OTR Thermostat

[buswarrior]

You tore out the interior and the ducting.

You didn't do any of that gently.

the wiring has been jerked, all bets are off.

Did you cut the cable before or after the disconnect plug?

Was there a resistor taped into the sensor bundle wiring?

How many wires ran to the temp sensor that was installed?

Does the sensor version match the control version?

"D" model buses had several variations of controls retrofitted and re-developed, because they failed regularly. Even after-market ones were out there.

The first line carriers were changing these things out like mad, before the coach even made 8 years old... there's no telling what the lower tier operators did after that.

In some matters, the books for the bus aren't going to help when the bus doesn't match anymore.

It might be easier to design a fix and add it in, than trying to work backwards?

The displayed numbers don't mean anything anyway. Yeah, I know, that'll make some generations heads spin, just ignore the damned numbers after a lifetime of counting on them...

Can you get the interior to a controlled comfortable temperature by making small adjustments to the control and wait for the resulting change in performance?

happy coaching!
buswarrior

[Jcparmley]

That's all very good points.  I guess the numbers don't really matter for me.  But try to explain that to my wife!! The idea of making a new system work rather than working backwards is also a very good point.  All of this is moot anyway if the OTR system is removed.  At the moment it dosen't work.  I have read so many posts about wether to keep the OTR or remove it.  Some say to get it working and others say it's not worth the cost to maintain especially if you aren't going to use the bus but for some weekends each year.  I am seriously considering taking out the OTR AC and just leaving the heat/defrost function or the OTR.  If that's the case then the numbers on the controller is even less important as long as I can turn the system on and produce heat.  I haven't had the ability to test the heat yet. 

[Jim Blackwood]

JC, unfortunately I can't offer MCI specific advice but I do know these devices well from industrial use. If we could just make the assumption that MCI followed industry best practices it'd make things fairly easy but unfortunately we have seen that they may not have always done that. So bearing that in mind, and also considering that I have not yet been into the sensor side of my climate controls please take this in the spirit in which it is offered which is to say advisory but otherwise YOYO.

Good chance we can expect the connectors MCI used to be thermocouple approved, that is to say that they will not change the very small current generated by the thermocouple in any way. If the temp you are reading is abnormally high it is an indication of either a mismatch or a calibration error. Normally you would not have any resistors in the circuit as that would just degrade the signal.

If I recall correctly, the thermocouple requires a fairly specialized sort of balanced bridge circuit which is sensitive enough to tell if microcurrents are being generated and in which direction. Standard practice is to run single strand paired thermocouple wire  but in a bus you might find stranded. So in your case what I'd do first is take the new thermocouple and see if I couldn't find a way to connect it directly to the back of the controller, or as directly as is practical and see what your readings are. Odds are good that you've damaged the wiring somewhere and this should give a proper reading, allowing you to track down the problem. Avoid using jumper wires to make your connections if at all possible other than to hold a couple of wires in contact with each other.

If it still reads high then I'd question the new thermocouple as possibly being the wrong type. If you have the remains of the old one and the junction at the tip is still intact you can still use that to double check, it doesn't make any difference how long the wires are.

Now, each type of thermocouple uses a distinctive wire cover. Most common (K type I think but it could be J) is brown with a black stripe woven through it, then one wire has red insulation and the other has white. These are woven heat resistant insulations. See if your new part uses the same colors as your old one, if not, there's your problem.

Jim

[daddysgirl]

Well hello there everyone!
I have missed you all.

FWIW...the book for the MC8 has a split year for locating those sensors, and not a lot of detailed information. Mine was on the passenger side sitting on the floor.
But...would that particular sensor cause a -40 reading on the dash?
I know the buses are different, but I figured it can't hurt to ask.

And my 2 cents on the OTR air: Keep it! I have found it to be worth it's weight in gold when driving in the summer, into the sun whilst getting sunburn.

 

[peterbylt]

Hi Peterbylt

Did you ever figure out why your dash read -40?   The snow is finally melting and I am looking forward to getting back into the bus to figure this out.  Thanks

I have not got it to work yet, What I have done so far.
 
I pulled the sensor mount out of the top of the first bay (shown in picture) only to find just the holder, no sensor.
 
Contacted Luke, who tracked one of the sensors down for me (expensive little buggers!)
 
I installed the sensor into the holder and reinstalled it into the Duct.
 
End result, no change still reads -40  …….
 
Peter

[Jim Blackwood]

Pete, have you tried hooking it up backwards? (Can't hurt it by doing that.)

I'll be hooking up an Omega controller in a couple days so I need to review the books anyway. The thermocouple colors are:
J= red and white (2nd most common)
K= red/yellow (most common)
T= red/blue
E= red/purple
N= red/brown
R=red/black
S=red/black
B= red/black

So most of these you can differentiate by the wire insulation color. The red is always the (-) terminal.

Jim

[peterbylt]

Pete, have you tried hooking it up backwards? (Can't hurt it by doing that.)

I'll be hooking up an Omega controller in a couple days so I need to review the books anyway. The thermocouple colors are:
J= red and white (2nd most common)
K= red/yellow (most common)
T= red/blue
E= red/purple
N= red/brown
R=red/black
S=red/black
B= red/black

So most of these you can differentiate by the wire insulation color. The red is always the (-) terminal.

Jim

So, I went back in and checked to see if I had installed the sensor reversed as suggested, Found you could only install it one way, wide and narrow connectors.
 
The wire is a “J” red and white.
 
I traced the thermocouple wire back up to the dashboard where the display/controller is.
 
This is where it gets interesting, I found the other end of the  thermocouple wire, with special connector, not connected to anything and no place to connect it into the  display/controller.
 
I examined the controller and current wiring, all look to be nice tagged MCI wiring, I cross-referenced with the MCI wiring diagrams, It looks like my bus has the AC wiring and controllers of a three year older 96A3 that uses a different type of cabin sensor.
 
That is why the sensor was missing.
 
To further complicate the matter, when I purchased the Bus it had a Welch Industries Auxiliary air conditioner in the back that I removed, that is probably when the problems started, the Air Conditioning worked great on the initial ride home before I started tearing things apart.
 
I did not run the bus for at least a year after I purchased it while I did a lot of the heavy conversion work.
 
I now believe Welch changed out the controller and some of the wiring to conform to wiring they had for the Auxiliary air conditioner they installed.
 
I now have to figure out what Welch used for a cabin temperature sensor, and any other changes they may have made, I would currently settle for bypassing the temp control and running the AC full on, controlling the temperature with the on off switch.
 
Peter 

[Jim Blackwood]

You can get small and relatively simple temperature controllers on ebay for about 10-15 bucks, or more complex and complicated ones for about 25, both of which can use thermocouple inputs.

Jim