Starter motor staying on

[impact30]

Hi All
A clubmate called me over to-day to look at his starter motor. It is on an old yanmar 1gm10 and is a pre-engage type. Basically it is staying on. Removed all wires except earth battery cable and feed from solenoid to motor. fed 12v to coil of solenoid and engine turned over, removed 12v from coil and it kept turning. There is now 6.8 volts at unconnected coil terminal. Removed connection from solenoid to starter motor and engine stopped turning, reconnected and it did not start again. This is the second, albeit used starter exhibiting this behaviour and it does not do it on the bench.
It has me scratching my head.
Has anybody seen this before?
TIA
Padraic

 

[William_H]

It does sound like the solenoid (relay inn this case is sticking closed even when power is disconnected from the solenoid coil.
I would suggest dismantle the solenoid and clean it up as far as possible.

The voltage you read I am guessing, was with a digital multimeter. These things have very high internal resistance which means that they can measure a voltage through a very high resistance. Try putting negative wire on -ve of a battery and positive onto your finger then put your finger onto the +ve terminal. You will read almost 12v via your finger.

What this means is that almost any leakage via moisture or corrosion will show a voltage. (6.8) If you connect a small light bulb in parallel with the meter you will get a truer reading. ie some current drain by the bulb will mean voltage measured is only real voltage with a current flow.

A DVM can be misleading in low voltage 12v circuits. olewill

 

[2Tizwoz]

Some solenoid winding formers, the insulating tube which the wire coil is wrapped around, swell over time and cause the solenoid to jam. this can cause the solenoid not to move or to jam.

Alternatively burned contacts overheat and can weld together.

Replace the cap with the contacts or the complete solenoid according to which problem you find and your inclination to dismantle the solenoid and repair it.

 

[impact30]

Thanks both of you for your replies. I will check the residual voltage again with a small load. The solenoids on both starters react fine when bench tested. They may both be duff though!!