Toilet
A guided diagnosis for RV techs. Each step is a check, a reading or a question.
- Toilet complaint? Most RV toilets are a ball or blade seal over a straight drop into the black tank, with a pedal-operated water valve and a vacuum breaker behind the bowl. Expect: Bowl holds a couple of inches of water for days, blade moves smoothly, no water at the base
- Close the blade and look at the seal. What do you see? A flashlight and a wet finger tell you most of it.
- Is water reaching the toilet? Close the toilet's shutoff, loosen the supply fitting at the valve a little and open it back up.
- When and where does it leak? Dry the area and watch while someone flushes.
- When does it smell? Black tank gas has three ways in: past the seal, out a vent that isn't venting, or pulled in by a fan.
- 12V at the macerator motor during a flush? Meter the motor leads while someone presses the flush switch. Expect: 12V at the motor while flushing
Causes and repairs
Debris on the seal
A single sheet of paper lets the bowl drain overnight.
Tools: seal conditioner
Clean the seal and blade edge. Condition the seal with a toilet seal lubricant (not petroleum jelly, it swells the rubber). Fill and recheck in an hour.
Seal worn out
Dry, cracked or set seals stop sealing. Common on coaches stored without water in the bowl.
Tools: hand tools, seal kit
Replace the seal kit for the model (Dometic 300/310/320, Thetford Aqua-Magic). Teach the owner to leave water in the bowl.
No supply to the toilet
Shutoff closed, the pump off or not building pressure, a frozen line, or the line still full of antifreeze air after winterizing.
Tools: none
Open the shutoff. Check the pump builds pressure at a tap. Thaw the line. If the whole coach has no water, see the water pump guide.
Water valve not opening
The pedal linkage isn't opening the valve, or the valve module failed (often after a freeze).
Tools: hand tools
Watch the linkage move the valve. Replace the water valve module.
Inlet screen or vacuum breaker
A screen at the valve inlet clogs with debris. A sticking vacuum breaker restricts flow.
Tools: hand tools
Clean the inlet screen. Replace the vacuum breaker if it sticks or leaks.
Water valve not closing
Debris in the valve or a cracked valve body, very common after a freeze.
Tools: hand tools
Replace the water valve. Check the line for other freeze cracks.
Vacuum breaker leaking
The anti-siphon valve behind the bowl splits or its seal fails and sprays when flushed.
Tools: hand tools
Replace the vacuum breaker.
Floor flange seal or cracked bowl
Water at the base when flushing is the flange seal under the toilet or a crack in the base.
Tools: hand tools, flange seal
Pull the toilet, replace the flange seal, tighten the two bolts evenly. Inspect the floor for rot while it's up.
Roof fan pulling tank air up
An exhaust fan running with windows shut puts the coach in a vacuum. When the blade opens, tank air comes up the toilet.
Tools: none
Crack a window when the fan runs, or run the fan in intake. Nothing is broken.
Vent pipe or tank
A black tank vent blocked at the roof (wasp nest, cap), a vent pipe loose inside the wall, or a caked tank.
Tools: flashlight
Check the roof vent cap and clear the pipe. Look for a loose vent joint behind the toilet's wall panel. Dump, flush and treat the tank.
Pedal or blade mechanism
A dry blade, debris in the track, or a broken spring or cable.
Tools: hand tools
Clean and lubricate the blade. Replace the pedal assembly or spring.
Macerator jammed or failed
Something hard in the chamber, or the motor is done. Power off before you reach in.
Tools: hand tools
Clear the chamber. Replace the motor if it still won't turn.
Flush switch, fuse or control module
Fuse at the panel, the flush switch, and the control module between them.
Tools: multimeter
Fuse first, then 12V through the switch, then out of the control. Replace the dead part.