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MG MGB Technical - Brake lights and fuel gauge off

I am working to repair my brake lights. I had the brake light leads hot connected and working yesterday...and ordered a new switch from Moss. However, I started the car this morning with the battery charger connected for about two minutes before removing the leads! Now the brake light wires have continuity but do not light up the brake light bulbs when I make the hot connection. The new switch arrived and is intermittent, so I will exchange it. Also, my fuel gauge was working is now not working. So far I tested the rear bulbs and they seem good. I tested the fuses in the fuse box and two fuses in the harness I could see near the heater valve and they have continuity. Is there another fuse inline somewhere that may be bad and I have missed? What did I do? What happened?

Simi Valley, CA
Stephen Lofaro

No more fuses. But check the grounding of the rear light units, the brake lights draw more current and will "tax" a poor earth. I always connect an earth lead to the front and rear units. Also check bullets (green and green/purple) at bulkhead. When you say "hot" connection I assume you mean shorted out switch.
As for fuel gauge first assertain whether it is poor connections, the gauge, stabiliser or sender. i.e., remove "live" (green/black) wire from sender unit and ground it. Gauge should then read full. If no go to the front check the bullet connections between front and rear looms coming up r/h bulkhead. The fuel gauge works off 10volts and on the bulkhead inside, near the wiper motor there is a voltage stabiliser. Check connections there, AND that is mounted correctly, they are position sensitive. A meter on the output side should give a varying voltage which is supposed to average out at 10 volts Putting a PP9 9v battery across the fuel gauge terminals should give movement towards full. If not it's the gauge.
Allan Reeling

If neither brake light is working then it could be two earth faults, but more like one common fault if they have been working in the recent past.

What year is this? Replacement hydraulic switches at least are very poor and not up to the job, one has to fit a relay, and even that needs a diode quench across the winding.

You need to operate the brake pedal with the ignition on, then test both sides of the switch with a voltmeter to work out what is happening.

If you have little or no voltage on both wires, then there is a bad connection back to the green circuit fuse.

If you have 12v on the green but little or no voltage on the green/purple then the switch is bad.

If you have 12v on both the green and the green/purple, then you need to check both sides of that wire where it joins the green/purple in the rear harness, at the mass of connectors by the fusebox.

If you have 12v both side there then you need to do the same thing with the green/purple by the right-hand rear light in the boot (three connections this time).

If the problem is in the fusebox that would affect the fuel gauge as well, but also the tach. Otherwise as Allan says using a known good earth wire on the green/black to see if the gauge moves, and if it does then to a clean point of the body of the sender. If both points cause the gauge to move then it's bad earth at the tank. If the green/black moves it but not the sender body then it's the sender.

The original sender pulses between 12v and 0v once every couple of seconds, and whilst it averages about 10v over a period of time you wouldn't see that on a meter.
Paul Hunt

Thanks to Allan and Paul for their input. I am making some headway on the electrical connections on my MGB-GT.
Cleaning the connectors on the fuse box seems to have been the most helpful, so far. All my gauges are actually working, though intermittently. Perhaps I need to crimp one of the green connectors down just a tad. I am searching for that one.

I am still having no luck replicating the brake lights on when shorting out the switch. I have checked the grounds in the boot and they seem fine. I have checked and cleaned the bullet plugs adjacent to the heater box. I have not run any voltage tests on those leads because I am not exactly sure how to do that.

My son installed a new turn signal switch and flasher unit. At one point, when I engaged the turn signal up or down, all the lamps are flashing simultaneously and it is causing the gauge displays to fail. Now I cannot get the flasher unit powered up again to send to the turn signals. I am not sure if the small harness plug on the turn signal switch can be put on in more than one configuration, or how to test the flasher unit itself. Any thoughts are appreciated on these issues.

Stephen Lofaro
Simi Valley, CA

Stephen Lofaro

The rear light clusters do not use wired earths.

Intermittent gauges indicate there are still problems, which could be in the fusebox, and hence affecting the brake lights. Indeed, it's quite possible that it's the additional load of the brake lights is causing a weak point in the fusebox to break down and stop the gauges working. Also indicated (no pun intended) by the gauges stopping working when you use the indicators.

What do you mean by "all the lamps are flashing simultaneously"? The indicators both sides? Or brake lights/tail lights are flashing with the indicators? The latter does indicate a bad earth, which can be where the light unit bolts to the wing, or inside the light unit.

For the brake lights, if they are still a problem, you need to connect your voltmeter between the point being tested and a known good earth.

Points to test, all connections made as they should be, all tests done with the ignition on (disconnect the coil to stop it overheating) and the brake pedal operated i.e. switch contacts should be closed, all should show 12v:

Green wire on the brake light switch. If voltage low here the problem lies back towards the fusebox.

If that's Ok then green/purple wire on the switch. If voltage low here then the switch is faulty.

If that's OK then the back of the bullet on the green/purple from the rear harness where it joins the green/purple from the switch or main harness at a 2-wire connector in the mass by the fusebox. If low here that bullet connection is bad.

If that's OK then the back of the three bullets on the green/purple wires at the 4-way bullet connector by the right-hand rear light. One wire goes to the right-hand light, the other two are in the rear harness, one from the front and one to the left-hand light. Unless the rear harness has been damaged under the car somewhere at least one of these should show 12v. If not don't show 12v that connector is faulty. Come back when you have done all those, if there is still a problem.
Paul Hunt

Thanks to Paul Hunt and Alan Reeling. The update on the MGB-GT wiring...turns out the flasher switch was bad and needed to be replaced to get the turn signals separated. Finally, I have the electrics and gauges working.

I am going to start a new thread to get some advice on a 'cold starting' issue that seems to be one of the next problems to resolve.

Steve Lofaro
Stephen Lofaro

This thread was discussed between 28/02/2014 and 24/09/2014

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