[Richard]

A fellow guru is having a bit of an issue on a trip because apparently the mechanisms for allowing the aux compressor to relieve head pressure after cycling have been removed in the new to him coach. Primary symptom is the compressor just hums when trying restart. He has already burned up one compressor on this trip. The owner can release the head pressure on the compressor, and it starts and performs as intended. 

To the purists, the following recommendation is NOT the HWH designed system to relieve pressure from the dessicant bowl. Those part numbers can be found in other posts. 

I post this info here to get the young man back to flushing potties at night as quickly as possible. By posting here it may help someone else in the future. 
First, a compressor unloader is required. https://www.amazon.com/Compressor-Pressu...B0B14TVP77  I have used this one in three applications with no issues. You can pay more for an Ingersoll Rand if you like. 

Second, here is the schematic for piping it in. 

AUX comp setup.pdf Size: 85.34 KB  Downloads: 69

This will replace the SquareD well pump switch for the compressor. 

Pay close attention to the order of the tee and the check valve in the line.

[encantotom]

nice writeup richard. thanks

tom

[BusNit]

Saved in my wish list!
Thanks Richard!

[Poweron]

Richard

Thanks so much. Trying to find a place to have one shipped ahead.

[Doug Musick]

What pressure settings do you recommend on the pressure switch?
My coach's 110 V compressor is currently set to turn on at 70 psi and off at 90 psi.  I don't have a 12V compressor in my system.  The 90 psi doesn't want to lift the air bags very well.  I was thinking of going with 90 and 110 psi for the pressure switch settings when I replace my compressor.  I'll be using a pressure switch w/ unloader valve like in your diagram, with a California Air Tools rocking piston compressor rated up to 125 psi.

[Richard]

In the original scheme of things of that era, Newell’s were generally equipped with three compressors. The obvious engine driven one, a 120V one, and a 12VDC one.

Newell set up cascading pressures 12VDC 50 to 70, 120VAC 70 to 90, and engine 90 to 125. Kind of clever logic to keep the compressors from competing with one another.

A couple of things to keep in mind are that it really takes about 115 to 120 psi to raise the rear air bags. That is a factor of the weight of the coach and the air bag diameter. So, yes you are spot on, 90 psi will not level the coach. The HWH system comes with a bypass electrical switch. It is about the same size as the well pump switch, and it is usually close by the well pump switch for the 120V compressor. When the HWH system calls for leveling such as when pushing an up button, or if left in Autolevel mode, the system wakes up and tries to level the coach, the HWH control box sends a signal to the bypass relay and turns on the 120V pump BYPASSING the pressure switch. If you disassemble all that you will see it’s done with parallel electrical wiring.

I had a problem with that system, in that early in my ownership, I left it in Auto mode and the pressure went so high as to explode an airbag. It is supposed to have a high pressure relief in the system, but mine did not function as intended. Bottom line, I am not a fan of this feature, and have since disabled it. But it works great for leveling the coach if you are not in a hurry.

Sorry for all that context. You can certainly up the pressures on the well pump switch to enable the 120V to raise the coach. The only downside will be slightly shortened life of the compressor and slightly higher leak rates. You will notice that raising the coach on the 120V is agonizingly slow.

[Doug Musick]

Great information.  I didn't know about the HWH bypass, I'll have to look for that.  Yes the 120V to level is really slow.