On a 97 Legacy Wagon, I have a large voltage drop on the ground side of the both headlamp circuits. I know the ground wire is switched at the steering column cluster, but where is the actual ground point to the body?

Voltage on the downstream side of the load (the light) should be 0 vdc if there is only one load in the circuit.

 

If there are mulitiple loads in the circuit, you are reading the voltage drop across the single load.

 

There are mulitiple grounds in that harness, I'm not sure which one is the actual ground.

 

- Main ground up front on the radiator support in front of the battery

- Ground on each side of the lower frame rail, towards the front

- 2 grounds on each front strut tower.

Yup... I think you need to clarify how you made the measurement.

 

If the ground is bad, you should be able to measure the full battery voltage (13.7V?) from the positive side of the lamp to the negative terminal of the battery, BUT you'll get a measurement larger than 0 V between the negative terminal of the lamp and the negative terminal of the battery.

It is none of the the ground points under the hood. Subaru wouldn't come back out of the passenger compartment with the ground wire, its not cost effective.

 

For the measurement... it was taken from the negative side of the low filament to the negative battery terminal, thus testing the bulb terminal, wiring & crimps, headlight switch, ground point to the body and negative cabling to the battery.

 

In just writing this, I realize that if the voltage drop is only the low filament side and the high is ok, then it is the low side of the expensive switch and not the ground point. Ground point failure would affect both.

Check the socket. Lot's of times they are not as tight as they should be and cause high resistance. I've seen the plugs melted.