MCI coach heat relay question

[bevans6]

I have never seen anything like that box, but that means little...  Looks like a set of resistor bridges, which could conceivable work in conjunction with that mercury switch.  It strikes me that if your electric water valve still works you could eliminate the whole rheostat/resistor/mercury switch/heater deal and replace it with a simple bi-metallic strip thermostat.  That would close to call for heat, while the mercury switch closes to stop calling for heat, so all you would do is connect the thermostat between pin 2 of the relay and ground (after ripping out what's there now) and switch the connection to the electric water valve from pin 3 to pin 4 on the relay.  That would work.  All that mess with the resistors and the mercury switch is is a thermostat, at the end of the day, just a more sophisticated one with finer control.   Your tell-tale light would work backwards, too, so if you wanted to retain that you would switch it from pin 5 to pin 6.

[bevans6]

This got me curious, so I went out and looked at my heat control relay.  My schematic doesn't show it as a relay like the 102 or the MC-9 schematics do, it shows it as a module with wires going directly to it (not to screw terminals like on the other schematics).  So mine is the normal square plastic relay, same as most others on the bus, but is mounted on a printed circuit board with the resistors and so on also mounted.  Below it is a large diode, and below that is a fuse holder with no fuse.  There is a burnt trace on the back of the PCB that the relay socket is mounted on.  That would be both hard to troubleshoot and hard to modify, I don't think I would bother unless I really had to, and you'd have to do it by getting at the wires on the terminal studs in the electrical panel.  Most of the wires from that module terminate on numbered studs, per my schematic.  FWIW:

[jraynor]

bevans,

Now this is what I expected to see, Not exactly like that but I have seen that type of item online for sale and it had the relay plug in ring and I was wondering where the actual relay was but your picture makes it clear as you would have to add the relay to it. So my question is now, which is the same question that im going to ask MCI tomorrow when I call them, What point in time did it change from a relay, which my bus should have, and went to this little circuit?

also, Where is that mounted at?

[bevans6]

On my bus it's located in the front electrical bay under the driver's window on the outside of the bus, on the right hand side as soon as you open the main cover.  I have a row of relays - the horn relay, the lighting relay, Driver's Defrost relay, the water valve for the driver's heat, and then on the right the heat relay assembly.  My MC-9 manual shows similar, but several more relays, Heat relay still far right.  Above that row of relays there is a fold down cover for the wiring with all of the stud terminals, breakers, buzzers, etc.  My bus is earlier than yours, so at some point they may have changed to a more normal relay mount with screw terminals, as is implied by your diagram.  Your bus is a TMC, while mine is a MCI, so I can definitely see the possibility of detail changes like relay mounting.

Open up your electrical bay cover, and see what's in there along the bottom of the bay.  Should be labeled.

[chessie4905]

Would be nice if you could change it to mechanical relay. Electronic ones aren't as forgiving and will probably be unobtanium down the road.

[jraynor]

Open up your electrical bay cover, and see what's in there along the bottom of the bay.  Should be labeled.

Yeah when I open my front Junction box its only electrical stuff, no water valve in this area. I have the 65 stud box. I've figured out what every relay that will be left does and this was the last one I was interested in why 1 lead wasn't connected. so I think ill have to call MCI to find out why the schematic shows a relay but I have a circuit instead. May have been an upgrade but id rather know before I just change or reconnect the last lead

Would be nice if you could change it to mechanical relay. Electronic ones aren't as forgiving and will probably be unobtanium down the road.

That would be what id rather do. Ive seen one online so I know I can buy it to do so, but Im curious to as was this an upgrade to that type? or did someone replace it with a new setup when it went out

[Oonrahnjay]

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...   

       Very clear explanation, Brian.  But the main thing I take away is that it's both beautifully simple and monstrously complex all at the same time.  Good luck with your work, Jonathan. 

[jraynor]

So here is an update: I contacted MCI about this mystery circuit and I gave them the part number of the relay that should ideally be where it connects. They went off and after about 3 days of digging came back and gave me a part number that is probably an upgrade to the circuit that I have: Drum roll please ... its the same part number I gave them to begin with. So really they couldn't tell me nothing about why my bus had this little circuit on it.

My conclusion, the actual relay control board went bad, someone took it out and found something to replace it and they made it work.

My solution: I purchased a coach heat relay off Ebay and ill be installing it. Now remember the whole reason that I've been digging into this: My coach heat light is always on and should only be on while the coach is under 82 degrees. I'm in Florida and its still hot here (has to be around 90-100 inside the bus near the temp probe) so this light should be out. When It comes in, ill connect this relay up and get the bus started and I would imagine that the coach light would be off. Ill report back with the update

[bevans6]

Cool, who would have thought that you could get that so easily.  Obviously don't forget to install the actual relay in the socket!

[jraynor]

Cool, who would have thought that you could get that so easily.  Obviously don't forget to install the actual relay in the socket!

Yeah I had seen it online previously. so that also started my thinking about it being different. And yes I will be using the relay from the retractable step that no longer exists on my coach. I took that out since its not needed so I have a use for the relay. Would have rather had a spare relay but oh well, Im sure ill get a spare someday

[jraynor]

Relay Base came in yesterday. So this weekend I would like to do a test. I need to get a infrared thermometer, get the bus running to air pressure so the air will run. Get a temperature of the probe in the duct and see what it reads and possible pull some voltages to determine what state its in. All of this with the old mystery box installed. Then swap it out with the new base and relay and repeat.

Anybody want to make an assumption of what might happen? Remember, what I'm trying to do is determine why the coach heat light is always on when the bus is hot from being in the sun.

I'll report my findings as things progress.

As for the relay base I received, its slightly difference than the schematic as the contacts in the schematic are opposite of the actual circuit. (as in, what connects to 1,3,4 are connected to 8,6,5 and vise versa. Same functionality, just odd that it doesn't match factory drawings

[Dave5Cs]

Ice Cube relay.
I actually took mine apart to make sure the contacts were not welded together which happens. They were fine now have to test the relay itself.