MCI OTR heating control?

[bevans6]

My MC-5C has the stock OTR heating system and it works great. So great I can only run the fan for about 5 minutes at a time when on the road at freezing outside temps.  The stock electric on/off valve in the passenger side rear engine bay is disconnected.  I tried turning the manual valve down but even with it barely cracked open the heat was too much.  Is there an easy way to reduce the fan speed?  It seems to be one speed - high - only.  Is there a controllable valve that could be installed to control the hot coolant flow?

Thanks, Brian

[Nick Badame Refrig/ACC]

Hi brian,

You can slow the fan down and others here will tell you how but, I think a better idea would be to

install a 24 or 12v selonoid valve controled by a t-stat to stop the coolant flow to the coils. This way

you would have total control right at the drivers seat. I'm sure you can find the right parts to do this

with a simple web search.

Good Luck
Nick-

[buswarrior]

What Nick says.

Just get the stock valving back up to snuff and you'll be very happy.

happy coaching!
buswarrior

[busshawg]

I have exactly the same problem, the wire terminal is broken on my original valve. Does anyone have a source for buying even an good aftermarket solinoid valve? What exactly does it do? I would replace my exsisting valve with this?

[buswarrior]

The electric valve opens and closes to regulate the flow of hot coolant through the heat exchanger.

Very difficult to regulate the interior temp any other way that doesn't involve big temp swings, blasts of really hot air, followed by a cooling period.

By balancing the amount of coolant flow to match the need for heat, you get lovely blended temps, without surges in temperature coming out of the vents.

Measure the pipes, price some alternatives, then price MCI and see who can sell you one cheapest.

happy coaching!
buswarrior

[bevans6]

I thought that the MCI valve was on/off, controlled by a temp sensor that the PO may or may not have trashed...  Are you saying that it is indeed a variable flow control, not just an on/off?

Brian

[buswarrior]

The valve is an on-off valve. Sorry for the confusion.

The system's beauty is that, once the coach interior is up to temp, the valve only opens briefly, then shuts, then opens briefly again, so that the heater core is only hot enough to maintain the interior at the set temp.

There are no wild temp swings when it is working properly, you get nice, comfortable, just warm enough air all the time.

Three key parts, the solenoid valve, the rheostat by the driver to set the temp, and the temp sensor down in the air intake.

All things being equal,

I have been told that the temp sensor is likely the first culprit if you aren't getting the temps/performance you want. The 4 wire plug that never comes apart can be easily cut out and replaced with one intended for box trailer lighting use. Label your wires!

Trouble shooting the valve is easy, test the wire for voltage, feel the pipes on either side for heat indicating mechanical valve movement and water flow. Run a jumper if no voltage, to test the valve.

I have been told the rheostats were an infrequent repair, but that is from first line fleet mechanics, they don't see stuff that is much older than 12 years, so reliability may deteriorate as more years go by.

happy coaching!
buswarrior

[bevans6]

Thanks very much for the explanation, makes a lot of sense.  I still have all the pieces, maybe it's going to be as simple as connecting the wires back up!  I thought the sensor was for the AC, which didn't work and is removed, not the heater. 

Cheers, Brian

[Jerry32]

I don't heat the rear of my coach while on the road and the temp sensor must be in the rear. Can anyone tell me where it is located and could it be moved to the front area? Jerry

[buswarrior]

Jerry32, in my MC8, it is in the air intake at the front of the coach.

Depending on the vintage, it is behind a block-off plate in the metal ductwork that runs against the floor/wall, earlier on the passenger side, later on the driver's side, IIRC.

It will be mounted in the hole where the return air gets sucked down through the floor into the machinery.

If the duct work is not properly sealing the feed and return air flows, the temp sensor is being fooled. There is both feed and return sharing that duct work. If it has been disturbed and the seal between the "floors" inside not carefully installed, you get a short circuit in your HVAC, the sensor is satisfied for temp inside the duct work, while you freeze/boil inside the coach.

So, the parts might be good, the duct install might be bad.

happy coaching!
buswarrior

[Jerry32]

well I have the digital system with a temp readout and the indicated temp is never close to the actual temp. Jerry.

[Tony LEE]

Just to help with the fault-finding, the main solenoid valve is normally open so it needs to be energised to shut it off. If it is disconnected then you will get full heating.

On my MC8, the original temperature control has been replaced with an electronic version with a new temperature probe and it controls the temperature in both heating and cooling modes very accurately.

BTW in cooling mode the AC unit runs continuously and the heating coil is controlled to regulate the temperature. Might make for accurate control, but seems a bit wasteful of scarce engine horsepower to run a large compressor on full load all the time. One day I will get around to modifying the system so that the three compressor cylinders are progressively unloaded to vary the cooling capacity with perhaps the heating being used for fine control for accuracy.

[buswarrior]

The big compressor should be staged to come on progressively?

Unless a previous owner has ganged the cylinders together to save on repair costs...

happy coaching!
buswarrior