JCB 3CII starter solenoid connection

[Dave-OLW]

Why does the solenoid have two connections to the starter motor. What does this second connection do, from the workshop manual it appears to connect to part of the winding.

[Jeremy Rowland]

Hi Dave; I'm certainly no electrical expert but my best guess would be a connection to the motor field coil windings?

Jeremy

[gecko.cx]

A couple of possibilities come to mind regarding the four terminal solenoid.
1. it allows you to use two batteries that would normally be isolated.
2. it's an easy way to double the current rating of the contacts
3. the 'Aux' terminal isn't connected to anything internally and is only provided as a distribution point

In the first pic, what I assume it the brushes connection may be for convenience as it would allow the brush assemble to be easily removed for maintenance.

I've had a look at the 3c Mk2 manual in the files section and the starter motors shown are quite conventional.
Which circuit do you think most closely matches your machine?

[Dave-OLW]

It is connection 5 lug in the picture, the solenoid connects opposite terminals and because of the link all get connected to the battery +. if link was removed you could connect another battery.

[gecko.cx]

Ok - Jeremy had it right.

Do you have a part number for the starter and/or solenoid?

[Dave-OLW]

Do not know starter part number, Starter solenoid (37H7114)

[gecko.cx]

Seems that part is still available. It's listed as fitting a Lucas M50 and a GEU436, but I can't find anything with the twin field connections.
I can't find an exploded diagram of the solenoid, but based on your description, I'm pretty confident the four terminals are to provide two sets of contacts and hence double the current rating.


https://www.charnleys.com/part/leyland_heavy/bau1779/lucas-starter-motor

[kaos]

A bit of googling turned up this. It's a part of the service manual for Ford 2720 industrial engines, but it includes a detailed description of the Lucas M50 starter.

The takeaway is:
1. Yes, the motor actually does have two positive terminals (the bolt on the side of the body and the pigtail coming out of the end).
2. Yes, the solenoid has two corresponding contacts. Not merely for power doubling though. Apparently, one contact will always "make" when the coil is energized, but the other only once the starter pinion has fully engaged. Thus, should the teeth of the starter pinion butt against the teeth of the ring gear, only low power will be applied to the starter to move the pinion and allow it to engage, and only then is full power applied.

Ingenious, but the first time I've come across such an arrangement. Ah well, you always learn something new.

--
Regards, Kári.

[gecko.cx]

I tip my hat to you.
It hadn't occurred to me to chase exotic M50's as I have two machines with them and they are quite conventional.
Technically it is a "better" system, but has obviously been found to be not worth the added complexity.
I suspect a bevel or lead-in on both the pinion and ring gear teeth has been found to be just as effective.
Thank you to both of you for a surprisingly interesting question and the correct answer!

[Dave-OLW]

Many thanks, that answers the Question. Have just replaced faulty solenoid, Tractorspares in UK, supply this as the correct one for a Leyland4/98NT engine with Lucas M50 starter. Was trying to understand how it worked when all diagrams on the internet only showed one connection to starter motor.

Thanks David