Holding tanks

A guided diagnosis for RV techs. Each step is a check, a reading or a question.

Tank complaint? Three tanks: fresh, grey, black. A monitor reading full when the tank isn't is the most common call. Expect: Monitor tracks the tank; valves pull smoothly and seal
Which monitor, and what does it read? Probe systems have screws through the tank wall. SeeLevel and similar use a strip glued to the outside of the tank.
What happens when you pull the black valve? Hook up a clear elbow so you can see what comes out.

Causes and repairs

Probes coated

Paper and waste coat the black tank probes and bridge them to the full level. Grease does the same in the grey.

Tools: tank rinser, multimeter

Flush with a tank rinser, fill with water and treatment, drive or let it soak, flush again. Still full: disconnect each probe wire at the panel; the reading that drops tells you which probe is bridged.

Probe circuit open

The common (ground) probe wire is off or corroded, or the panel's ground is open. Every level reads empty when the common is open.

Tools: multimeter

Check the common probe wire at the tank and at the panel. Clean the probe screw terminal.

SeeLevel OPn / Err

OPn is an open between the display and the sender strip: a broken wire, a loose screw terminal on the back of the display, or a corroded ground. Err is a sender fault.

Tools: multimeter

Check the display terminals, then the wire to the strip, then the strip's ground. Replace the sender strip if the wiring is good.

Strip sender reading poorly

Strips read through the tank wall. A strip lifting off the tank, water between the strip and tank, or a strip cut to length wrong gives jumping readings.

Tools: flashlight

Check the strip is bonded flat. Confirm it was cut to the tank height per the instructions.

Valve cable or rod broken

Remote-cable valves break the cable or pull it out of the handle. The valve never opened.

Tools: hand tools

Open the valve at the valve body by hand if you can reach it. Replace the cable or the valve.

Pyramid or packed tank

The black valve was left open at a campsite: liquid ran out and solids piled under the toilet. Or not enough water with each flush.

Tools: tank wand

Close the valve, fill the tank with water and treatment, soak a day or two, use a tank wand through the toilet. Teach the owner: valve closed until the tank is two-thirds full.

Grey tank slow

Grease and hair in the tank or the shower P-trap. A shower that backs up when the sink drains is a full grey tank or a blocked vent.

Tools: none

Dump. Hot water and a grey tank treatment. Clear the shower trap. Check the vent at the roof.

Vent problem

Inside: a blocked roof vent, a vent cap that lets wind blow down the pipe, a dry trap, or a loose vent pipe in the wall. Outside: normal when dumping, or a valve leaking past.

Tools: flashlight

Check the roof caps and clear them. Fill dry traps. Inspect the vent pipe joints behind the walls.

Dump valve

Leaks past: worn blade seals. Won't pull: debris, a bent rod, or a frozen tank.

Tools: hand tools

Replace the valve seals or the valve. Lubricate. Replace a bent rod or cable.

Fresh tank

Leaks at the fittings or the drain valve, or a cracked tank. Won't fill: a kinked fill hose or a blocked vent (it burps water back out the fill).