Electrical help

[advinnie]

Hi all, I have a 1971 Vw beetle which I’ve just brought. It has three aftermarket gauges. A rpm, oil temperature and oil pressure gauge. Now all three gauges work just fine BUT ever time I touch the break pedal all three gauges stop working and full back to zero until I take my foot of the brake pedal. Now the break lights are coming on when I touch the break pedal but as soon as they do the three gauges fail.
Any ideas on what’s going on?

[ashman40]

From the trunk side of your dash, identify the ground (-) and power (+) wires running to your gauges. I'm guessing they will be in some way tied together (single ground wire and single power wire daisy-chained between all three?). Make sure the ground wire goes to a good ground point behind the dash. Make sure the electrical connection to this ground is clean and not corroded. Follow the power wire and make sure it goes to the fuse box and does not splice into another wire.

Find the harness of brake switch wires that runs towards the left front of the car. It will be the only harness of wires with a black/red wire. This black/red wire runs from the brake switches on the MC all the way to the rear of the car. There is a junction in the trunk but this wire should not connect to anything in the trunk. No wires should be spliced into it.
In the same harness of wires coming from the MC switches you should find a solid red wire. Solid red wires are normally constant 12v+ from the battery... all except for this single red wire which runs from the MC switches to the Brake Warning lamp in the dash. This wire should have no splices on its way to the Brake Warning Lamp.

One scenario that would make sense based on your symptoms... your gauges are grounded to the black/red brake light wire. While the brake lights are not ON, the gauges ground thru the rear brake light bulbs. When the brake pedal is pressed, the MC switches power the black/red wire and your gauges no longer have a path to ground so they stop working.

A common mistake when working on Beetles is to think black wires (black/red in this case?) are ground wires. Black wires on Beetles are part of the #15 ignition switched 12v+ circuit. They are powered by the ignition switch. They are never ground... unless someone used the wrong wire color.
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AshMan40
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'67 Beetle #1 {project car that never made it to the road }
'75 Beetle 1200LS (RHD Japan model) {junked due to frame rot}
'67 Beetle #2 {2019 project car - Wish me luck!}

[advinnie]

Thanks for the reply mate.
All three gauges take their 12v supply from the ignition live fuse connection, two of the gauges shear the same earth but the throes gauge has its own earth point. I’ve removed all the gauge earths from the earth point behind the dash and earthed them all to a door hinge screw just to see what would happen. Doing this hasn’t change anything, still all gauges work until the break pedal is touched. Could I have a positive supply going to earth when the break lights come on sending a +12 v to the earths of the gauges ?

[ashman40]

[advinnie]

I will get back to you with answers to your questions tomorrow mate when I’m back at the car.
And thanks you for your help 👍