S130 mystery warning light fault

[checkrail]

I'm still trying to solve the fault with my brake warning light on my '71 +2 , it's a three fuse box model. The P.O. has removed the fail safe system, I just have a normal cap on the master cylinder but when I turn the battery isolater switch on,. the red fail light shows on the dash, but turning the ignition switch on to the start position deletes it.
Everything else electrical is working as it should, I have removed all the under dash trim, searching for a possible earth or wire fault but no luck.
The last time I drove the car it was perfect, it baffles me as to why this should happen sitting in the garage, luckily that I have a battery master switch.

I would really appreciate some help with this annoying problem

Thanks very much John

[JonB]

I’d pull the bulb.

[checkrail]

If I pull the bulb there's an outside chance that I could forget to turn the master switch on.
Result flat battery?
John

[JonB]

I doubt that. Unless you have an electrical fault (in addition to the lamp) that causes the car to draw power when the ignition is off.

Anyway, proper diagnosis is the correct action here. You’ll need to trace the wiring with respect to the wiring diagram for your car. IIRC the indicator wires exit the loom under the master cylinder. I wonder if they are still there, maybe loose and shorting against something?

[checkrail]

I have found the wires buried under the master cylinder, they have good insulation with 12v at the terminals.
There is no sign of any shorting, the wiring diagram shows a connection with the heated rear screen which is fine.
I will struggle on.

[Donels]

The warning light comes on when the fluid level is low. This happens when the contacts in the SOVI connect, so if the PO removed the SOVI and connected the wires it will be on.

[checkrail]

I have owned and used the +2 for four years now and this has only just happened, so this is what the mystery is as nothing has been altered.
Why does the light go out when the ignition is switched to the running position?

[billwill]

I have owned and used the +2 for four years now and this has only just happened, so this is what the mystery is as nothing has been altered.
Why does the light go out when the ignition is switched to the running position?

It's weird but what you are describing is, I think, the standard behavior of an alternator's charging light.

[JonB]

I have owned and used the +2 for four years now and this has only just happened, so this is what the mystery is as nothing has been altered.
Why does the light go out when the ignition is switched to the running position?

It's weird but what you are describing is, I think, the standard behavior of an alternator's charging light.

Not quite. The alternator light goes out when the engine is running and it is supplying electrical power, not when the ignition is turned to the running position (actually, that's when it comes on).

[Spyder fan]

I have owned and used the +2 for four years now and this has only just happened, so this is what the mystery is as nothing has been altered.
Why does the light go out when the ignition is switched to the running position?

It's weird but what you are describing is, I think, the standard behavior of an alternator's charging light.

Not quite. The alternator light goes out when the engine is running and it is supplying electrical power, not when the ignition is turned to the running position (actually, that's when it comes on).

Jon, the OP states the brake warning light comes on when the battery isolator is switched on, but goes out when the ignition is switched on. So there is obviously an open circuit present that is closed when the ignition is switched on.

[Spyder fan]

I think that Bill is on the right track in that failing alternators do all sorts of strange things to vehicle electrics. When my own alternator failed the ignition light remained on when the ignition was switched off, which is a common symptom of fried components in the alternator. Looking at the wiring diagram for an S130 I can see that the Sovy and warning lamp are in the same circuit as the ignition.

I’m guessing that this can be tested by disconnecting the alternator…. Bill?

[checkrail]

Thanks for all the comments, of the two wires of the warning system the earth goes to the solenoid earth on the bulhead and the + wire goes down into the harness towards the front of the car. I presume it ends up at the centre fuse box
As I have already commented the heated rear screen which is on the same feed is working correctly

John

[checkrail]

I removed the fuse for the warning light/ rear screen, but the fault is still the same but no power for the screen.
I pulled the plug on the alternator, that's made no difference.
There's power getting to that light from somewhere?

[billwill]

It's possibly getting power THROUGH another bulb somewhere that is not earthed properly.

[checkrail]

Thanks for the suggestion, that will keep me busy!