I am fitting a Mark 2 control panel (Thetford part no. 51810/ No. 46 on Thetford spare parts diagram) to a Thetford C-402C bench toilet. I am assured by the suppliers that this part replaces the original control panel (part no.?). I'm aware from other forums on the web that this is not a particularly unusual situation however, most 'cures' seem to be an interminable list of replacement parts often not leading to any change in the situation. In the end, I'm prepared to bypass the electronics all together and put in my own microswitch that simply provides a 12V supply straight to the pump when pressed. We don't have any need for a warning system for when the tank is full, we simply empty it frequently! For two pins I'd swap the entire toilet out for a hand pump flush model! I'm also aware that many owners simply flush with a bucket and spray with toilet cleaner and are perfectly happy to do so.

Nonetheless....here we go....

The wiring to the control panel has been tested for continuity/voltage and measured at 12V right up to the board connecting plug. The wiring is in the correct order as far as I can tell - Left to right from the top of the board connector plug: from the 12V supply: black, red - then to the pump: blue, brown. (The additional ‘chocolate block’ connectors in the photo have been fitted to correct a previous owner's modification and have been tested for continuity. They restore the original circuit plan. Don't ask!!)

I have also checked that the 3amp blade fuse and its connectors (housed next to to the reed switch) are good.

Once the control panel is connected, I have depressed the flush switch on the control panel but there is no response from the pump. To check the pump is working I have bypassed the control panel and connected the 12V supply directly to the load. When this is done the pump works immediately and at full power. This also shows that the 3amp fuse and its connectors are working.

The 12V supply is measurable in the ‘pins’ of the board connector plug and, when the plug is connected to the PCB, the 12V supply is also measurable on the board’s 12V input contacts. There is no voltage coming through to the contacts on the PCB that connect to the wires to the pump when the PCB microswitch is either on or off.

All of the above remains the same whether the reed switch wires are connected to the control panel PCB or not. I have inspected the reed switch housing, the PCB and the connector plug to the reed switch PCB. They are all visually in good condition with no corrosion or signs of damage. The wires connecting the reed switch to the control panel have been checked for continuity and are fine.

Therefore either:
1. A system is operating within the toilet that prevents the 12V supply being connected by the switch. EG. The reed switch not only triggers a warning light on the control panel but also prevents the 12V supply to the pump when the holding tank is full or removed. If this was the case then maybe the reed switch is damaged and therefore is effectively stuck in ‘pump off’ mode. There is no ‘tank full’ warning light showing on the control panel.
2. The float box in the holding tank is broken or misaligned and is therefore not triggering the reed switch correctly.
3. The new PCB in the new control panel is broken or incompatible with the toilet model.
4. The microswitch on the new PCB is faulty or the flush button itself is not depressing the microswitch effectively.

Questions:
1. If I temporarily bypass the reed switch, by connecting the two reed switch contacts on the control board PCB, will the pump work - could I damage the PCB?
2. Does the reed switch in the holding tank cavity prevent pump operation if it is either registering that the tank is full, is faulty or if the fuse has blown?
3. Is there another part of the system that would cut out power to the pump if it was faulty?
4. Could the new control panel PCB be faulty, if so, how could I test it with a multi-meter?
5. Could the microswitch on the new PCB be faulty, if so, how could I test it with a multi-meter?
6. How could I test the reed switch with a multimeter to find out if it is working?
IMG_1435.jpeg - Bild entfernt (keine Rechte)

Thanks Aaron - bit concerned not to fire up the bat signal to JohnE too much as he’s kindly responded to two of my interminable threads already!

I can’t understand Eriba deciding to fit these electric models from Thetford - seems to fly in the face of so much of the ‘simplicity’ of the van itself. I’m also mystified about the practical advantages (apart from providing a never ending revenue stream of price inflated and poorly designed spare parts). As I say, for two pins....

Apparently, I'm fully toilet trained. I even have a certificate awarded by Thetford to prove it (although Mrs E insists I must have beeen on a fag break during the 'Lowering The Seat When Finished' seminar).

Anyway...

1. If I temporarily bypass the reed switch, by connecting the two reed switch contacts on the control board PCB, will the pump work - could I damage the PCB?
2. Does the reed switch in the holding tank cavity prevent pump operation if it is either registering that the tank is full, is faulty or if the fuse has blown?

Forget about the reed switch for the moment, Pete - its only function in life is to illuminate the LED on the control panel when the holding tank's full - it's contacts are 'normally open'.

3. Is there another part of the system that would cut out power to the pump if it was faulty?
4. Could the new control panel PCB be faulty, if so, how could I test it with a multi-meter?
5. Could the microswitch on the new PCB be faulty, if so, how could I test it with a multi-meter?

The PCB's circuitry comprises passive components and a couple of transistors configured as a solid-state relay. This means there's no way to continuity check the path from the supply through the switch to the pump.
But, from experience, I doubt your new panel is faulty.

The main cause of grief on these toilets is corrosion on connectors, fuse blades, fuse holders and pcb tracks on the control panel.
And, of course, jammed and burnt-out pumps.

If you've driven the pump from the supply from those chocolate-block connectors then, as you say, continuity through the fuse and holder must be sound.

The only thing that perhaps you may not have checked so far is the PCB connector which has the 12V supply and pump wires connected to it.
Those wires are pushed into forks on top of the connector's terminals, much like some BT telephone connectors. These IDCs (Insulation Displacement Connectors) rely on the pin's fork to puncture the wire's insulation to make contact.
The connector has a hinged top - if you unclip it you'll be able to check them.
Also check that the connector's blades (where they make contact with the pcb tracks) are clean and corrosion free.

To prove the integrity of the connector, plug it back onto the PCB and with your meter probes touching the 0V & 12V PCB tracks, operate the switch - an appreciable voltage drop shown on the meter indicates a high-resistance connection somewhere in the loom.

6. How could I test the reed switch with a multimeter to find out if it is working?
Check the reed switch closes when a magnet is held next to it.

Lastly, Pete, are you confident the connections on the PCB connector are correct for that panel - could you post a picture of the panel, showing the edge connectors?

Quote: JohnE wrote in post #4

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Forget about the reed switch for the moment, Pete - its only function in life is to illuminate the LED on the control panel when the holding tank's full - it's contacts are 'normally open'.

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Quote: JohnE wrote in post #4

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The only thing that perhaps you may not have checked so far is the PCB connector which has the 12V supply and pump wires connected to it.
Those wires are pushed into forks on top of the connector's terminals, much like some BT telephone connectors. These IDCs (Insulation Displacement Connectors) rely on the pin's fork to puncture the wire's insulation to make contact.
The connector has a hinged top - if you unclip it you'll be able to check them.
Also check that the connector's blades (where they make contact with the pcb tracks) are clean and corrosion free.

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Quote: Pete Marchant wrote in post #1

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The 12V supply is measurable in the ‘pins’ of the board connector plug and, when the plug is connected to the PCB, the 12V supply is also measurable on the board’s 12V input contacts. There is no voltage coming through to the contacts on the PCB that connect to the wires to the pump when the PCB microswitch is either on or off.

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Quote: JohnE wrote in post #4

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To prove the integrity of the connector, plug it back onto the PCB and with your meter probes touching the 0V & 12V PCB tracks, operate the switch - an appreciable voltage drop shown on the meter indicates a high-resistance connection somewhere in the loom.

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Quote: JohnE wrote in post #4

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Lastly, Pete, are you confident the connections on the PCB connector are correct for that panel - could you post a picture of the panel, showing the edge connectors?

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I am having an identical issue with the electrics on the Thetford 402c in our Triton and wonder how you finally solved it in yours Pete? As it seems impossible to simply buy a replacement PCB (I know I can buy the whole unit with cover at £50+) I am very inclined to discard the PCB altogether and fix a direct wired microswitch in its place. This would mean the level indicator not working - but who cares... Has anyone else gone down a similar route?