No power from ignition switch to coil (sometimes)

[P27]

The engine runs, but sometimes stops. Or sometimes doesn’t start;
We have traced the problem to power (ie electricity) not getting to the coil.
As a temporary test, we ran a cable direct from battery to the coil - and then engine runs fine.
We read that the Lucas ignition switch is ‘temperamental’ at this age. We also read that old land rovers have similar issues, and users have solved it with wd40 in the key lock.
Has anyone experience of these?

We would rather not have to remove the instrument panel, as we might create more problems than we solve;)

[SHyslop]

I had this on two cars but the problem was the control box. On one, being a bit short of time, I put in a separate cable from the battery at the solenoid, to a switch, then to the coil. Everything worked. The separate switch meant I could turn the ignition off or on in the car. The original ignition switch can fail too but if you have an original, or old, RF95 control box, there is an internal connection that seems to give up the ghost. The temporary fix was replaced in due course with a new control box and the ignition switch is still the original so it wasn't the switch.

Now, if you suspect the ignition switch and don't want to disturb the dashboard, here's what you can do. First, do your sidelamps work? If so, then the A connection from the battery to the light switch is working. The A terminal on the light switch is connected to one side of the ignition switch so although the problem could be in the wire from the lamp switch to the ignition switch, you have narrowed the supply side problem to a very short piece of wire. If the side or headlights don't work when the car stops (of its own accord), the the problem could be the one I have mentioned above connected to the control box.

Next, connect a multimeter or an earthed bulb to A3 on the control box. Turn the ignition switch on and off. Does the bulb light or the meter show 12v or close ? Try this between 5 or 10 times and if it works, check that the ons and offs correspond with the turning of the key. If that works ok, the switch is maybe not to blame.

If you think about what the key is doing in the lock, it is turning a set of tumblers to make the barrel turn in its mount. Turning the barrel will be turning a brass set of contacts , probably on a plastic /Bakelite support, to connect A with IG. The WD40 may be seeping through to help clean some corrosion off, if corrosion inside the unit is a problem. However, what it won't do is repair a break in the rather small components should that have happened. The switch does not come apart easily.

I hope that maybe suggests some things to do without really taking anything apart.

[P27]

Thanks for your very informative and useful reply.

Yes - the lights turn on.

But the wd40 sprayed into the ignition lock, did not solve the problem ;(

So we will use your suggested temp fix of taking power from the voltage regulator (or somewhere) and wiring it to the coil via a temp switch fitted on/near the instrument console.

I’m looking for a mobile auto electrician to do it, and will post the results here (eventually).

[P27]

Some more help needed, as I’m not at the car.

1)
SHyslop: you say you took a feed from the solenoid.
Where is the solenoid located on a 1937 p2 hp12?
(I’m especially confused as some answers from google (ai) tell me they didn’t have solenoids).

2)
have a copy of the wiring diagram, but I only have an iPhone here to read it (((
Is terminal A on the voltage regulator (aka control box) always live (ie can we take the feed from there, as it’s adjacent to the coil)

[SHyslop]

My reasoning for not taking a supply direct from A is that this bypasses the ammeter and means that as well as not having fuses (unless you install one, perfectly possible and recommended) you could have a short circuit with no visible warning until it was maybe too late. However, in my case I was losing power to all the electrics whereas you seem to be only losing power to the ignition. That makes a difference in that I was having (temporarily) to power the whole of the electrics but if it is only affecting your ignition circuit, you can remove the cable coming from the ignition switch to the coil and only power the coil. Tape up the terminal on the end of the wire you have removed.

Although the coil is physically close to the control box and solenoid (more on this in a moment), I would not recommend that you hotwire the coil direct from a continuously live supply without an accessible switch. The coil should not be left on with the engine not running for longer than is necessary and in the event of something untoward happening, trying to disconnect a live terminal while something was smoking or burning may be risky.

I am attaching a photo of where the solenoid is on a 46/47 car and I think the layout will be similar on your 1937 car. The solenoid is the round black item below the control box. There are three terminals on it. Two are larger, the third is for the starter solenoid. The terminal you want to connect to is the one with two cables going to it, one larger and the other smaller. Before doing this, disconnect the battery. The other larger cable goes to the starter.

Pre 1937 Rovers can have Startix solenoids and other makes can have cable operated ora large finger or foot operated starter switch but all cars fitted with electric starters have some sort of switch and if it is not mechanical, it is a solenoid.

I hope you can find an auto electrician conversant with this age of vehicle. We have a 95 year old member looking for an electrician to help with his coil but he says finding someone who can work on a car without a computer connection is difficult.

solenoid2.jpg (39.21 KiB) Viewed 39 times