I hope someone can clarify if I have diagnosed the fault that I think Ive got.

A few days ago the central heating started to operate in a strange way. The only way to get the pump to turn on was to have the hot water turned on at the timer. And have the cylinder stat calling for heat.

I believe the problem that I have is that the mid position valve is not working properly and is not energising the orange wire as it should. To test this I have removed the valve wires from the house circuit and done the following to see how the valve reacts.

Blue connected to neutral, earth to earth then I energised white and grey as follows and monitored the voltage on the orange wire and noted the position of the valve.

White_Grey___Orange__Position
__X____X_______X_______W
__/____X___see below___M
__/____/___see below___M
__X____/_______X_______W

X=not energised
/=energised
W= Port B (hot water
M= Port A&B (mid position/both)
H= Port A (central heating)
ignore the lines (_) they are just to space out the table


The voltage on the orange wire was 240v as the valve moved to position M but then it fell to around 82v. And the valve never made it to the H position (ie the central heating position) even though I think it should when the grey and white wires are both energised?

My wiring is (I'm fairly certain) as shown on page5 of the following pdf,

http://www.screwfix.com/sfd/i/cat/pdf/20/p4734620.pdf

the valve is a acl-drayton 679H340-30LO

Any help or advice appreciated.

Cheers
Neale

Neale, November 2008

Thanks for the helpful post. I followed this and found dirty contacts on one of the microswitches. I first tested the microswitch with the meter and found no continuity when the switch was closed. I was able to open the microswitch with a knife and clean the contacts with some wet and dry.

M Smith, April 2014

Just in case anyone else wants to know what my problem turned out to be this is what I did.

I replaced the valve motor with one from screw fix and had the exact same problem. So I took the circuit board out to see if anything had burned out. And discovered that both micro switches where faulty.

They both had very dirty contacts and one of them had a bent contact causing it to arc when energised. I cleaned them both up resoldered them to the circuit board plugged it all back together and its all working again.

For anyone with an acl-drayton 679H340-30LO valve, ie the old type without the removable actuator, it IS possible to completely disassemble it as follows;

TURN OFF ALL POWER TO THE CENTRAL HEATING IF YOUR NOT SURE HOW TO DO THIS YOU SHOULD NOT READ ANY FURTHER. CONTINUE AT YOUR OWN RISK.

1. Undo the screw and take off the cover.

2. to remove the motor there are two screws, ones probably hiding behind the orange wires. remove the screws and pull the motor away from the body with the earth plate and heatsink/cover then carefully unplug the connectors from the circuit board. If you need to replace the motor just plug and screw in your new one.

3. to remove the circuit board, slide off the white cover, and undo the two small screws that go through the micro switches. You can now remove the circuit board. (If you havent already you'll also need to remove the cable clip).

4. if you want to go any further, (I didnt), you can remove the actuator bits and springs and there are two screws to remove the actuator body from the valve body. And thats pretty much it.

Neale, November 2008