No power to gas water heater switch

Do you have any idea where the power to the switch comes from, since there is no water heater circuit on the panel?

It has to be on one of the DC fuses.

I would measure between both switch contacts and ground.
12 volt on one. zero or close to it on the other.. OPEN SWITCH
12 on both Problem down line
zero on both Problem UP line.

Thank you, that’s very helpful. That’s the schematic I have from the service manual. Have to guess which is the + wire coming into the switch since it’s “field wired” with no color code. The unit seems to function fine now after no effort on my part! Do you have any idea where the power to the switch comes from, since there is no water heater circuit on the panel?
Thanks everyone for pitching in to help. If it fails again I’ll be back.

Your panel should have a switch and a light. The red light will come on when you turn on the switch and turn off when the burner flame is verified. 12V is at the switch (+) and ground is at one of the two wires on the light (-).

144Grayling wrote:
Thanks. It’s testing the power at the switch that is the challenge. It’s on the kitchen wall. Not much exposed metal there.

Oops, sorry. Lost my concentration on that one! :o

So when checking the switch, either use the Ohms function, with power off, or check across both tabs with power on and using the 20VDC function. If the switch is working, AND both thermodiscs are good, and with power on, you should get 12VDC on the switch when it is switched off, and 0VDC when it is on. I would switch to voltage when checking the thermodiscs.

Thanks. It’s testing the power at the switch that is the challenge. It’s on the kitchen wall. Not much exposed metal there.

144Grayling wrote:
What’s a good ground in the interior of a travel trailer? Everything is plastic or fiberboard!

There is plenty of grounded metal in the water heater itself. Just firmly put the black prove into the metal that makes up the heater. There will be a green wire that has a screw in it, that is the actual ground for the water heater. It is right in front of you in the heater.

Outside of a lamp socket!

What’s a good ground in the interior of a travel trailer? Everything is plastic or fiberboard!

144Grayling wrote:
Thanks for all the replies. It is a suburban SW6D. I got the service manual on line and it said to check the voltage across the switch terminals so that is what I did. (Meter in parallel with switch) Zero voltage. The manual says if no voltage to check the 12 volt power input to the switch but I’m not sure how to do that. If it were a car I’d just go to ground. Does that work on a trailer? I can unplug the wires and try that way but there is almost no slack in the wires and I’m afraid of losing them in the wall. There is no fuse in the control center for water heater. The switch is mounted to the wall so I have no idea where the wires come from or go to. The service manual schematic shows a positive wire coming into the switch and a wire out the other side to a junction box but I have no idea where those are in the trailer itself. Where could this circuit get its power if not from the control center? (Note there is also no fuse in the control center for the LP detector and it’s not getting 12 volts either.)

Weird. When I went to check on it this morning it was working! When before the red light did not come on at all, it did this morning, then of course it goes out when it starts. So I might have an intermittent failure situation.
Again any suggestions would be appreciated.

Very similar to a car test. Keep the black lead on ground and use the red lead to verify presence of 12v at the switch. Both tabs with switch on. Then use the red lead to check the two thermodiscs located under the black foam cover. There are two in series. One is the thermostat that sets water temp, and the other it the "over temp" safety thermodisc. If you have the black lead grounded, you should get 12v at all four of the disc connectors. If you lose it, one of the discs is bad. This is, of course, when you have cold water and the heater is calling for heat.