S30 Grounds and Ground Refresh

Signs of a Bad Ground

• Dim or flickering lights, especially headlamps or tail lamps that get brighter when you wiggle something. The factory headlamp trouble chart lists a poor ground as a cause of dim headlamps, and a missing ground as a cause of headlamps that won't switch beams.
• Lights that affect each other: the turn signal makes the tail lamp glow, or the dash lights flicker with the brake lights. When a lamp loses its ground, current finds a way back through another bulb.
• Gauges that read wrong or move when other things are switched on.
• Slow cranking or a starter that clicks, even with a good battery.
• Fuel injection problems on the 280Z: the EFI sensors and control unit measure small voltages, and a poor ground throws the readings off.

Where the Grounds Are

Ground wires are solid black (B) in the Nissan color code. The main return paths on a 280Z, from the factory service manual and circuit diagrams:

From the 1975 280Z FSM, section BE (harness descriptions, Fig. BE-16 and BE-22) and section EF. Exact bolt locations vary by year; trace the black wires on your own car.

Voltage Drop Test

An ohmmeter won't find most bad grounds, because a connection that reads fine with no current can still fail under load. A voltage drop test measures the connection while it's working:

1. Set a digital meter to DC volts.
2. Turn on the circuit you're testing (for example, the headlamps, or crank the engine for the starter ground).
3. Put the black lead on the battery negative post (the post itself, not the cable clamp).
4. Put the red lead on the ground side of the part you're testing: the lamp's ground terminal, the engine block, the body near the ground bolt, and so on.
5. Read the meter. It shows how much voltage is being lost on the way back to the battery. On a good ground it should be close to zero, a few tenths of a volt at most. The more current the circuit draws, the more a bad joint shows up.
6. Move the red lead step by step toward the battery (wire end, ring terminal, bolt, body). The point where the reading drops is the bad connection.

You can test the positive side the same way: red lead on the battery positive post, black lead on the part's power terminal. The factory manual uses the same idea for the headlamps: measure the voltage at the headlamp terminals, and if it's low, check the charging system and the connections.

Ground Refresh

1. Disconnect the battery, negative cable first.
2. Battery cables. Clean the posts and clamps until they're bright. Check the cables for green corrosion under the insulation, a swollen cable or a loose crimp at the ends. Replace a bad cable rather than cleaning it.
3. Go around the car and remove each ground bolt one at a time, so you don't mix up the wires.
4. Clean to bare metal. Wire-brush or sand the ring terminal on both faces, the bolt, and the body under the terminal until they're shiny. Paint, primer and rust don't conduct.
5. Check the terminal. If the wire is corroded where it enters the ring terminal, or the crimp is loose, cut it back and crimp on a new terminal.
6. Reinstall tight. The factory checklist says every grounding bolt must be tight. A serrated or star washer under the terminal helps it bite into the metal.
7. Protect it. Once it's tight, coat the outside of the joint with dielectric grease or a spray protectant so moisture can't get in. Don't put grease between the terminal and the metal.
8. Retest with the voltage drop test.

Extra Ground Wires

Adding a few extra ground wires is a common, inexpensive upgrade, especially if you've added a higher-output alternator, brighter headlamps or a modern stereo:

• Engine to body: a heavy cable from the engine block to the body, in addition to the stock ground.
• Battery to body: a heavy cable from the battery negative to a clean spot on the body.
• Front lamps: a dedicated ground wire from the headlamp and front lamp grounds to the body or the battery negative.
• Rear lamps: a ground wire for the tail lamp assemblies to a clean spot on the rear body panel.

Use wire at least as large as the original, ring terminals that are crimped (and soldered if you like) and covered with heat-shrink, and bolt each one to clean bare metal.