headlights on, brake pedal pressed & loss of current

[uestvega]

My 81 spider has developed a few old age habits....when the headlights are on and I press the brake pedal, there seems to be an excessive current draw I notice that if I have the turn signals on, they suddenly blink almost 2x as slow when I press the brakes. If just the parking lights are on, this does not happen.

Also, the light for the gas gauge must have come loose as it stopped working, but what I notice is that once i turn the lights on, the fuel gauge suddenly jumps to full and pegs the needle...(now I know this is a great way to beat the gas price blues, thinking that your tank if alway full), as soon as I turn the dash light dimmer down the gas gauge needle moves tot he left a little and if I click the dash lights all the way off with the dimmer the gas gauge functions normally, so I am assuming that the light is touching one of the contacts of the gas gauge behind the cluster. Thoughts.

Also, while we're at it, what is the simplest way to reconnect the speedo cable to the back of the unit.

Thanks

[spider2081]

I have changed the instrument light bulb that is controlled by the dimmer in my gas gauge from under the dash. So you might be able to see if the bulb is in place or not. The fuel light bulb is harder to remove and replace.

To reconnect the speedometer I pull the entire instrument cluster out. I remove the 6 screws holding on the steering wheel and the steering wheel. Also the plastic cover for the steering column switches. I do the to make room for the dash to come out. Some folks drop the entire steering column. Then in the engine compartment I pull any spare length of speedometer cable up from tranny and push it into firewall to give a little extra length behind the speedometer. Very tight working area

The light dimming and slow flashers sound like poor grounds. Clean and tighten all you grounding pods.

[seabeelt]

Check the grounds/grounding pods in the trunk that go to the tail light circuit boards. Also check the circuit board connections and bulb sockets. Heavy draw in current suggests trying to overcome more resistance in the circuit. I.e. The ground loop.
R/

[GeorgeT]

Have the alternator output checked as well.

[bradartigue]

Are you sure the speedo cable isn't broken?

Cars with bad grounds find the shortest route to ground, which is often the speedometer and clutch cables. Cars that break these with frequency have bad grounds. Check the engine-to-firewall ground, the instrument-to-dash ground, the trunk grounds, etc.