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MG MGB Technical - Continuing the flasher/brake light problem

Hello Gentlemen, It's time to search for more help on my '77B roadster electrical woes.
God Bless Lucas, without them we would have nothing to worry about.

The story starts about 15 months ago when I bought my B. Apart from things that I could easily determine, like a bad alternator, Need new suspension, stuff like that, I havd a problem with indicator lights and brake lights.
The symptoms showed that when I used the indicators they were dim and when I also used the brakes, both brakes and indicators showed no illumination. In addition, when I applied brakes, the Tach showed a reduction in revolutions until I released the brake pedal and if I used the indicators, the Tach would reduce/return to normal in synch with the flasher. I have done extensive research into this problem and would like to thank Rick Astley for his in depth study into the electrical system of the MGB. I have printed his whole web site and used it as an electrical manual.

OK. So the problem would be either a bad earth/ground or a sneak circuit. Great, well I have removed, cleaned and reconnected every ground that I can find. I have removed the Dashboard Harness, re-soldered all ground wires, Tested every green wire for insulation to ground and still I have my challenge. I am about at the end of my rope, except for the fact that at one time it must have worked correctly or else it would not have left the factory.
I have check just about every voltage point I can come up with and the only thing that I have an issue with is that the voltage output from the Instrument Voltage Regulator never shows me anything like 10 volts. When I remove the green wire from the VR, it shows 12V +/- 1V.
When I connect this to the VR the VR output fluctuates between 0 and 3V, it never gets to 10V. Should I replace this with an electronic version as in Rick's Manual. (put a new VR in when I was first researching this problem)

Does anyone have any ideas where to look next?
I WILL NOT STOP UNTIL I FIND IT. This has become a quest.
I have been able to fix just about anything electronic since I was 15 (38 Years) but this one is the best challenge I have ever had.

Thanks Gents,

England vs Argentina Friday 7:30am Don't miss it.
Bob Anderson

Bob - Your symptom of the tach dropping revs whenever you stepped on the brake is the same as I had on our MGB on two trips that we took to California a number of years ago. Both times it turned out to be the result of my camera case having been stuffed in the fender well in the trunk and pressing the wire to the brake light against the shell of the socket, causing a short to ground. Stepping on the brake would apply 12 volts to the short, load the circuit down and cause the tach rpm to drop AND no brake light. I finally fixed the problem for good by making and installing shields around the tail/turn/brake light sockets. Have not had any problem since regardless of how tightly I pack the trunk. When you say that you have checked all the grounds, I assume that you checked the one under the license plate mounting bracket bolt in the trunk. I have had this one cause dim tail light and brake lights and even cause the fuel pump to quit. I don't know where to tell you to look for the VR, I have never had a problem with that. Good luck - Dave
David DuBois

Just by saying that DAvid, now you will.:)
Luigi

Yes Luigi, we must remember that these cars have their own minds. Thanks Dave for your thoughts. I will check the leads going to the lights and make sure that they are well insulated from Ground. I will not quit until I find this. I refuse to give in to a machine. If we do that they could end up taking over the world.

Bob
Bob Anderson

The brake lights, tach and turn signals all operate off the green circuit. If anything in this circuit is giving a bad connection then the extra load of the brakes or turn flashers can cause the tach to drop. Start at the fusebox, 2nd fuse up, check the voltage with the brakes on and the brakes off (or the turn signals). Check the connector on the white wires, the fusebox spade, the fuse end-cap, the green spades, and the connectors on the green wires first of all. If you see almost the same voltage (there will always be a small variation of a few tenths of a volt) both on and off then move through the many double connectors in the green circuit. If the voltage drops significantly even on the connector on the white wire coming from the ignition then move back towards the ignition switch and its multi-plug. But it is more likely to be around the fusebox. There is a rivetted connection on the back connecting the spades and the fuse holder that can corrode as well.
Paul Hunt

Thanks Paul,
All good information. I think that I am leaning towards the fuse panel. I have been checking voltages all over the green circuit trying to get a handle on the voltage drop but I have not thought about the fuse panel itself being a possible cause. I'll play with that today, (if I can convince the trouble & strife that I don't really need to cut the grass) and let you know what I find.

Bob.
Bob Anderson

This thread was discussed between 06/06/2002 and 08/06/2002

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