TR6 Starter wiring question

[Ribbs]

I'm in the process of reinstalling my engine, along with a new hi-torque starter (Moss# 541-555). I'm not understanding something about the intended wiring of the starter; In my head, it seems that the ignition starter switch feeds 12 volts to the solenoid, which in turn starts the starter motor. But why is it the one and only terminal on the new starter/solenoid (aside from the main battery feed) is grounded through the body of the starter (and therefore chassis ground). To me, that would result in a dead short when 12 volts is fed through the starter switch. I went back and checked the old stock starter/solenoid and the wire intended for the starter switch is grounded too. How could this not cause a dead short when 12 volts is applied through the starter switch?

I'm also scratching my head over the fact that there was a second wire connected to the old starter solenoid with a smaller spade connector (than the obvious wire from the starter switch). This does not go back to the coil as a resistance bypass wire would, but I have no idea where it goes at this time and thinking I'll just tape it off and assume that the previous owner modified wiring at some point.

 

[poolboy]

The year would help because there are 2 years (74 and 75) that have a starter relay and there are pre 73 years without ballast... but the resistance wire if you have one on the coil does not come from the starter solenoid....the ballast resistor wire comes from the ignition switch.
IF the coil is being fed from the starter solenoid, it would be the full 12 volts when the starter is engaged

 

[Ribbs]

It's a 73. I did confirm, using an ohm meter, that the wire feeding the coil is not a ballast resistor wire. Likewise, I don't think there is a Starter Relay, unless this was part of the starter mounted solenoid. Where was the starter relay located anyway?

My original solenoid had one large spade connector and one small spade connector, in addition to the two large in and out studs. As mentioned, the larger of the two spade connectors seems to be common to ground through the body of the starter, but that's also where I assumed ignition switch fed 12 volts to the solenoid.

 

[poolboy]

No just the the 74 and 75 had a starter relay...which was mounted next to the horn relay. which were near the fuse box.
FWIW, the starter solenoid is activated by a White/red (W/R) wire.
The 73 had 2 spade connectors on the solenoid...one was for that W/R wire and the other was for a White/Yellow (W/Y) wire that feeds the coil 12 volts when using the starter.
Once the engine starts and starter is disengage, the ballast resistor wire from the ignition switch assumes the duty of feeding the coil...
That's a 73, but there is always the possibility that some one in the past has bypassed the resistor which was a fairly common modification
Do you have the REAL wiring schematic for your year car ?
If not you'll find one here:

ADVANCE AUTO WIRE

And this book will take a lot of the mystery out of the TR6/250 electrical system and components
It's worth every penny in the time it'll save you for things like this.

Library | Triumph TR250-TR6: Electrical Maintenance Handbook

Electrical Maintenance Handbook

mossmotors.com

 

[Ribbs]

Thanks much, that's helping to clear things up. The wiring diagram you linked is a much better representation of my car's wiring than any other wiring diagram I've seen (I've been using the one from Bentley). I do have the second White/Yellow wire at the solenoid, however the ballast resistor wire has been bypassed/removed on my car. So the one remaining question I have is why is the bigger spade connector on the solenoid (the one that engages the solenoid from the ignition switch) going to ground? If I feed 12 volts to it via the White/Red wire I would think I'd get a dead short.

 

[poolboy]

What makes you think it's grounded ?

https://mossmotors.com/media/instructions/541-555.pdf

 

[Ribbs]

Doh! I was using an ohmmeter - going from the spade (for the signal from the ignition key) to the body of the starter - and got zero resistance. But of course, I forgot a coil would have zero impedance for DC signals, and an ohmmeter would naturally show zero resistance across the solenoid coil. Thanks again for all your help Ken!