A couple of weeks ago the signal lights and brake lights on my MG TF stopped working completely. By going to an MG newsgroup, we determined the problem was probably in the electrical relay under the hood. We removed it, cleaned it up and adjusted the contacts. We installed a new brake lamp switch from NAPA and a new flasher. The car tested out perfect -- with the headlights off.
Last night we took a pretty long ride in the car. The brake lights were still working and the rear signal lights worked fine. But the front turn signals didn't work properly with the headlights on. They were kind of "surging" a bit when we put the switch on, but not really blinking. However, with the headlights off, they worked properly again.
Most people might have expected a "short" in the wiring or blamed the problem on Lucas electricals. British car owners love to laugh about "Lucas: Prince of Darkness"
But Lucas had nothing to do with this problem. The solution was very simple. The parking light/directional light bulbs were in the light housing sockets upside down. With the headlights off, the wrong filament was doing the blinking and could handle the load. With the headlights on, it could not handle the load, so the blinking action was slow and weak.
As soon as we flipped the bulbs over, all of the lights worked fine. We also noticed that the green indicator light on the dashboard was working stronger, too.
So don't blame Lucas for all electrical problems. By and large, the old prince does a pretty good job.