Diagnosing a headlight issue - Low beams inoperative

The purpose of this post is to help give people an idea of how I approach diagnostics to assist with their own troubleshooting.

Scenario: 78 280Z with no low beams

Background: My friend was working on another person's 280Z and found that the headlights were not working. He checked the high/low beam switch and found it need of repair. However, after repairing the switch, he could only get the high beams to work. He asked if I could assist with the diagnosis and repair.

Diagnostic process:

First I verified the problem description. The high beams would work, but the low beams would not. I backprobed the connector (inserted the meter probe on the back of the connector) at the steering column with the other probe connected to the negative post on the battery. I checked for resistance to ground and found both the high and low beam positions showed acceptably low resistance on both sides of the connector. Note: Since the resistance to ground readings were good on both sides of the connector, it lowered the probability that the problem was corrosion at the steering wheel harness connection.

Next I split the connector at the steering wheel and confirmed that there was not an issue with corrosion. I turned on the headlight switch and confirmed I had good voltage to ground at the red/black wire (low beam circuit). I then connected a jumper wire to the red/black wire in the dash harness and grounded the other end of the wire while monitoring the voltage. The voltage dropped to zero, but the low beams were not illuminated. Thanks to @Captain Obvious, I knew that meant there was corrosion in the circuit obstructing current flow. (As he has mentioned in other threads, voltage readings show potential, but that doesn't mean the path can support the current flow.)

I referred to the service manual to find which connector linked the red/black wire from the engine harness to the dash harness. I found it was connector C-2 (see below). (The 260Z and 280Z manuals have great break downs of the lighting circuits in the Body Electrical section.) This connector is located near the firewall in the passenger footwell. I found that the size of the connector would not allow me to backprobe with my full-sized meter probe, so I opted to use the back probe kit I got from Harbor Freight to take a voltage reading at the connector. I inserted the probe on the engine harness side of the connector and operated the headlights. There was still voltage to ground. Since this wire is the path to ground, it should not have voltage. That indicated there was an issue at this connector. I confirmed by connecting the back probe to ground and turned on the headlights. I could see the low beam, so this validated my theory. I opened the connector and found many terminals were green and white from corrosion. Polishing the terminals with a Dremel and wire brush did not remove enough corrosion. Unfortunately the AMP style 1 connectors/pins are no longer available, so the solution was to remove the affected wires, put on 6.3mm open barrel spade terminals, and insert those terminals into a new connector.

When I was finding the issue at C-2, my friend noticed that only the passenger headlight illuminated on low beam. We tested the headlight bulb sub harness and confirmed that the other 2 wires at the connector were good. He had another sub harness, so we installed that and confirmed that the issue was with the sub harness. He depinned the low beam terminal from the headlight connector, but I could not see any issue. That left me to believe that the problem was at the connector between the sub harness and engine harness. I depinned the red/black wire from the connector and installed a new terminal on the wire. We re-tested and found that both low and high beams were working.