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Author: Subject: Brake Light Woes
turnipfarmer

posted on 10/8/20 at 02:10 PM Reply With Quote
Brake Light Woes

Hi Folks...

I've done a search for this one but can't find anything.

It's a brake lights problem on a seven style. There's a combination light unit on each rear wing, with 4 wires for indicator, brake light, tail light & an earth. Brake lights operate via a pressure switch. The loom is from PWS. Here's the situation...

Am getting 12v to the pressure switch. Also getting 12v from the pressure switch when brake pedal is pressed, but only 0.5v down the wires to the brake lights themselves. Indicator & tail lights work OK though. It's the same on both sides. The wiring on one side is also completely independent of the wiring on the other.

Replaced the pressure switch - no difference. Replaced all fuses - no difference. Brake light bulbs both OK.

Can't see what would affect both brake lights & nothing else. Before I start tearing the interior apart to check out every single inch of the wiring, can anyone else think of anything else I might try first?

Got to be something obvious, but I just can't see it.

Thanks.

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40inches

posted on 10/8/20 at 02:31 PM Reply With Quote
quote:
Originally posted by turnipfarmer
Hi Folks...

I've done a search for this one but can't find anything.

It's a brake lights problem on a seven style. There's a combination light unit on each rear wing, with 4 wires for indicator, brake light, tail light & an earth. Brake lights operate via a pressure switch. The loom is from PWS. Here's the situation...

Am getting 12v to the pressure switch. Also getting 12v from the pressure switch when brake pedal is pressed, but only 0.5v down the wires to the brake lights themselves. Indicator & tail lights work OK though. It's the same on both sides. The wiring on one side is also completely independent of the wiring on the other.

Replaced the pressure switch - no difference. Replaced all fuses - no difference. Brake light bulbs both OK.

Can't see what would affect both brake lights & nothing else. Before I start tearing the interior apart to check out every single inch of the wiring, can anyone else think of anything else I might try first?

Got to be something obvious, but I just can't see it.

Thanks.

Run a test wire from switch to light?

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loggyboy

posted on 10/8/20 at 02:50 PM Reply With Quote
Try bypassing the pressure switch, or swapping for any type of press switch. They are notorious for not working due to air locks etc.





Mistral Motorsport

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nick205

posted on 10/8/20 at 03:00 PM Reply With Quote
If I'm understanding right you've got a pressure switch in the rear brake hydraulic circuit?

If so I fitted this when building my MK Indy. The brake lights came on, but only when you gave the brakes quite a bit of effort (brake pedal quite a push). If building again I'd use an electric switch at the brake pedal for better brake light response and better warning to drivers behind.

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adithorp

posted on 10/8/20 at 03:27 PM Reply With Quote
Won't be the switch if you have good feed out of it (though they are a common fail). Must be an issue somewhere between there and the lights and as you say both are the same then it must be before they split. Check the loom between the switch and where it splits to feed each side. My money is on that splice/joint where it splits bring corroded.





"A witty saying proves nothing" Voltaire

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steve m

posted on 10/8/20 at 03:42 PM Reply With Quote
Has it ever worked, or is this a new installation ?

If your only getting '5v at the bulb end, it must be shorting out some where?





Thats was probably spelt wrong, or had some grammer, that the "grammer police have to have a moan at




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britishtrident

posted on 10/8/20 at 06:44 PM Reply With Quote
Bad ground





[I] What use our work, Bennet, if we cannot care for those we love? .
― From BBC TV/Amazon's Ripper Street.
[/I]

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adithorp

posted on 11/8/20 at 06:28 AM Reply With Quote
quote:
Originally posted by britishtrident
Bad ground


Possibly and my initial thoughts but I'd have expected faults on other lights as well as they're combo lights with a shared earth.





"A witty saying proves nothing" Voltaire

http://jpsc.org.uk/forum/

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turnipfarmer

posted on 11/8/20 at 01:44 PM Reply With Quote
Thanks for all the replies guys.

For your info...

I also discounted the earth for the same reason as above.

Bypassed the switch - still nothing.

Will try running a test wire from switch to light.

If that doesn't work then it's got to be between switch & lights, which means dashboard & instruments out. Ah well - just have to bite that bullet if needs be

Lights have been working fine for last 6 years.

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gremlin1234

posted on 11/8/20 at 02:40 PM Reply With Quote
the next test I would do:
use a meter, on the feed side of the switch, and check its 12v, then activate the switch, (or bridge with a screwdriver or similar) and see what voltage is (ie read under load)
if its now significantly less than 12v the problem is before the switch,
if remains 12v, the problem is after that test point.

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MikeR

posted on 11/8/20 at 09:39 PM Reply With Quote
I'd follow gremlin advice first, then run a test wire, then (whilst using the test wire) run a dedicated earth. Then use jumper wires to feed 12v into your temporary cable.

You've then tested the switch, feed to the switch, wire from switch to lights, earth from lights and the lights themselves.

Somewhere in that lot it should work and therefore you know what is failing.

Good luck

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turnipfarmer

posted on 12/8/20 at 11:52 AM Reply With Quote
Now disassembled the dashboard & think I may have found the problem (hope so anyway!)

Did some more testing last night & noticed I wasn't always getting 12v on the output from the switch. This happened more & more frequently until it dropped to 0v completely.

Then I noticed that the output wire colour was different at one end to what it was at the other. That can only mean I had to lengthen it at some time in the past.

So there has to be a connector somewhere along the way that's hidden by cable sleeving, and if it's failed, it would account for the problems I've been having.

Let's hope so, & once more thanks for your suggestions.

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turnipfarmer

posted on 22/8/20 at 06:05 PM Reply With Quote
Let there be light - and there is - finally!!

After many more episodes of yes it's working! - No it isn't!, I've finally abandoned the idea of trying to identify this mysterious fault, and just fixed the problem by wiring the output from the pressure switch straight to the brake lights.

Thanks to everyone for their kind help and suggestions.

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