Servis M9310 Motor 'stuttering'

[davehills]

I have a fairly modern Servis M9310. (yeah, I know, but it was cheap and I needed something quick to replace our old machine).

All was fine until about a month ago when it developed a problem. During the wash cycle the drum would 'stutter' slightly, almost like a slight 'misfire' on a car.

I took the back off and the belt was in a bad way and rotating the drum by hand you could feel it wasn't running right. After a quick inspection of the brushes and a new belt the machine was put back together and ran OK.

However, the fault has come back, worse than before! I have inspected the belt-still good as new. I have checked the brushes again (Tons left) and cleaned/inspected the commutator. Still the problem persists.

The fault is almost as if the motor is not receiving power on an intermittent basis. Occasionally today the motor has attempted to intermittently run at high speed during the wash cycle, ie much faster than is normal during a wash cycle. I think this might be a clue.

This has led me to conclude that the motor is essentially OK but there's an intermittent fault with the speed control mechanism. Is this a fair assumption?

How does the speed control on these machines work? I know there's PCB which controls the motor-does this actually sense the speed of the motor? If so, how does it do it? Is there a speed sensor on the motor itself?

If there's no speed sensor then I'll check the PCB for dry joints/damage.

(The machine has always been very sensitive to poor loading and often won't go into a full spin when washing towels etc. How does the machine sense when it's out of balance?)

[Whitegoodshelp (Andy)]

It sounds like a problem with power or a faulty connection, the pcb receives feedback from the tacho coil on the end of the motor. The main armature turns the belt at one end, and the other end has a small magnet attached, which revolves past the tachol coil. The pcb can read every revolution of the armature through the tacho coil and magnet.

If the magnet or tacho coil have a problem the pcb looses all control of the motor. In the old days this resulted in the motor going instantly into a full spin which caused a lot of damage as it flung the tub around. These days manufactures have sensibly designed them so that the motor just stops turning instead and should produce an error code.

It's possible the motor has a fault but check all the connections in the motor plug in case one of the pins is slightly pushed in and not making proper contact. Other than that it could be a dry joint or faulty component on the pcb.

[davehills]

I didn't let it run long enough to see if it automatically shut down!

No error codes I'm aware of; I woudn't have any way of decyphering them even if there were.

I'll check everything you mention and post back when (If?) I resolve it.

[Whitegoodshelp (Andy)]

This might help - Displaying error code or lights flashing -

[davehills]

Have had the motor out.

The tacho coil on this model is build into the actual motor and looks like it's only possible to remove it by taking the motor apart! Have traced the wires back to the connector and I get a steady resistance across these, no matter how much I wiggle the wiring. Also, if I put a voltmeter across the terminals and spin the armature by hand I get a fluctuating voltage. Obviously, I don't know if the voltage is correct, but it proves that the coil isn't open circuit and that the magnet hasn't fallen off!

I have done a continuity check on the wiring between the tacho and the control PCB and this appears OK.

I have removed the PCB and inspected it and can't see anything wrong, no burnt components, no dry joints. I have cleaned the terminals on the PCB and the motor, for good measure.

Still the problem persists. No error codes or flashing lights.

Obviously, it'd be nice to confirm the fault by substituting parts but a new control board is ~£75 and a secondhand motor is about £50, neither of which I can really justify on such a cheap machine!

(I have video'd the machine misbehaving and might upload it later)

I suspect I'll have to end up breaking the machine and selling the bits to recoup my outlay; shame as it's in A1 condition and hasn't seem much use.

Edit: It doesn't exhibit the issue during 'tumble dry' mode, nor does it do it on 'spin', although it doesn't spin if heavily loaded.

Control module= Minisel 546052300-002

Motor= 512011603 (The newer square version, not the old round type)

[Whitegoodshelp (Andy)]

It would be risky to try buying parts speculatively. Have you checked the pins in the motor plug? I don;t think it's anything to do with the tacho coil, was just explaining how that works as you asked how the speed of the motor is controlled.