Classic car starter wants to re-engauge

[classicnut]

Hello
Starter turns over engine but wants to re-engage when running.
Trying to establish if wiring, solenoid or starter fault.
I understand system is simple- current from battery to seperate solenoid which is activated by current from ignition switch ( when switched on ) to send current to starter which is earthed to complete circuit.
what I don’t understand is why I’m getting continuity between the two main solenoid terminals when the battery is disconnected and ignition off. Surely this would indicate that that solenoid is permanently on ??
I get the same conductivity with another car but that works fine.
I can only assume that my thinking is flawed and wonder if someone could put bye straight on this and perhaps heop guide me to finding the fault

 

[James]

You are reading continuity because on lead is connected to the starter motor +ve
this then traces through the motor windings and then down to ground (chassis)

the other lead is connected to battery +ve
this in turn is connected to all the other loads in the car
some via switches but others that are permanently powered,
any of the circuits that are powered will give you a continuity reading to ground

so both leads of your meter are connected through different things to ground, hence the continuity reading.

 

[classicnut]

You are reading continuity because on lead is connected to the starter motor +ve
this then traces through the motor windings and then down to ground (chassis)

the other lead is connected to battery +ve
this in turn is connected to all the other loads in the car
some via switches but others that are permanently powered,
any of the circuits that are powered will give you a continuity reading to ground

so both leads of your meter are connected through different things to ground, hence the continuity reading.

 

[classicnut]

Hi James
thanks for your reply. Will look at it in detail tomorrow when I hope will be able to fully digest what you have said
thanks
phil

 

[classicnut]

Hi James
thanks for your reply. Will look at it in detail tomorrow when I hope will be able to fully digest what you have said
thanks
phil

 

[Lucien Nunes]

There is another trap for the unwary explorer here, which is that the solenoid in a typical pre-engaged starter has two coils. The hold coil connects from the control terminal to ground, but a heavier, momentary-rated pull-in coil connects from the control terminal to the motor positive. When you turn the key, both coils are energised at first. The pull-in coil is in series with the motor, which puts most of the 12V across the coil but enough voltage across the motor to start it trundling round as the pinion goes into mesh. Then when the main contacts close at the end of the solenoid stroke the full voltage appears across the motor, and the pull-in coil is effectively shorted out as both ends see battery positive albeit via different routes. The hold coil remains energised with the full voltage all the time.

Neither coil connects to the solenoid battery terminal, so any continuity you read there while it's still fitted in the car is likely via other loads from battery positive to ground as @James explains.

 

[brianmoooore]

I've answered a similar question numerous times on a certain car forum. If the wiring has been disconnected then reconnected again, I usually find that the alternator output has been connected to the solenoid terminal that goes to the starter motor.
This leaves the alternator trying to spin the starter motor when the engine is running.

 

[classicnut]

You are reading continuity because on lead is connected to the starter motor +ve
this then traces through the motor windings and then down to ground (chassis)

the other lead is connected to battery +ve
this in turn is connected to all the other loads in the car
some via switches but others that are permanently powered,
any of the circuits that are powered will give you a continuity reading to ground

so both leads of your meter are connected through different things to ground, hence the continuity reading.

Hi James
thanks for your reply. Will look at it in detail tomorrow when I hope will be able to fully digest what you have said
thanks
phil

Hi james
I had disconected battery but forgot the other wires that might give produce continuity so thanks for pointing that out. On further examination I’m 99% sure fault lies in ignition switch. Very grateful for your comments and those of others who helped get this problem sorted.
phil

 

[classicnut]

There is another trap for the unwary explorer here, which is that the solenoid in a typical pre-engaged starter has two coils. The hold coil connects from the control terminal to ground, but a heavier, momentary-rated pull-in coil connects from the control terminal to the motor positive. When you turn the key, both coils are energised at first. The pull-in coil is in series with the motor, which puts most of the 12V across the coil but enough voltage across the motor to start it trundling round as the pinion goes into mesh. Then when the main contacts close at the end of the solenoid stroke the full voltage appears across the motor, and the pull-in coil is effectively shorted out as both ends see battery positive albeit via different routes. The hold coil remains energised with the full voltage all the time.

Neither coil connects to the solenoid battery terminal, so any continuity you read there while it's still fitted in the car is likely via other loads from battery positive to ground as @James explains.

Hi Lucian
thanks for your suggestions. Mine isn’t a pre gauge starter but on further investigation I’m 99% certain fault lies in ignition switch. Your contribution was very much appreciated and helped me become more familiar with the subject
Phil

 

[classicnut]

I've answered a similar question numerous times on a certain car forum. If the wiring has been disconnected then reconnected again, I usually find that the alternator output has been connected to the solenoid terminal that goes to the starter motor.
This leaves the alternator trying to spin the starter motor when the engine is running.

Hi Brian
thanks for your thoughts . I had wired them correctly but it’s good point you raised iI’m sure had I done as you suggested it would have bought about the scenario I described so your input was much apprecited. On further examination I’m 99% certain that fault lies in ignition switch

 

[mainline]

Hi Lucian
thanks for your suggestions. Mine isn’t a pre gauge starter but on further investigation I’m 99% certain fault lies in ignition switch. Your contribution was very much appreciated and helped me become more familiar with the subject
Phil

What type of starter motor is it ?
Is it a bendix type where the rotation of the starter throws in a gear which connects to the flywheel ? These can have problems where the gear doesn't disengage properly and spins the starter motor.
You can usually tell as to the type you have, as the solenoid is separate from the starter motor, usually bolted on the bulkhead.
The simplest way to test is with a lamp tester, as you can easily disconnect the ignition supply and or solenoid /motor supply to test.