Hoover Performa 1100 motor runs full speed in wash cycle

[2CVman]

This is a motor speed problem on a Hoover Performa 1100.
Motor brushes replaced a few months ago.
Motor should be turning drum slowly in wash cycle but is turning at full speed instead.
Rectifier diode on main board (IN4007) has slight blackening around it, and reads infinite resistance one way and 3.28 ohms the other way, dropping over a few seconds to 2.65 ohms. Is this likely to be the culprit?
There is also a coil or something on the back of the motor, could be to do with the speed? two orange wires from it. This is showing resistance gradually climbing to about 19 ohms.
Any advice welcomed!

 

[fidom]

Yes there is a device called a tacho on the back end of the motor which is used to monitor the motor speed. It consists of a magnet on the motor spindle and a coil on the non moving part of the motor. The symptoms you describe occur if the magnet comes loose or the very fine wire in the coil breaks internally. You need to check that the magnet is secure. 19 ohms sounds very low to me so perhaps there is a problem with the coil, although they usually fail with an infinity ohm reading.

 

[2CVman]

Thanks for the advice. I have measured the resistance of the coil again and it settles at around 18-19 ohms. This resistance reading drops as the drum is spun by hand, which suggests there is still some communication between magnet and coil. Maybe its the electronics on the circuit board which are at fault? I will try dismantling the assembly to check that the magnet is secure and not slipping or loose.

 

[2CVman]

I took the coil off the back of the motor, cleaned the carbon dust from it, and put it back securely.
The magnet is secure on the motor shaft.
Reassembled and tested, still the same- the motor only runs at full speed.
The connections from the tacho coil seem to run in to some pretty complex and delicate-looking electronics on the main controller circuit board; I guess if this is where the fault lies there's little chance of diagnosing and fixing it without trying another board, unless you have any other ideas?
I plan to dismantle a friend's old washing machine tomorrow (still in her back garden as it packed up a while ago and we haven't got round to taking it to the tip yet), I might be able to rob a few bits from it, although it's a different make. I just object to dumping a whole washing machine for the sake of a couple of components which would probably cost next to nothing to replace. A symptom of our unsustainable throw-away society I guess! Thanks for your advice.

 

[fidom]

The other part that sometimes causes these symptoms is the triac in the motor speed circuitry. This is a large square transistor looking device, usually screwed or clipped to a heat sink on the main circuit board. These sometimes fail by going short circuit. If you have soldering skills and equipment it is fairly easy to change the triac. You could check it with your meter with it still in place then remove it and check the resistance out of circuit. You may indeed get a replacement from a different make of machine as they all use similar spec. triacs for the speed control. Alternatively they are quite cheap from shops like Maplin.