Need help with Heat Control on 96A3 (1985)

[Brian Diehl]

I need some help!  I'm trying to get my coach heat control to work correctly.  Last year the coach tell tale started acting up.  It started flickering while it was lit.  In troubleshooting the flickering I found several loose solder joints on the control board where the solenoid is connected.  I re-soldered all loose joints and put things back together.  Upon running the coach I find it is now running at least 25 degrees hotter than it was before.  This is so much hotter that the coolest setting is too hot.

So, I am now troubleshooting this problem.  On the control board are two resistors and one diode.  Both the resistors and the diode check out and function correctly according to my multimeter.  The rheostat on the dash control also appears to work correctly going from 2.2 ohms to 4900 ohms.  This appears to be according to spec.  I have checked my wires at the heat control in the duct work.  I have good power and grounds.  Since everything appears to check out I thought I would try and eliminate the heat sensor as the problem.  According to the wiring diagram the sensor closes on heat rise.  So, if I connect pins "A" and "C" together with a jumper cable this should be the same as the heat sensor doing the same thing.  Well, when I connect pin "A" to "C" nothing happens.  This should be the "calling for cooling" mode.  The "calling" for heat mode is pin "A" grounded to pin "B".  If I do the same thing with a jumper cable again nothing happens. 

The heat control solenoid should close when the heat sensor closes on heat rise.  However, I can't get it to do that with the jumper cables.  While troubleshooting I also checked voltage at pins "A" and "C" as both appear to always have voltage on them according to the wiring diagram.  Pin "C" always had 24.8 volts on it.  Pin "A" had 25.0 volts on it when the heat control solenoid was in the "calling for heat" position.  If I manually closed the solenoid in the "no heat" position then pin "A" would drop to 24.5 volts.

I am now at an impasse.  I have checked everything I can think to check and still don't understand what isn't working.  Can anyone help me troubleshoot this further?

I am uploading a picture of my wiring diagram.  I will have to reduce the quality to upload the pictures.  Email me if you want me to send you the higher quality photo.

Thank you in advance for any help you can provide.

[Brian Diehl]

wiring diagram ...

[buswarrior]

Those were a pain in those series of coaches. Many did not leave their first line fleet without some sort of parts changing.

Finicky at best even when they did work.

Call Luke, he'll zone you right in on what to check, and what you need to simply replace.

1-888-862-2434.

happy coaching!
buswarrior

[Brian Diehl]

Thanks for the encouragement buswarrior!  I wish I understood the theory of operation a little bit better on these.  I'll give Luke a call on Monday and see what he has to say.

[Brian Diehl]

I did some more troubleshooting yesterday.  I discovered the diode on the relay control board I thought was good was not.  It was allowing up to two volts of reverse voltage when it should have been 0.  I removed the diode and replaced with a new one.  So, the control board appears to be working correctly now.  I then turned my attention to the temperature probe.  I pulled the glass mercury thermometer looking probe out of the brass tube it is mounted in and found the mercury is not one continuous column anymore.  I think this is now my remaining problem.  I did some searching and found an electronic replacement.  These guys sell them:  http://3-gproductions.com/.  The part is the second item on their catalog page.  Here is the direct link to the part:  http://3-gproductions.com/new/thr_124etherm.html.  Has anyone ever used one of these?  How well did it work?  I'll call tomorrow and get some details and pricing on it.