Automotive Air Conditioning Information Forum

One way to tell whether you're getting any reheating is to feel the hoses on the side of the valve that go to and from the heater core after the engine is up to normal operating temperature and the heater is turned off at the dash. If one or both hoses are warm to hot, the heater valve has failed.

Been reading a lot about these water valves on an RV forum and a lot of folks have installed a ball valve in one of the hoses to just completely cut off the the flow of coolant to the valve in the summer. Come winter they just open up the ball valve. I am strongly considering this - if following some troubleshooting it does show that my heater valve is bad.

Just trying to figure out the best place to install the valve.

Good idea. In fact, another member of this Forum has done exactly that on a pickup truck. I forget which topic it appeared in, but it was recent and secondary to the main discussion.

On many GM vehicles, the inlet hose that sends hot water into the heater core measures 5/8" ID and runs from the top of the engine to the bottom of the core. The outlet or return hose measures 3/4" ID and directs the cooler water from the top of the core to the suction side of the water pump.

I'm not sure whether your motorhome has a similar configuration, but I suggest putting the ball valve somewhere on the inlet or hot-water supply hose, usually the smaller of the two.

OEM heater water valves are notorious for poor function. You just cannot beat an inline ball valve. An easy install. I have done this on a vintage p/u and one other car. If your low side is 30 and your vent temp is 50 (for example) then be looking at reheating.
In fact my thanks to JohnHere for adding this to our bag of tricks!

I just checked again - at idle - Hi side 235 Lo side 20. At >1800 rpm - Hi side 275 Lo side 14. Vent temps never get lower than 58 and my thermometer in front of the condenser is reading 97F (although it's not that hot here today - 85F with 45% humidity). That's due to the big V10 and no electric fans.

I went about pinching off the heater hoses - in a couple of different places - and while the hoses were warm (hot??) it did not make a difference in my vent temps. On the RV forums folks say it doesn't matter where you pinch off, but maybe it does?

Added maybe 3 ozs r134a - raised the pressures at idle to Hi 250 and Lo 25 and at >1800rpm Hi side went to about 275 and Lo side about 16. No difference in vent temps.

Once I shut the RV off it took about 30 minutes for the Hi and Lo sides to equalize (EXPANSION VALVE, maybe)?

According to the Evans book - posted earlier - I'm going from 97F in front of condenser to 60F coming out my vents. Maybe that's the best the system can do????

One other item - after about 30 minutes pressures had stabilized to 150 psi, overnight the hi and lo pressures are at 75 psi.