S30 Headlight Relay Upgrade
On a stock 240Z, 260Z or 280Z, all of the headlamp current runs from the battery, through the fusible link, the ammeter circuit, the light switch, the fuse block, the long harness to the front of the car, and back through the dimmer switch. That's a lot of old wire, connectors and switch contacts, and every one of them loses a little voltage. The result is dim headlamps, and over time burnt switch contacts and a cooked fuse block. A pair of relays fixes this: the stock switches only turn the relays on, and the relays feed the headlamps from the battery through short, new wires.
*WARNING* - Disconnect the battery negative cable before wiring, and fuse every new power wire close to the battery. Do this at your own risk. We are not responsible for any issues of your own doing.
How the Stock Circuit Works
From the 1975 280Z headlamp circuit diagram (FSM Fig. BE-16):
- Power comes from the battery cable at the starter, through the black fusible link and the ammeter shunt, to the light switch.
- With the switch in the second position, power goes through two 10A fuses in the fuse block, one for each headlamp, then up the red (R) wire to the headlamps.
- The two beam wires (RW and RB) run from the headlamps back to the dimmer switch in the combination switch, which connects the selected beam to ground. In other words, the dimmer switches the ground side, not the power side.
- Stock headlamps are 50W high / 40W low sealed beams (SAE 6012). Two high beams draw about 8 amps, which all passes through the light switch and the dimmer contacts.
The factory says to measure the voltage at the headlamp terminals: below 12.8V means something is wrong. On many old Zs it's well under that with the engine running. Measure it before the upgrade so you can see the difference after.
Parts
- Two 12V automotive relays, 30/40 amp, four or five pin (Bosch-style), one for high beam and one for low beam, with sockets.
- An inline fuse holder for each relay (20A each) or one 30A fuse feeding both.
- Two headlamp plugs (three-terminal H4 / 6012 style) to connect the new harness to the headlamps.
- Wire: 12 or 14 AWG for the relay feeds and the headlamp wires, 12 AWG for the new headlamp ground, and 18 AWG for the relay coil wires.
- Ring terminals, crimp connectors, heat-shrink, loom and zip ties.
Several Z parts vendors sell ready-made plug-in relay harnesses that do the same job without cutting the stock wiring.
Identify the Wires on Your Car
Wiring changed over the years and many cars have been modified, so test the stock headlamp plug before you build anything:
- Unplug one headlamp and turn the headlamps on.
- Put a meter's black lead on a good ground and check each of the three terminals in the plug. The one with 12V is the power feed (red, R, on the 1975 280Z).
- Now put the meter's red lead on the power feed terminal and the black lead on each of the other two terminals. Switch between high and low beam. The terminal that shows 12V on high is the high beam wire, and the one that shows 12V on low is the low beam wire. Write them down.
- If you find 12V on the beam terminals themselves instead (powered, not grounded), your harness switches the power side. Wire the relay coils the other way around: terminal 86 to the beam wire and terminal 85 to ground.
Relay Wiring
For the factory ground-switched circuit, wire each relay like this:
| Relay terminal | High beam relay | Low beam relay |
|---|---|---|
| 86 (coil +) | Stock power feed wire from the old headlamp plug (R) | Same stock power feed wire (R) |
| 85 (coil -) | Stock high beam wire from the old plug | Stock low beam wire from the old plug |
| 30 (power in) | Fused wire from the battery positive (20A) | Fused wire from the battery positive (20A) |
| 87 (power out) | New wires to the high beam terminal of both headlamps | New wires to the low beam terminal of both headlamps |
Then run a new ground wire from the ground terminal of each new headlamp plug to clean bare metal on the body or straight to the battery negative.
When you turn on the headlamps, the light switch powers both relay coils through the stock wire, and the dimmer switch grounds the coil of whichever relay you've selected. That relay closes and feeds its beam straight from the battery. The stock switches and fuse block now carry only the relay coil current, a small fraction of an amp.
Installation
- Disconnect the battery negative cable.
- Mount the relays in the engine compartment near the front, away from exhaust heat, with the terminals pointing down so water can't collect in them.
- Run the feed from the battery positive through the fuse(s) to terminal 30 of each relay.
- Build the new headlamp harness: the two 87 outputs to the headlamp plugs, plus the new ground wire. Route it along the stock harness and keep it away from sharp edges and moving parts.
- Connect the coils to the stock headlamp plug wires. Splicing into one stock plug, or using a plug-in adapter so the stock plug stays original, both work.
- Reconnect the battery and test low beam and high beam on both sides. Then measure the voltage at the headlamp terminals again.
With the extra voltage, the stock sealed beams get noticeably brighter. If you're also changing to higher-wattage bulbs, the relays are a must: never run high-wattage bulbs through the stock switches.
Further Reading
- S30 Helper: Electrical, which covers the S30 lighting circuits and the headlight relay upgrade.
- S30 Grounds and Ground Refresh and S30 Fuses and Fusible Links on this site.
Sources Used
- 1975 Datsun 280Z Factory Service Manual, section BE: bulb specifications (BE-10), headlamp circuit diagram (BE-11, Fig. BE-16), fuse block (BE-5) and the headlamp trouble chart (BE-31)