Macerator Operation, Troubleshooting, Tips, Modifications
The Thetford macerator used on many Class-B RVs is a garbage disposal type
pump enclosed in a plastic housing. Effluent from the RV’s gray tank and black
tank flows into the housing when the individual valves are opened. The motor is
powered by a momentary switch, and the sewerage is pumped out through the
expandable dump hose. The dump hose itself is 21 feet long and is housed in an 8- foot-long storage tube. This gives you about 12 to 13 feet to reach the sump station. (You will need to pull rather hard to get the hose extended all the way.)
The undermount macerators frequently experience broken inspection covers
and/or housing due to speed bumps or other obstructions, and occasionally the
pump motor becomes inoperable.
A motor with a four-bladed impeller inside the macerator grinds and pumps
waste out the dump hose. There is a thermal cut-off switch that breaks the
power circuit if the motor gets too hot. An O-ring (that is too long!) seals the top cover.
The macerator has several plumbing connection ports around the housing for black and gray water. There are two wires for power, black
(positive) and white (negative ground), and two blue wires for the thermal cutoff switch in the motor.
The macerator is controlled by a momentary rocker switch housed in the external water box. The switch connects to the coil of a 30 amp relay that is housed in the wire cabinet in the rear wheel well and is in series with the motor thermal switch. The relay is wired to activate by a connection to ground. The motor receives electricity from the relay via a fused (30 AMP) circuit.
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Macerator Isn’t Working:
OK, we will assume you have full tanks and have opened the appropriate dump valve, and when you hit the macerator switch… no
“googada-glagada” noise and no goop surging out the hose. We will also assume the coach battery is fully charged and the main
power switch is in “USE” mode, not “STORE”.
There are several possible failure points related to the macerator: Fuses, Relay, Switch, Macerator Thermal Switch, Macerator Motor,
Clogged Impeller
1 – Loose Connection:
A surprising number of people have been reporting their macerator quit working due to the spade connector to relay blade not installed properly. Instead of the connector going on the relay blade, the blade is jammed between the connector insulation and the connector. This issue has also occurred with the yellow spade power connectors to the refrigerator. Others have reported bad connector crimps onto the wires and the wires pull out of the connector.
2 – Fuse:
Check the 30-amp fuse.
If the indicator light is on, the fuse is bad.
If the light is not on, just for fun, pull the fuse and test it with an OHM meter.
There is a second in-line 30-amp fuse connected at the relay that could also be burned out.
3 – Relay:
Is there a clicking sound?
While an assistant outside flips the switch On/Off/On/Off… Do you hear the relay clicking in the rear wheel well cabinet?
YES – If you hear clicking, the relay is OK (most likely), the switch is OK, and the motor is probably NOT OK.
TIP: Now that you located the clicking relay, this is the perfect time to slap a label on the relay for future reference
NO – clicking, Oh oh! It could be any of the failure points listed above.
The macerator relay is a five-pin relay (center pin unused), with the coil energized by connecting the blue wire to ground. The two red wires are +12 VDC (fused 30 amps) and supply power to one side of the coil and one side of the switch contact. When the relay switch is energized, the contacts close and direct power to the red wire on the far side, which goes to the macerator motor (black+ wire on macerator).
Testing: Check the 2 red blades for 12 vdc. Use a jumper from ground to the blue blade and the relay should energize. When energized, test for 12 vdc on the far red blade.
TIP#242: Volt Meter – A volt meter is a valuable addition to the RV tool kit. It doesn’t have to be an expensive one, a $7 meter from Harbor Freight is more than adequate for testing batteries and probing the wiring rat’s nest.
4 – Switch:
Test the switch by removing it and shorting the two wires to see if the relay clicks or the motor runs. If it runs, the switch is bad (it does happen). If the motor doesn’t run, use leveling blocks or ramps, crawl under and test for power at the pump.
5 – Thermal Switch on the macerator motor:
Disconnect the blue wires at the macerator, and using an OHM meter on the resistance setting, check for continuity between the two wires. There should be zero ohms, indicating the thermal switch is closed.
6 – Wiring Break: (This rarely happens).
Check the spade connectors to see if they have come loose. Under the van at the macerator, using a voltmeter, disconnect the black (positive) wire, and while someone is operating the switch, check for voltage from the wire to a bare area of the van chassis, such as a clean bolt head. If there is 12 VDC, then the motor is probably dead.
7 – Motor:
Test the motor with a 12-volt DC jump battery. Connect the motor wires to the battery, black positive, white negative. (The thermal switch’s blue wires are not needed with this direct connection.) If the motor doesn’t run, you have a bad motor and a new and exciting repair job on your hands.
8 – Clogged Impeller:
If you hear humming from the motor. Remove the clear access cover on the bottom of the macerator (Oh Yuck!!) and check to see if the blade turns freely. If not, there could be an obstruction. (Those darn kids and their Legos again! I told them not to flush those things.) Some owners have started with this step and found their blades are clogged with toilet paper.
Now, don’t you wish you had installed the gravity dump valve on the spare port?
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Replacement parts:
Amazon sells the macerator housing, motor, and cover as a complete assembly. They also have the slinky dump hose.
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Motor Replacement:
Remove the hang strap that supports the slinky dump hose storage tube.
Loosen the hose clamp and remove the slinky hose from the macerator barbed fitting.
Disconnect the wires.
Disconnect the black and gray sewer pipes using one of the methods outlined below.
Remove the macerator from the mounting bracket.
Remove the macerator top cover.
Push the wiring rubber grommet tube into the housing.
Remove the four motor mounting screws.
Lift the motor straight up, pulling the wires through the hole.
Install a new motor:
Feed wires through hole, install new motor and mounting screws, push the wiring grommet thru the housing wall.
Re-install the cover O-ring (ours was challenging since it was too long) and install the cover.
Some Options for disconnecting macerator from black and gray sewer pipes:
Replacing Motor Only:
Option 1 – Pipes Intact: Remove the hang straps from the pipes. Remove bolts holding the dump valves together and separate the halves of the valves. Drop the macerator from the mounting bracket with black and gray pipes still attached to the macerator.
Option 2 – Cut the pipes and re-connect using rubber/metal coupler and hose clamps or glued couplers.
Replacing Housing and Motor:
Option 1 – If you are replacing the macerator housing and motor as a complete unit: Drop the macerator and pipes as described above.
Cut the gray portion of the macerator housing pipe and use a “pipe saver” to ream out the gray macerator pipes from the inside of the black sewer pipes. Install the new macerator.
We purchased a pipe saver to test it on some of the old pipe. It looks like a paint mixer. The washer disk fits inside the pipe that will be removed. The cutting blades are the diameter of the hole after the inner pipe is removed. The inside will need a little sanding after reaming.
NOTE: It does take some power to spin the cutter so a 1/2 inch drill is required. If you don’t have a large drill, you can rent them at Home Depot. The pipe saver is sold on Amazon.
Option 2 – Cut the black pipes and re-plumb with new elbows and pipe.
NOTE: the black pipes are ABS plastic and should not be mixed with PVC. ABS isn’t used much and finding pipe and elbows may be challenging.
Option 3 – The option we selected was to replace the gray pipes to gain extra room for the addition of an electric dump valve.
New pipes were reconnected to the housing using rubber/metal hose band couplings.
This gives us the option of just disconnecting the hose band couplings if, in the future, we need to replace the housing or motor.
(Like many of our ideas, this may not work out in the long run. We will update the page with any failures as needed.)
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Macerator Dump Hose:
The Thetford Macerator hose is available on Amazon as well as the
complete housing assembly.
When fully extended, the hose is 21 feet long and 8 feet long when compressed. The storage tube under the van is also 8 feet long so you should be able to extend 13 feet (…in theory!). The storage tube has a couple 90 degree corners that make extending and retracting the hose a bit harder.
Replacing the Slinky Dump Hose:
Disconnect the hang strap on the storage tube to gain more free-play. Our storage tube had a glob of glue holding it to the macerator housing that needed to be cut away. To get more free play from the head-end of the hose, we removed the 2 screws holding the valve support plate to the van (and replaced them afterwards with stainless). Push the storage tube away from the housing and you will see a hose clamp holding the slinky on a barbed fitting glued to the housing. Loosen and move the hose clamp and pry the slinky off the barbed fitting (not an easy task and we finally just split the hose with a box knife.)
IMPORTANT: Before pulling the hose from the storage tube, tie some paracord to the macerator end of the hose . This will provide a way to pull the new hose into the storage tube.
Disconnect the head (dump end) from the old hose by loosing the hose clamp. Install the new hose.
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Additional UPGRADES:
Gravity Dump Valve – If you are replacing the macerator housing, it is the perfect time to add a Gravity Dump Valve. This allows you to use a standard stinky-slinky dump hose without the macerator. It also allows easy access to rinse out the housing. See Modifications page for Gravity Dump Valve.
Stainless Screws – The screws securing the macerator to the mounting bracket were badly rusted. We replaced them with stainless steel screws (#14×1).
Electric Dump Valves – another project, since you are under there, is to replace the cable pull dump valves with electrically operated valves. (Ours did require moving the gray pipes to gain room for the valve motor). See Modifications page for Adding Electric Dump Valves.
Electric Gray Valve Mod – one modification we made to the electric gray valve was to use a threaded spacer to couple a threaded rod to the valve. A double nut and lock nut are on either side of the motor’s push lever. In the event of a valve motor failure, the double nut can be backed down to the coupler and the threaded rod pulled manually to open the valve.
Dump Valve Lubrication – As long as we were replacing the dump valves with Camco valves, it presented the perfect opportunity to modify the valves for easy lubrication. A small hole was drilled to allow a tube to be inserted to spray lithium grease in the valve housing. A tiny screw plugs the hole to keep dirt out. See Modifications page for Valve Lubrication.
Rust paint – In addition to using stainless screws, we cleaned off the rust from the mounting bracket and sprayed it with Rust Reformer spray paint and any other rust we found under the van.
Aluminum Cover – The macerator viewing cover is a fragile clear plastic cover that seems to hook onto speedbumps and other hazards. See Modifications page for Aluminum Cover
TIP243: Umm Donut – Sand, pebbles, and other debris can collect inside the dump hose storage tube and wear holes in the slinky hose turning it into a sprinkler hose. Adding a rubber donut reduces to opening diameter at the hose head. One drawback is the hose clamp doesn’t fit so would need to be replaced with strong tape or other method.
Stronger Hang Straps – After reading of other owners breaking pipe hang straps, and having two broken straps ourselves, we replaced them with much heavier material. See Modifications page for Plumbing Hang Straps
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