Earth calling

[Darren Sharp]

Hi guys can anyone answer the following question about Solenoids?

Heralds and early Spits have an underbonnet start button on their solenoids. All the wiring diagrams I can find show a seperate earth on this type of Solenoid. This earth is through the mounting plate. Later types of solenoid fitted from Mk IV onwards do not have the button and all the wiring diagrams do not show a seperate earth although they are affixed to the bulkhead in the same fashion. Sometime ago I had an earth drain on my Mk IV that was eventually cured by fitting a new solenoid. This was not before I had gone through various Dynamo's and starter motors, leads and even a Battery.

Yesterday I attended a show where a Herald was stranded with no apparent power to the starter. I noticed that both the Starter Motor and the Generator had been replaced. I checked the ignition wire to the solenoid for continuity which was fine. I then operated the button starter and the car sprang into life. I assume this was due to the action of pushing the button agitated the fixing screws and made the earth and hence the circuit.

I am confused as to why improving the earth to the solenoid mountings cures both types of issue I.e the power drain on my MK IV (which is not supposed to be earthed) and  the starting issues on the Herald / early Spit. The later is easy to understand if the seperate earth forms part of the solenoid switch.

Thanks guys

Darren

[JensH]

The starter solenoid is a high current relay; nothing else.
Earth on starter solenoid is only for activating the relay - and if you take one apart you will see its has an earth via the housing.
On my mk 3 spitfire; its has the earth via a black wire connected to the fixing bolt (eye-connector), but my guess is often most of the relative small current is going via body tub.
As you can guess, dirt, rust etc. is an issue.

On the earlier ones (two types, square and cylinder form) with a push button, the relay is activated mechanical and has nothing to do with your ignition switch/earthing. But pressing the button could cure a dodgy connection...