An interesting electrical fault...

[Steve Priest]

Evening all,

My TR4 suffered a slightly bewildering electrical fault on the way home from work this evening.  Bear with me, I have some questions at the end...

I'd just joined a fairly fast B road when there was a small clunk and then the sound of an electric motor running at high revs.  My first thought was that my Kenlow fan had come off the motor!  Very odd I thought.  There was nowhere to stop so I carried on for a bit.  I then glanced at the ammeter and saw that it was pinned at about +30A.  Still not able to stop I switched the ignition off and then back on again, which stopped it.  It then happened again a very short while later but luckily I was able to pull over.  I stopped the engine and noticed that the key was bit warm as was the starter solenoid.  I started the engine and it was clear what the problem was - the starter motor continued to run when the key was released.  I pulled the wire off the starter solenoid and this sorted it out until I got home.

So, I've just been in the garage running a few tests.  There's clearly a fault in the ignition switch.  With the ignition on, there's about 13 ohms between the starter terminal on the switch and +ve on the battery.  This drops to zero when the key is turned to start.  Therefore, the starter solenoid is powered to a degree all the time.  Now interestingly, this 13 ohms means that there is not quite enough juice to pull in the starter solenoid but there is enough to hold it in.  So how did it manage to engage while I was driving?  Perhaps a bump in the road was enough to jolt the solenoid closed, who knows.  Anyway, I'm relieved that nothing appears to have been damaged and I didn't melt the starter.

Anyway, as promised, here are the questions:

Is it worth trying to repair the ignition switch?  If not, there are plenty out there, some Lucas branded others not.  Any recommendations?

Is it possible to swap the lock barrel from my current switch into a new one?  On a scale of 1 to Brexit, how difficult is this to do?

Cheers,
Steve

 

 

[Lebro]

I would clean the old switch up, & re-measure the resistance, if now high then carry on using it. if it remains at 13R (or thereabouts) the look for a replacement.

Yes the barrel can be changed over, they are usually held in place with a pin which can be pressed out. You do need the key to be in to do this.

Bob.

[tthomson]

I have had a faulty ignition switch in the past, and I took it apart and repaired it. It's fiddly, but doable, however I always work on the premise that if it has failed once, unless I can see why it failed and can properly prevent it for the future, I would replace it.

Good luck

TT

[PodOne]

I'd be inclined to replace the switch as once they begin to fail they generally beyond salvage and 30A is a lot of juice to be running though 50year old wiring.

Andy

[Lebro]

I would hope the ignition switch is not passing anything like 30A 

Bob.

[Steve Priest]

Thanks for your comments.  I shall probably line up a decent replacement switch when I finally fix this properly.  The starter circuit is currently disconnected from the switch with a temporary button wired in so I can keep using it.

I have dynamo but a +/-50A ammeter and it was a little over half scale.  I assume that current was just through the ammeter, rather than the switch, as the dynamo desperately tried to replenish what the starter motor was drawing from the battery.  I was a little surprised that my dynamo managed to output that much.

Cheers,
Steve