MC-8 OTR A/C Performance

[DavidInWilmNC]

I'm getting ready for a trip in my MC-8 next weekend and I'm expecting it to be HOT in Georgia.  I tried out the OTR air and it didn't seem very cool - supply side air was about 72°.  It's 91° outside and the bus is about 80° inside; I had the roof A/C on for a couple of hours.  I'm not really familiar with working on the refrigerant side of A/C systems, but I do have a gauge set.  It was at around 35psi.  My gauge says this is the low end of charged, with 45 psi being the high end of the suction side.  I put in about 4 cans of 134a and got it up to around 43 psi.  The supply air got down to 60-61°.  I have the water valve in the engine bay turned off, as I'm not sure how well the temp control works.  Plus, if I happen to get too cold, I'll just turn the A/C off.  The engine is on fast idle.

Is this what  I should expect from this system, or should it get colder?  It seems that with the volume of air those blower put out, 61° air might be sufficient.  I know that my Avalon, idling in traffic in 92° temps has an air outlet temp of around 39°.  The air from the OTR blowers come straight through the floor with a grill over it at present.  As I block the opening a bit, the blower gets much quieter and the air temp drops some.  I'm sure those blowers are designed to have a little more resistance to air flow that what I currently have.  I'll duct them to the middle and back eventually.  Any input from those who kept their OTR A/C would be appreciated!

David

[Tony LEE]

Hi David,
Sounds like I have a similar setup to yours. One way of lowering the fan speed slightly is to connect the fan motor auxiliary terminal to the supply in the cooling mode just as it is in heat mode.
I just have it blasting out of the floor and will be throttling it back a bit as you suggested because the previous dimple plate air distribution system would certainly have been much more restricted than blasting straight out.

I'll take some temperatures when I get a bit more north (winter here in the south) but it certainly drags the temperature down very rapidly.

I won't be bothering to duct the air anywhere else because the front half of the bus is well covered from the two vents and there is little point worrying about cooling the back half since it is not used on the road.

[Beatenbo]

The MCI Bus air should hold 18-20 lbs of freon from empty. While running the freon level should be 1/2 way on the sight glass in the condenser  compartment. Don't try to charge with door open. Open barely to view sight glass and close back. ALSO WHEN PUTTING IN FREON MAKE SURE CONTAINER IS UP RIGHT PUTTING IN GAS. do not turn over allowing it to to dispense as liquid. Big trouble for your system. The big or suction line at the compressor should be cold to the touch and 40-45 on your gauge.

[Nick Badame Refrig/ACC]

Hi Guy's,

Charlie is right on the procedure.

One note, R-134A refrigerant is a blended gas, all blended gasses need to be dispensed in a liquid state to avoid separation.

With that said, the proper way to fill a system is to charge from a 30 inch vacuum and scale in the exact amount stated on the system.

However, I have never seen a carrier 04G compressor get damaged by adding liquid while the system is running. I have added to a low charge many times.

Good Luck
Nick-

[DavidInWilmNC]

Thanks, guys.  I know I put some of the 134a in as a liquid, but I had the valve open slightly on the can and never allowed the pressure to get very high.  The suction line is very cool to cold.  I imagine I'll get a bit better performance when the bus is actually being driven and the rpm's are higher.  So, is an 18° temp drop acceptable at idle? 

I really would like to reduce the fan speed some, but my motor doesn't have a lower speed connection.  I replaced it with a good used one that was identical to my old one (from a MC-9), except the 'new' one worked.  Maybe some of these didn't have a low(er) speed, for some reason. 

David

[niles500]

David - If the ambient was 91 and the output was 61 - depending on other factors - that's pretty good - I think that Nick will tell you a slightly undercharged system is way too inefficient - that last 10% of charge makes a big difference in cooling capacity - FWIW

[buswarrior]

Hello David.

Nice to see another rebel keeping the stock HVAC!

Outside air intrusion?
What is the condition of the intake side of your system?
Blocked off the outside fresh air intakes on the sides of the coach?
Interior air returns blocked and forcing outside air into the system?

The system was rated at 80 000 BTU when new with R12.

Mine is converted to 134a and does a lovely job of pulling down the interior after being parked in the sun all day.

If the temp control works for heating, it will work for cooling.

happy coaching!
buswarrior

[DavidInWilmNC]

Buswarrior, I don't see any reason to reinvent a good heating and cooling system ('til it becomes too expensive to maintain, that is!).  I'm curious as to the capacity of the system on 134a.  I'll check and see if it needs a bit more before the trip. I definitely want to be cool and comfortable - I'm gonna be nervous enough driving the bus (it's only the second time)!

I've closed off the side intakes when I removed the interior panels and insulation.  I've replaced the ramp with wood and there's a step in the front.  The space between the floor and step is for return air, as are the intakes along the inside walls.  I'll eventually pull up the front floor and change some of the return air intakes.  I probably need to seal the blower access panel in the front bay a little better, though.  I know it has to be sucking in some air from that bay.  I have no clue if the temp controls work for heating or cooling, as I've never driven the bus in the winter.  Will the water valve that regulates temp. fail open (hot) or closed (cold) if it doesn't work?  I'll probably turn the gate valves on and see if there's any heat being applied to the heater coil when it's turned from the coolest position.

David

[mc8 tin tent]

David
  I have found chassis a/c sys. will take a little longer to start cooling after the bus has been setting for approx. 3 wks. or longer, however It will cool down
very well after 5 min.of hi way speed.
  FWIW
    Dwayne

[buswarrior]

Hello David.

Open the water valve and try it out.

There is a dashboard indicator that the temp control valve is open, light on, water flows.

Do not be alarmed that it wants to flow water. This system is designed to re-heat the frigid AC air coming off the coils. So you do get some water flow when the AC is turned on.

Gives pleasantly blended air, instead of blasts of on and off cold, once it reaches temp.

The temp control valve is normally open, so it takes power to close it. The temp sensor is in the air return on the passenger side in mine. Sometime late in the MC8 production, it was moved to driver's side intake.

The control valve is in the front baggage compartment, curb side, up high against the drop channel, against the front wall. It lives behind that bump in the corner.

happy coaching!
buswarrior