Brake Light Weirdness

[trymes]

Well, as I've been able to get the car out and about a bit more, I've had cause to use the lights a few times, though I avoid it, as they currently do seem to drop the ammeter into negative territory.

Anyhow, tonight I got into the car after dark, and turned the key on, then put my foot on the brake pedal before starting. Whoa! The panel lights all came on! Looking a little more closely, the side lights all came on, too. Something's not wired right! Seems a bit much for a sneak ground, but I'm guessing that somehow the brake light voltage is backfeeding into the sidelight and panel light circuit, maybe the result of the tail-lamp and blinker upgrades that Robin did to the car many years back.

It gets weirder: When I arrived at my destination, I pulled into a parking space, foot on the brake, and turned the key off. No change. The engine just kept running. You guessed it: I took my foot off the brake, and the key worked normally.

Digging a bit further:

1. If the key is on and I press the brake pedal, the side lights come on.
2. If the side lights are on and I press the brake pedal, It's as if I turned the key on. The ignition light comes on, and the electric fuel pump starts going.
3. If I have the lights on and the pedal depressed, turning the key off does nothing (same issue as #2)
4. My brake lights are barely brighter when the side lights are on. I suspect that the wires to the rear lamps are reversed, such that the brake lights are illuminating the dimmer of the two filaments, but I don't see how that could cause the above behavior.

Now to investigate what’s crossed up and where - it seems to me that pressing the brake pedal causes the side light circuit and the white "switched" circuit to be connected somewhere. The same thing happens in reverse if the key is on and the lights are off: pressing the pedal causes the lights to come on. The issue almost certainly was introduced when the previous owner added different taillight housings and flashing indicators, but then again, maybe I caused the problem when I modified the wiring to allow the flashing indicators to be controlled by the trafficator switch? Why not both?!

Here's a quick video I made showing this behavior: https://youtu.be/HrovceqSPBQ

Tom

[Mike Allfrey]

G'dday Tom,

This concern sounds like the brake light switch is somehow wired into your side lamps and panel lights circuit. I think your Item 4 proves this. Also Item 1 is similar.

The difficulty in troubleshooting the wiring is where someone has altered connections - by installing new rear light assemblies and using different colour code wires from the Lucas system.

Question, has the wiring been changed at the brake light switch? Mostly, we do not know who or what has been there before.

In addition to that, it is vitally important that there is a really sound earth through the car's body from the earth cable at the battery.

I do not know what type of rear light assemblies you have, but it would be a simple matter to remove them, disconnect the wires to the bulb (globe) holder and, with everything switched off, check with a multi meter which wire comes up with 12-volts when the ignition is switched on and the brake pedal is pressed. Once this wire has been identified, it should be connected to the brighter filament's connection in the bulb holder. The other coloured wire should then be connected to the other connector, it being part of the side/panel lamps circuit. Finally, make sure the earth wire (black) is in continuity with the lamp bodies - with the bulbs removed.

I think that is where your concern is located.

The electrics on my Jupiter have been very reliable, a bit different from a Rover P6B I had which had all sorts of problems until i sorted them out. Once sorted, it matched the Jupiter for reliability. In ten years ownership, I never did find a common earth post that was shown on the wiring diagram.

Regards,

Mike Allfrey, Melbourne, Australia.

[Forumadmin]

I had similar symptoms which turned out to be a rear light bulb holder where the brake light was connected to the side light.
Take a bulb out of each rear light in turn and see if the symptom changes.
It could also be that one rear light bulb holder is not earthing properly but you should then see dimmer than normal lights but with both filaments on.
Another check is to take the bulb out and test the ohm resistance to the chassis battery terminal (positive battery terminal on unmodified car) of each of the bulb contacts in the holder. Maybe safer to disconnect both battery terminals from battery! Do this with the side light and brake light switches off. Then check the resistance from the bulb holder case which should be less than 3 ohms and preferably less than one. You could do similar with the other battery terminal but disconnected from the battery so that you check continuity with the barke switch and side light switches switched on in turn.

[trymes]

Thank you for the advice, I'll start digging in. I find wiring to be generally pretty straightforward in an MGB, as the wire colors are generally present, but in my Riley (1959) and the Jowett, the fabric wire covers are all faded and I can generally never tell what wire is which color. Unfortunately, a lot of the non-original wiring I have found is all blue, and not always terribly well thought out, IMHO.

A test light and multimeter should get me more or less straight, I hope. Truth be told, this was pretty funny when it happened, especially the "turn off the key and the engine stays running" bit. The "no brake lights when side lights are on" part is less funny.

Tom

[Srenner]

As long as you're tracing blue wires to where they're grafted to the original loom, might want to consider re-wiring with the correct color code. My source is britishwiring.com. IIRC, located in PA.

[Keith Clements]

What happens when you have the earthing issue on the brake holder is that the juice from the brake light switch when closed goes through the faulty holder's brake bulb filament then through the side light filament then through the other side light filaments to chassis. The lights are dim because the voltage across them is about half. But I suspect there may be another fault.

[trymes]

What happens when you have the earthing issue on the brake holder is that the juice from the brake light switch when closed goes through the faulty holder's brake bulb filament then through the side light filament then through the other side light filaments to chassis. The lights are dim because the voltage across them is about half.

This is what I have always thought was called a "sneak ground", where current from one circuit, lacking a proper ground, follows its way back out through another circuit to find its way to ground.

But I suspect there may be another fault.

You and me both!

[trymes]

As long as you're tracing blue wires to where they're grafted to the original loom, might want to consider re-wiring with the correct color code. My source is britishwiring.com. IIRC, located in PA.

Thanks, Scott. I've used them before, though I think they may be more expensive than ordering directly from Autosparks in the UK, IIRC, and assuming the order is big enough to defray shipping costs.

Any hints as to how to decode colors on older fabric looms? They always just look like muddy colors to me. Could be red, brown, blue,or some other color.

[Srenner]

Yes, a certain shade of weak coffee brown seems to be the norm, possibly with just a hint of a darker stripe for the wires that carried a second color. Faded true color might be gleaned if any of the loom woven cloth still hides the wires, but often age has killed the lacquer everywhere.

Given the goofiness of your issues, I would remove connections for any add-on lighting and such, just to isolate the original loom and see of any problems went away. Then move to the white wire circuit as either there is feed back when switched off or the switch is defective.

Is the 8-way relay box on this car? This was the Lucas solution to having the turn/brake/running lights combined. It would be suspect to me as getting any wires on the wrong terminal could light up the wrong things. If a flasher unit has been added without this box, then all hell would break loose in the lighting.

In any case, the answer to me is to follow the wiring diagram for each light/appliance to verify the origin and terminus per the diagram. I have alligator clips on the ends of 10 foot jumper wires just for this purpose. My cheap Chinese multi-tester comes in handy. Hey, free from Harbor Freight!

Just did a Lotus Europa this way where the P.O. had a spool of white and a roll of gooey plastic tape. Who needs connectors? Cheap bastard didn't even pop for black for the grounds (earth to the Poms).

Have fun!

[trymes]

Thanks, Scott. I’ll figure out the mess. I don’t think this car has the strange 8-way box, which my MGA does have.

Add to the Europa problems the fact that the body is fiberglass, and I imagine you have quite the challenge on your hands!

[Mike Allfrey]

G'dday Tom and Others,

I think there could be a market for a small Lucas colour code tag to clip onto the wire's braiding in cases where the colours have faded. This assumes we know which wire is which in the first place! More work for a multi-meter and an assistant.

Hope the concern gets sorted,

Mike A.

[Keith Clements]

I bought a pack of wheels of numbers when I rewired the SA . You can then wrap these around each end to identify the wires. Especially good if you are colour blind which 10% of people are. e.g.
https://www.cef.co.uk/catalogue/product ... f-adhesive
or
https://cpc.farnell.com/hellermanntyton ... dp/CB18313

[trymes]

Coming back to this topic, I was able to trace this back to a ground issue, as predicted by many of you. When Robin (the prior owner of this car) fitted the Volkswagen tail lights to the car, he seems to have done a good job of adding a strong ground to the lamp body, so as to avoid these sorts of issues.

Unfortunately, he also repainted the backing plate, which contains the light fixtures themselves. As a result, the bulb sockets had some paint on them that might have been interfering with a reliable ground connection, and the screw that connects the tail light housing to the backplate, and that provides ground to the entire fixture, did not have good, reliable, contact with bare metal.

At this point, I have eliminated the issues reported above, but I’m going to keep an eye on it and perhaps connect the grounding wire directly to the backing plate where the screw affixes it to the housing.

[Srenner]

Nice that it wasn't a tear-the-loom-out fix.