[Archived]

Our 1998 XLV chassis has two fan speed rheostat knobs; one for driver, one for coach. We also have to temp knobs; driver and coach.

Looks like you guys with the early 2000’s chassis are referncing digital something or other. Do you have digital temps that you can set? Can you see that temp the coach is?