continuity issue full rewire

I'm in the process of a complete rewire of my 55 chevy pickup. Most of the wires are routed, connected and what not. I decided to test my turn signal indicators and found something odd. I had continuity between the front left and front right turn signals. These are isolated leads that don't run through a fusebox, or anything. The only theory i have is that there is a ground loop of sorts causing the multimeter to pick up its signal as it passes down one wire, through the bulb, to ground through the other bulb and back up the other wire. I just can't think of how to test this or fix it. Any ideas?

Are you sure you are not checking the parking lamps? They are common with each other.

No, I'm checking the turn signal wires under the dash. I installed a complete head to tail rewire so I know its the turn signal wires.

You can't go +ve thru a bulb to GND and up another bulb to +ve.
The only way that can happen is if the GND does not exist (ie, is a resistance similar to or higher than the bulb resistance(s)).

That is a ground fault or break - not a ground loop.
And it is the reason some dual-filament bulbs circuits act funny - eg, a combined 7/21W tail/stop bulb with no GND works fine as a tail, or looks like a tail when only the stop lights are on, or the bulb extinguishes totally when both tail & brakes are on.
In other cases (including single filament bulbs), a ground break can light entire banks of lamps (but dimmer than normal).

Why are you measuring continuity? What is the resistance?

If you measure the resistance from one turn signal trigger wire to the other turn signal trigger wire you are correct they are connected but through 2 resistors (light bulbs). If you want to verify that is what you are seeing measure the resistance and if it is above 20 ohms or so you know you have some sort of load connecting them. To further verify it is the bulbs connecting the circuits together through their common ground remove one of the light bulbs - this should cause the circuit to "open" up.

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Kevin Pierson

I'm using the continuity setting on a multi-meter to check for shorts?

DMM Continuity tests usually indicate "continuity" for a resistance under ~50 Ohms.
Light bulbs of 3W and higher are below 50 Ohms.
Hence as KP wrote, if measuring continuity between left and right (flasher) light circuits, you will get "continuity" (unless the bulbs on at least one side are removed).

Ok continuity test aside, if i connect a power wire to the left turn signal wire under the dash where the steering column wires attach to the main harness both front turn signals light up.

Are they (both) full brightness?
If not, ensure they have a proper ground.
If they are, then their must be a bridge (short) across the 2 sides.

If a faulty ground, then removing one bulb should extinguish the other.

Are the rears ok? Usually the front & rears are connected together except if the flasher filaments serve dual functions (eg - rear flasher & stop or reverse; or front flasher and DRL etc).
If they are joined, you may have to remove both/all bulbs on one side to break the ground to the "powered" other side (assuming an other side ground fault). But it does depend on how they are wired - eg, dual filament bulbs, or shared ground wire, etc.

Rears are ok near as I can tell. not at home for a couple of days so I'll try these suggestions and post results in a couple of days. Thanks All.

Ok so I had some time this week to work on my electrical system again... "THE LOOP" as I have come to call it, is present not only in the from marker / turn signal lights, but in head lights as well. "THE LOOP" is eliminated as soon as the bulb is removed from the circuit.

To save those of you just seeing this from reading the whole thing the problem is that my turn signal indicators both light when one turn signal or the other is activated (or the parking lights are on), and as I recently found the high beam indicator is activated when either the old beams or the high beams are connected. BUT AS I MENTIONED ABOVE THE PROBLEM IS RESOLVED BY REMOVING THE BULB.

My theory as I mentioned above is that power is passing the through one filament, to the bulbs common ground, back throughout other filament in the bulb and traveling backwards up the that supply line. In the case of turn/parking lights this means the parking light filament acts and a path of travel between the 2 turn signal circuits.

If I'm right the solution is to install diodes on each power line.

But I don't trust my own, informal, knowledge base.
PLEASE ADVISE

What you are describing is a broken or insufficient ground.
If the ground were intact, then as I wrote earlier, it is impossible for current to travel from +ve to ground and then "back up" another path to +ve.
However in the case of indicators/flashers, it is probably a case of bridging (interconnection) between the RH & LH sides.


All bulbs must be removed for a continuity test. A DMM's continuity test is only good for testing wire/circuit connectivity between 2 points - eg, RH output of flasher switch & RH +ve of the RH flasher bulbs versus RH out & LH bulbs or GND etc, and bulb GND to GND.

With bulbs etc in the circuit, measuring continuity between two +12V points (with the +12V source removed of course!!) does not mean there is a fault.
EG find a working brake and tail light but ensure both are of (unpowered)...
Measure the the continuity between the +12V of each bulb.
There will be continuity yet the 2 circuits work fine. They are not "shorted" together.