MCI Cabin Heat Control

Doug1968 · February 14, 2009, 06:59:04 PM

I have a 1986 MCI 102A3 that has a knob on the left side of the drivers seat that appears to be a potentiometer control? I also notice a shutoff valve for, I assume engine coolant in the forward bay on the curb side of the bus. Can someone explain to me how to operate the heat system on my bus? Should the valve in the bay be full on during the winter and shut off during the summer? Does the knob (potentiometer) control a coolant valve or does it control a fan? Do you turn it clockwise for more heat and does it act like a thermostat with more heat as you turn it clockwise? If someone can explain the proper operation and function of the bay valve and the knob marked "heat" that is on the panel to the left of the drive I would greatly appreciate your help?

Brian Diehl · February 14, 2009, 07:21:02 PM

Doug, the heat is turned up when turning the knob clockwise. You turn it counter-clockwise to turn down the heat. The knob operates as a potentiometer that adjusts when the coolant valve in the baggage bay turns on or off. If you don't have an electric valve in the bay then someone has "modified" the system as the valve in the bay should be electric. Is the valve in the bay manual as your post leads me to believe? If so, call up Luke and have him send you a new valve so you can get your system working correctly again.

Doug1968 · February 14, 2009, 07:34:20 PM

Brian, the valve appears to be manual? It has a round knob on it like a faucet you would have outside in the yard. So the potentiometer controls the heated coolant from the engine by opening or closing the valve as you move the potentiometer up or down. Without the automatic valve in the bay I would have no control of the heat, just as I am seeing. I will check with Luke on the original valve. Thanks!

Tony LEE · February 15, 2009, 03:09:19 AM

You may need more than the valve to get it all working again because there is a sensor in the return air duct and together with the potentiometer control, operate a special relay that then operates the valve. It is an obsolete system that is often replaced with an electronic controller.

Note - assuming it is a similar system to my MC8, there are two isolating valves in the engine compartment as well as the electric vlve in the front bay. Also note that the electric valve is normally open so the control energises the valve when the temperature gets to the set point.

There is also a manual valve in the driver compartment that isolates the water to the drivers defrost system.

Brian Diehl · February 15, 2009, 08:03:21 AM

Doug, you can check to see if you have the sensor in the return duct. On the driver side of the bus in the forward most baggage bay look at the tunnel. You should see a round "plate" with three screws holding it on. There will be wires going in through the plate. Take the screws out and then pull the assembly out. If you have the assembly and associated sensors then it might be worth your effort to get the system back to working correctly. You may also explore with Luke any other options available since you don't have all of the system right now.

Doug1968 · February 15, 2009, 08:23:53 AM

Tony - Brian,

I'll take a closer look to see if I have the sensor in the ducting. I also want to take a better look at the water valve in the front bay. As I recall it did not look as though it would be easy to remove?

I'll check this over today and get back to you guys. I appreciate your help. Thanks, Doug

Dreamscape · February 15, 2009, 08:28:00 AM

Doug,

Sounds like you're getting some good help with the heating questions.

Good Luck and I hope Lori is feeling better,

Paul

Jerry32 · February 15, 2009, 02:02:13 PM

I have a 102 and the thermostatic control is digital in the dash and shows the temp from the sensor in the duct. I also has a heat control on the left of the driver on the floor that pulls to turn on heat for the driver and defrost. jerry

Doug1968 · February 15, 2009, 04:08:42 PM

OK, here is what I found. It appears as though the ducting sensor is still in place. after removing a fiberglass cover I see that the automatic valve has in-fact been replaced with a manual valve? I'm not sure how the previous owner controlled the heat? There is a ground wire and a black wire (taped) near the manual valve. I assume that this would attach to the automatic valve solenoid.

Jerry, your 102 is an 88 and I think that MCI changed to the climate control design with the digital control on the dash after 1986.

Now, I have heard, as Brian noted that there may be an upgrade kit to convert to the later digital design. I would assume that if I replace the manual valve with the proper automatic valve that the system would work fine. Are you fellows that have the system that uses the potentiometer happy with the way it works? Please give me your thoughts. I would think that it will be cheaper to purchase the valve and stick with the potentiometer if this system worked well. Let me know which way you think I should go. Thanks

Jerry, I see that you live in Hermiston, Oregon. Sometime when possible lets get together as I would like to see your bus.

Brian Diehl · February 15, 2009, 04:57:54 PM

Doug, my potentiometer control works well. I had to move the duct sensor up to infront of the main heater core to get it to control the temperature better. Once I moved it to right infront of the heater core it has worked awesome. It maintains the temperature so well that I can not tell or notice any temperature swings. It does work really well. I considered upgrading to the digital controller myself, but couldn't justify the cost once I figured out to move the sensor to an area of greater air flow.

Jerry32 · February 16, 2009, 03:40:15 AM

I need to move my sensor too as I don't use the rear ducts anymore as they are blocked off. Drivers heat works but not enough heat so I need the main system too. Jerry

jraynor · July 19, 2019, 08:11:32 AM

Quote from: Brian Diehl on February 15, 2009, 04:57:54 PM
Doug, my potentiometer control works well. I had to move the duct sensor up to infront of the main heater core to get it to control the temperature better. Once I moved it to right infront of the heater core it has worked awesome. It maintains the temperature so well that I can not tell or notice any temperature swings. It does work really well. I considered upgrading to the digital controller myself, but couldn't justify the cost once I figured out to move the sensor to an area of greater air flow.

Hey Brian, do you have any pictures of where you moved the temp probe to? I know you said in front of the heat core. I'm assuming as the return air enters the bottom and right before it goes through the air filter.

I too have the manual heat control knob. Everything looks to be factory except that they changed the heat relay out for some type of circuit box in a aluminum enclosure that even MCI doesn't know what it is. I bought a new relay but have not hooked it up and my heat knob is not even connected. But I still get the coach heat light on the dash and it does come on and go off. So I've been working to update everything.

Thanks, Jonathan