Dash AC Heat - Temperature Control

[Bill R]

Since purchasing our coach about 1 1/2 years ago I have had off and on issues of controlling the temp of the dash air.  At times I would have the temp setting at 60F and nothing but hot air.  And vice versa at high temp settings.   By changing the digital temp controller up and down several times the temp would eventually change to where the temp coming out was close to the selected temp. 

I have determined the cause is that the heater control valve is not always moving when I change the temperature settings.  And sometimes the heater control valve moves significantly more with respect to the magnitude of the set temperature change.  Could this be a bad motor controller for the heater valve?  Should I clean the contacts on motor controller connector?

Is the system designed with a feed back loop that changes the heater control valve as the inside temp approaches the set temp?  Or does it require constant adjustment to get the temp set where you want it.  Right now when the heater valve moves to a position, it just stays there until I change the set point.  And when I change the set point from one temp, to another, and back to the same, the heater control valve does not go back to the same position.  The heater control valve is erratic at best.

Also, recently my digital display has also been freezing up.  No change when pushing the temp select up or down.  If I cycle the ignition switch, it starts to work.  Suggestions on a fix for this.  Clean contacts?

Has anyone experienced these issues, and where do you suggest to start troubleshooting?

 

 

 

[Gary Cole]

 Bill I don't have direct knowledge of your digital controller. However I can share a few troubleshooting suggestions. I couldn't discern from your description if your system uses a blend door or a modulating water valve. The systems are closed loop in that they typically monitor ambient, cabin, and sometimes coil temperature. I've only encountered coil temp sensors in industrial systems so I'm guessing you don't have to worry about that. By necessity there is a certain degree of hysteresis, time and temp band for example, built into the control system in order to prevent hunting. So the delay you noticed could be by design. Your digital display has 2 sources of power if it is capable of remembering previous settings. Try disconnecting both sources of power for 1 minute in order to bleed the capacitors down and clear your data registers. If that doesn't help then I would check to make sure you have a solid ground. Lots of electronic problems are caused by poor grounding. Then I would check the grounded output pin on your circuit board to make sure it is grounding external components. I always use a loaded probe when I'm checking low voltage grounds and 12V supply sources.  Next I would check the thermistors. They are 2 wire and plug into the circuit board. You are looking for 0 to 5 V to ground. Industrial systems do often use higher voltages however I've never encountered that in an automotive system. The output that you are looking for is almost always going to be inversely proportional to temp. The directionally proportional type is most often used in protective circuits. Something around 2 or 3 volts at 70 F would likely indicate a functioning thermistor. It would be helpful if you could find the manufactures chart plotting temp vs resistance. However they usually fail open and rarely drift out of range. You could also inspect the board for burned components or broken trace wires. Also pretty common.

Good luck.

Edited February 16, 2023 by Gary Cole

[Bill R]

@Gary ColeThank you for giving me some things to start checking on.  To answer your question, it uses a modulating hot water valve.  Below is a pic.  I will pull the unit out and look at connections and test voltages.  Have you ever known of the motor controller that modulates the hot water valve to go bad?

 

[jacwjames]

I was digging around in my dash in 2021 and found this laying behind the heater controls, took a picture to try and figure out what it was.   https://pdxrvwholesale.com/products/scs-servo-control-circuit-board-305-2537-rev-d

Not sure if you have one or not, when I looked at my wiring diagrams it did show the controller.

 

Edited February 17, 2023 by jacwjames

[Bill R]

Thanks @jacwjamesI'll find out when I pull out the unit.  Hopefully I do not have a problem with this board because I don't see them available anywhere. 

[Gary Cole]

Bill I don't have any experience with small modulating water valves such as yours outside of an industrial application.  The auto systems which I have played around with have used a blended door design. If your valve motor only has 2 wires then it is likely just a DC motor which stalls against a spring counter force and does not require a reverse function or position tracking.  I'm thinking a simple system like that might be more likely to gum up as you suspect. If the servo board is bad then a generic dc/dc pulse width modulated motor driver board can be had for about $40.00. If your motor has more than  2 wires then things get more complicated. That would indicate that the motor has a closed loop which tracks position. In that case it is probably a stepper motor. Hopefully not. Stepper motor driver boards are not that much more expensive and generally conform to a common protocol. 

 

Edited February 17, 2023 by Gary Cole