Dash heat/ac control valve.

[Stump Jumper]

Motorhome has a vacuum operated dash heat/ac control knob. When the motorhome key is on, the vacuum pump cycles on about every 15 seconds, which is quite annoying. If I pull the power from the vacuum pump, I only have defrost, no matter what setting the dash control is on. The vacuum pump and round ball shaped reservoir is located on the firewall in the front generator area. I can't detect any leaks there. I know that I must have a leak somewhere. What am I missing?

[GraySeal65]

You have a leak. easiest way I know of is to use a smokepro tool.

[fpresto]

You aren't missing anything. You have a leak and just haven't found it. There is probably a hose going to the ac/heat control that has a small leak. As you found out, if you didn't already know, defrost is the default position. It has been that way for many years in the name of safety so that if it fails at least your windshield will be clear.

[Stump Jumper]

Thanks for the input. I track the line from the vacuum pump & reservoir to the control unit. It seems that is the only place that I haven't checked yet. Hopefully a simple replacement of the vacuum line will solve the problem.