MCI coach heat relay question

[bevans6]

Question is - how the heck does that circuit actually work, anyway?  I see a funky relay box, with two resistors in it, a sensing unit that I think is a  temperature controlled variable resistor, a rheostat that should act in series with the sensing unit and the 920 ohm resistor inside the relay box, and somehow I think the resistors all add up to voltage dividers that somehow switch the relay, probably in conjuction with the diode bypass of the coil, to turn voltage on and off at the electric water valve.  But my electronics course in college was sadly lacking in relay control circuits like this...  I have no idea how it actually works and hence no idea on how to trouble shoot why it doesn't work...

a blow by blow explanation of the circuit would do me a world of good.  I am looking at a diagram from 1979, MC-5c  manual, but I bet this circuit never changed from the 1960's through the 1980's and the whole MC series of buses.

help, please!

Brian

[jraynor]

Hello All,

Didn't want to start a new post for a similar topic. I have a 1986 TMC 102A3 and I have a similar question for the coach heat relay. I have attached a picture which does apply to my coach so this is accurate. The issue that I am having is that my Coach heat tale tell is always on. even when the AC is on. As I was cleaning up the front junction box I came across this aluminum box to which I figured out was the coach heat. That picture is also attached. An additional issue is that the black wire is no longer connected. Not broke or anything, just taken off and someone put a piece of heat shrink on the terminal and let it hang. after looking at the diagram, it seems that the coach heat light is always on until the mercury switch reaches 82 degrees at which time the relay turns on and breaks the circuit. Is this correct? Would reconnecting the Black wire correct the issue?

[Geoff]

I dunno.  I took all that crap out and use three Coleman wall thermostats for heating and cooling.  Although the $17.96 Honeywell thermostats from Walmart would also work.

[jraynor]

I dunno.  I took all that crap out and use three Coleman wall thermostats for heating and cooling.  Although the $17.96 Honeywell thermostats from Walmart would also work.

Even for the over the road Heat?

[Geoff]

I got rid of the OTR HVAC,  but the wall thermostats could run that too.

[bevans6]

What a horrible circuit.  Reconnecting the black wire might fix something, but it was disconnected for a reason, and who knows what that reason is.   Or where it should be connected.  I will now, having gotten better with relays in the last 8 years, take a stab at unraveling this circuit.  I am basing this on my 1980 MCI schematics, which are older but still probably similar to yours.  The bit you've provided is electrically identical to my schematic, although drawn differently.  I also checked 1989 MC-9 schematic, it is also extremely similar (the 2250 ohm resistor had a value of 2700, which would change the control over the heater in the mercury switch).


Relay control is by pins 7 and 2.  Pin 7 is +24, and is also connected to pins 8 and 1, which are the common pins for the two sides of the relay.  Pin 2 is connected to the mercury switch, which closes at 82 degrees F (but cunningly has a heater element, so not necessarily 82 degrees room temp).  When the mercury switch is open, the relay is in the off state.  Power is present on pin 8, and is transferred to the normally closed pin 5, and turns on the tell-tale light.  All that side of the relay does is turn that light on and off.  The relay will turn on (and the Heat Tell-tale will go off) when the mercury switch closes at 82 degrees.  The heater in the mercury switch is controlled by the other side of the relay.  Power comes out of pin 1 to the rheostat (variable resistor, potentiometer) which has either a 5K ohm resistance at the counterclockwise position or zero resistance at the clockwise position (one presumes full off and full on).  The output of the rheostat is combined with the output of the 2250 ohm resistor connected to pin 4 of the relay to apply power to the 600 ohm heater in the mercury switch to warm it up.  At this point, the whole hot mess is in "heat mode" and power is off to the electric hot water valve back in the engine compartment, and hot water flows in the heaters, providing heat to the cabin heaters.  As soon as either the ambient temp or the combination of ambient plus the mercury switch heat reaches 82 degrees, the mercury switch closes, the relay turns on, and two things happen - first, the tell-tale turns off, and second, the hot water switch in the engine bay gets power, closes and turns off hot water to the heaters.  Thus is the thing controlled.  The control circuit is the rheostat (coach heat pot in this schematic, but I wrote this looking at my older schematic) in combo with the two resistors and the resistance of the heater in the mercury switch.  The various diodes are probably arc suppressing to keep relay contacts clean when things switch.

Now,  this circuit is powered by the Discharge and Blower Cut-in Relay which is itself controlled by the Relay terminal on the alternator, which is controlled by the low air pressure switch.  When air pressure is building, this circuit (along with the AC and heater fan motors) is disabled.  When air pressure is up above the cut-in point (usually around 95 psi) the air pressure switch closes, the alternator is turned on and starts charging, which causes the Discharge Etc Relay to close, which provides power to the whole heating and air conditioning system.  The Coach Heat/AC switch doesn't control the dash Heat Tell-Tale, only the mercury switch controls it (and the water heat valve).  Note one thing that you might have missed - when you are using the factory stock OTR air conditioning, it is full on all the time and coach interior temperature is balanced and controlled by the coach heating system, so coach heat is used to mitigate the cooling effect of the air conditioning to maintain a comfortable interior temperature.  Coach heat is also either on or off, it does not have a "half on" setting like the thermostat on your car, hot water is either flowing fully or it is not.  Note one other thing - the electric heating element in the air dryer purge valve is controlled by the coach heat/AC switch, it is on when coach heat is selected.  If you disable or remove your coach heat, remember to add a switch to turn the air dryer heat on.  (and the expello valve heater if you have one).

This is all I have on that.

Edit;  OK, I lied, I got one more thing.  The variable power to the 600 ohm heating coil on the mercury switch is provided by a variable voltage divider formed by the 2250 ohm resistor in parallel with the 920 ohm resistor in series with the variable resistance of the rheostat.  Basically 2250 in parallel with 920 plus whatever the variable resistance is (0 to 5K ohms).  Whatever resistance that turns out to be is divided over the 600 ohm resistance of the heater, and so the heater gets fed by a variable voltage, making it heat the mercury switch more or less.  Some electrical engineer in the 1950's was probably very proud of him/her self the day he/she drew this circuit...

Brian

[bevans6]

So all the junk I just figured out and wrote prompts a question to Jraynor - are you using the factory heating and air conditioning system over the road, and if not, do you want to?  If you are, you need to diagnose and fix whatever was broken to cause the temperature control relay to have been disabled.  If you are not, you can just pull the heater control relay, which is disabled anyway because the black lead has been disconnected (maybe - black is probably the lead to the 2250 ohm resistor and I think the system will work without the 2250 ohm resistor in the circuit, just differently) and the tell-tale will go off.  If you do plan to use the factory systems, then you should understand that the Heat Tell-tale is supposed to come on when the AC is on when the factory heat is being used to mitigate the power of the AC cooling and keep the interior temp at a comfortable level. 

[jraynor]

Thank you bevans,

I have a few questions to clarify: So basically the heat in the bus is used to keep the bus from getting too cold. I know someone has said that the AC is like 85k BTU. So that would make sense as my coach doesn't have the temperature unit to keep everything balanced. I guess during the summer when they want it colder is when they shut off the water valves (1. question here to make sure, left opens the valve and right shuts it right? 2. My valves turn very little so are they 1/4 turn valves?). As for the coach heat T/T, it is always on until it gets to 82 degrees. Unless of course you have the pipes closed and it won't ever get that hot. 3. So I assume everything is working since the T/T lights up? 4. Do you know where this Mercury switch is located? I'd like to get a temp reading on it to ensure it's doing the correct operation at 82°. As for the disconnected lead. I'm not sure why it's disconnected. It's strange to me but I plan to take the relay off and test it to enusure functionality. Just makes me wonder they they took it off. My initial thought was that it was the reason the T/T was always on.

In the end, if the heat is used to keep the coach comfortable versus getting too cold, that would make sense of why it's always on. I guess I'm too used to modern A/C /heat where it's either on or off not both on

[bevans6]

In normal operation in a stock bus, the heater water shut-off valves are all left open fully all the time.  That means you can get dash heat for the defroster, which you sometimes need even in summer, and control the AC temp.  There are three water shut off valves, as far as I am aware - the main return valve is down on the passenger side engine rail, after the electric on-off control valve.  The main send valve is up above the transmission and you reach it through one of the floor hatches.  The driver's heat return valve is up above the rear electrical panel on the driver's side.  And of course there is the driver's heat control valve beside the driver's seat.

Next - in the summer when they want full cold, the temperature control pot can achieve that by turning the mercury switch heater on fully. If it's full on, the switch is always closed and the electric water valve never opens, so no heat.  It's probably set up for a minimum coach temp of around 68 degrees or so, and that is controlled by that voltage divider with the resistors and the potentiometer.  The mercury switch is located in one of the cold air return ducts, mid-bus, on the floor.  I suspect a lot of converters rip it out and cover up the ducts, at the same time screwing up the factory heat/air.  You are unclear if your Tell-tale actually turns off at 82 degrees or not.  If it does, that implies the mercury switch is still in the circuit, so you are half way there.  If, as I suspect, the black lead is part of the 2250 ohm resistor circuit, that might disable or change the set point of the mercury switch heating circuit, which would remove or change the ability of the circuit to control the temp down below 82 degrees.  The whole 82 degree thing is a maximum temp setting not to be exceeded.  Finally, my manual on/off water valves are multi-twist deals just like an old fashioned hose faucet - turn maybe three or four full turns to fully open or close.  I've tried to use this to control the interior temp, like a variable flow deal, but honestly it doesn't work.  On my bus the AC is gone, the factory heat remains, the electric flow control valve is disconnected and I just turn the water on manually when it's cold and control interior heat by turning the fan on and off.  Now that you have prompted me to figure this whole deal out I may try to fix it... 

[jraynor]

My OTR is still intact and still works. Knock on wood. The temp probe is still in the duct. I may need to reposition it because I think I slide it back in it's bracket while replace the duct floor wood and probably forgot to put it back. But I should be able to adjust it via the access hole. What also prompted this is the fact that I'm sure it's 100 degrees in that bus since I'm in Florida and the heat says it's on. But maybe disconnecting the black wire has cause something to not switch off at 82°. That's my initial thought. So what I'll do is take the relay out and test it. I just need a 24v source to activate the relay which I need to buy. My power supply only goes to about 19V

[buswarrior]

Coach HVAC is a re-heat system, constant AC, moderated by adding heat, serves as an excellent de-humidifying function, with all those passengers breathing and sweating.

The control valve is powered close, so any failure that removes power from the valve, will really warm you up!

Complicated sounding, but this circuit gets the cooach interior temperature to the set point, and then keeps it there, the air flow kept close to "room temperature" without wild extremes of heat or cold as it cycles.

Most busnuts encounter cold running conditions at some point, the stock heating is a lovely system to keep functional.

If the boss ever finds out that the cold interior is because you removed a perfectly good heating system...

Happy coaching!
Buswarrior

[Geoff]

It sounds like a lot of wasted fuel to run the heat and the A/C to balance out the interior temperature.  Okay for 45 passengers and big buck companies, but why for a conversion?

[buswarrior]

For the same reasons?

Humidity is a big factor on hot days in much of the continent.

Many campers will tell you of the times that the unit was clammy inside, but the AC wouldn't stay on long enough to dry it out.

The newer buses have even more complicated HVAC controls that moderate the AC running as well as the heat for somewhat more efficient operation.

Those who maintain their complete stock HVAC system enjoy a drier coach.

Happy coaching!
Buswarrior

[richard5933]

It sounds like a lot of wasted fuel to run the heat and the A/C to balance out the interior temperature.  Okay for 45 passengers and big buck companies, but why for a conversion?

It was explained to me that in order to avoid destroying the a/c compressor on a GM it should only be engaged with the engine at idle. In fact, our 4108 has a safety system that only allows the a/c compressor clutch to engage when the oil pressure is below a preset point. Turning on the a/c compressor at highway speeds, if I understand correctly, would apply too much torque to the compressor too suddenly and basically cause it to fail. Some of the models have placards on the controls reminding the driver to only engage the a/c at idle.

Another reason I wouldn't want the compressor to cycle on and off is the amount of time it takes to bring the system up to pressure and start making cold. From a cold start on a warm day, it might be 15-20 minutes before the a/c is making cold air. The system has about 25 pounds of refrigerant, and it takes time to get things up to pressure. If the compressor cycled on and off it would take forever to start the cooling process again.

The manual also lists a reason the heat system is used to temper the cold air. When in passenger service the are 40 bodies on board, each adding moisture to the air. Without the a/c compressor running all the time the air would quickly become damp and uncomfortable, even if it was at the correct temperature. By keeping the compressor running and using the heater to temper the air temp moisture is continually removed.

I gotta add as well, as complicated as I thought the GM HVAC system was it sounds like you guys with the MCI coaches really need an engineering degree to figure things out. I've always been impressed with the utter simplicity of the systems used on the GM models. Sounds like MCI built a ENIAC computer from relays to run the system.

[jraynor]

So I opened the silver box to make sure everything was good and to see if there was a relay. Well I found this instead... A circuit. Not what I was expecting so now I'm trying to understand it's function. It was easier when it was a relay but my diagram does not show a circuit. I've seen one for a prevost for sale online so I'm wondering if they updated the design and went with that. My schematic is for units prior to March 1st 1988, and mine is an October 1986. So I assume that would be whats in my coach, but I'm not sure.

Has anyone seen one of these?