With Headlights on, Signals not working

[TheCommish]

So where should i start looking to fix this?

[azdave]

It would help if you would mention what vehicle you are working on. The circuits are quite different between some Corvair mocles.

In general...
I don't know what you have checked already but the first thing that comes to my mind is poor grounds. If you have poor grounds then the ground wires are not at zero volts which leads to many odd electrical issues. Power will accidenatlly feed through other related circuits which might make them appear to be working okay at times.

With a poor ground, when you turn on your headlights for instance, that could be raising the ground path voltage too far above zero volts so that the turn signals no longer pull enough current to blink or maybe even iluminate.

[bbodie52]

I'm assuming that you are talking about a 1964 Corvair?

Left-click on each image with your mouse to enlarge for better viewing...

1964 Combined Passenger Compartment & Engine Compartment Wiring Diagram

1964 Corvair Trunk Wiring Diagram

1964 Corvair Interior Wiring Diagram

1964 Corvair Engine Compartment Wiring Diagram

The turn signal switch is the hub of activity and switching. In the detail below, there are two sources of power: (1) The brake pedal brake lamp switch, and (2) The Flasher Relay in the Fuse Block. These two power sources are shown in the middle of the turn signal switch. The left and right front turn signal lamps are shown on the left side of the switch, while the left and right rear turn signal/brake lamps are depicted on the right side of the switch. (In the rear, the same filament in each 1157 (or 1034) bulb does dual-duty as both the brake and turn signal filament.

With the turn signal control in the neutral position, no power is connected to the front bulbs, and the brake pedal switch is connnected to the rear bulbs through the switch. If the left (or right) turn signal is actuated, the selected side in the front is connected to flasher power (as is the instrument panel turn signal indicator bulb on that same side). At the rear, the bulb on that same side is temporarily disconnected from brake lamp power, and is connected to the flasher power. The opposite side remains connected to the brake lamp switch. In this way, one side shows braking action, while the other side show turning action.

It is important to note that the current load produced by a circuit through all three bulbs is needed for the flasher to function properly. If something is interrupting the electrical current through one of these bulbs, the flasher may hang and not flash at all, or it may flash at a rapid rate. Since your turn signals are disrupted when the headlights are on, it is possible that the GROUND RETURN path is inadequate in the front, and that the operation of the headlights is disrupting the ground return for the turn signal sockets. This could place an improper load on the flasher which could cause it to stop flashing. DO THE BRAKE LIGHTS FUNCTION WHEN THE HEADLIGHTS ARE ON AND YOU STEP ON THE BRAKE PEDAL?

You might try wiring a better chassis ground to the headlights, and see if this restores function to the turn signals.

[Jerry Whitt]

There is a test that can be done with a volt meter. The test involves placing the positive terminal of the voltmeter to the "most positive" item in the circuit, and the negative cable attached to the "most negative" item.

In this case, attach the positive terminal of the volt meter to the turn signal outer case. Now attach the negative cable to the negative terminal on the battery. Set the volt meter at a low setting, 3 volts usually works.Then the turn signals are switched on, and the volt meter will read a number. Ideally this would be a really low number, like zero. What usually happens is the volt meter will show a reading of 1 to 2 volts or even higher.
This will confirm there is a measurable resistance in the circuit. This is called "voltage drop."

Now work backward thru the circuit, cleaning and tightening the various connections. The goal is, of course, zero resistance and zero voltage drop.