EFI (L-Jetronic)
The 280Z (1975–78) was Datsun's first US-market Z with electronic fuel injection — Bosch L-Jetronic, built under license by Nissan. It uses an analog ECU with a 35-pin connector, a vane-type air flow meter (AFM), six injectors fired in two banks (group fire), and a small cast of supporting actuators. There is no oxygen sensor and no closed-loop control; mixture is set entirely by the AFM and a few temperature inputs.
The good news: the system is simple by modern standards and is very diagnosable with a multimeter. The bad news: it's 50 years old, the wiring insulation is brittle, and the AFM's wiper track wears in exactly the spot you idle.
Wiring topology
Components
ECU
Analog board, 35-pin connector, mounted vertically behind the passenger kick panel (LHD; driver kick on RHD). Year-coded; 1975 ECU is not interchangeable with 1976+ without checking the part number. Failures are rare; capacitors do age but the board is generally bulletproof.
Air Flow Meter (AFM)
The single most important sensor. A spring-loaded vane in the intake tract is deflected by airflow; a wiper traces a resistive track to give the ECU an analog voltage proportional to airflow. The intake air temperature thermistor lives inside the same housing.
Failure mode: the resistive track wears under the wiper at the idle position, causing erratic mixture at idle. Repair: open the AFM, reposition the spring one tooth so the wiper rides on unworn track.
Coolant temperature sensor
Two-pin thermistor in the thermostat housing. Note: this is not the same sender as the dash temp gauge, which is single-pin. Don't confuse them; they're right next to each other.
Throttle switch
Two contacts: idle (closed at idle) and full-throttle (closed near WOT). Used by the ECU for idle enrichment and full-load enrichment. Adjustable; set the idle contact to just close at closed throttle.
Cold start valve + thermotime switch
An extra injector in the intake plenum, fired briefly during cold cranking to richen the mixture for cold starts. The thermotime switch in the head times its operation: it's a thermistor and an internal heating element, so it lets the cold start valve fire for ~5–8 seconds when cold and not at all when warm.
Injectors and dropping resistors
The 280Z uses low-impedance injectors (~2.5Ω each). To prevent overheating the injector windings or the ECU drivers, six dropping resistors (one per injector, sometimes called the "resistor pack") are wired in series. The pack mounts on the firewall or strut tower depending on year.
Fuel pump relay and EFI main relay
Two relays. The EFI main relay energizes the moment the key turns on, supplying 12V to the ECU and the injector circuit. The fuel pump relay is controlled by the ECU and only energizes when the AFM vane moves (engine running) or briefly during cranking. This is a safety feature: in a crash, if the engine stalls, the pump shuts off.
EFI sub-harness
The EFI wiring is a separate sub-harness that plugs into the main body harness via a single multi-pin connector under the dash. This is convenient for service — you can pull and replace the entire EFI harness without disturbing the body harness — but the connector itself is a notorious failure point.
Typical degradation:
- Brittle insulation cracking at injector and AFM connectors (heat-cycled for decades)
- Green corrosion inside the injector connectors
- Cracked solder joints inside the fuel pump relay
- Strands broken at the ECU connector strain relief
Pigtails for injectors, AFM, and CTS are sold by ZCarSource and others. Full reproduction harnesses are available but expensive.
Wire colors used in this circuit
| Wire | Color | Function |
|---|---|---|
| G | Green | Common ECU-controlled injector ground bus |
| G/B | Green / black | Bank A injectors (cylinders 1, 3, 5) |
| G/Y | Green / yellow | Bank B injectors (cylinders 2, 4, 6) |
| L | Blue | AFM signal output (vane potentiometer wiper) |
| L/W | Blue / white | Coolant temp sensor signal |
| W/B | White / black | EFI relay output — +12V to ECU and injector resistor pack |
| B | Black | Ground (chassis return — multiple body grounds) |
L-Jetronic in the FSM is sometimes called "EFI" and sometimes "Bosch fuel injection." The diagrams use Bosch terminal numbers (1, 7, 11, 14, 31, etc.) which can throw you off if you're used to wire-color-only diagrams. The terminal numbers are stamped on the AFM, ECU connector, and relay sockets — match them.
Diagnostic procedure
Cranks but won't start
- Confirm fuel pressure. Tee a gauge into the fuel rail. Key on, no crank — pressure should rise briefly to ~36 psi as the pump primes (a few seconds), then bleed off slowly. Cranking — should hold ~36 psi.
- EFI relay click. Listen at the relay when turning the key — it should click. No click = check coil voltage at the relay socket (12V on key-on).
- Fuel pump relay. If pump never primes during cranking, the issue is in the fuel pump relay or its trigger (the ECU pulls the trigger to ground based on AFM vane motion). You can jumper the relay to confirm the pump itself works.
- Injector pulse. Disconnect an injector connector and probe with a noid light. Cranking should produce a steady flash. No flash = ECU power, ECU ground, or rpm signal missing.
- Cold start valve. Pull the cold start connector and apply 12V briefly while observing fuel rail — the valve should spray. If it's plugged, no fuel goes into the plenum during cold cranking and the engine just won't catch.
Runs but stumbles / poor idle
- Vacuum leak. Old intake boots crack, especially the AFM-to-throttle-body boot. Spray carb cleaner around the intake; rpm change indicates a leak. Replace the boot.
- AFM idle wear. Probe the AFM signal wire (L) with the engine running. Idle voltage should be steady; if it bounces around or jumps suddenly when the throttle moves slightly, the wiper track is worn. Open and reposition (see below).
- Coolant temp sensor resistance. Two-pin sensor at thermostat housing. ~2500Ω cold, ~300Ω hot. Out of range = ECU thinks engine is colder/hotter than reality, mixture is wrong.
- Throttle switch idle contact. With ignition on, ohmmeter on TPS idle contacts — should show closed at idle, open as soon as throttle cracks. Adjust if needed.
AFM wiper repositioning (the "tooth bump")
The most common 280Z driveability fix in the community. Symptom: rough idle, stumble at part throttle, lean misfire on cruise, but full-throttle runs fine.
- Remove the AFM from the car. Mark the spring tension before disassembling.
- Remove the black plastic cover. Inside is a copper resistive track and a wiper arm on a spring-loaded hub.
- Inspect the track. You'll see a worn spot where the wiper rides at idle — usually a barely-visible scratch or shiny line.
- Lift the wiper hub off its post. The hub has internal teeth that engage the spring. Rotate the hub one tooth (advance the spring tension by one tooth) and replace.
- The wiper now rides on a fresh, unworn part of the track. Reassemble, reinstall, and verify smooth signal sweep with a multimeter.
Bruce Palmer's L-Jetronic guide (widely mirrored, originally hosted on Atlantic Z) is the canonical community reference for 280Z EFI diagnosis. Find a copy and keep it on the laptop in the garage.
Common failures
Cracked fuel pump relay solder joints
Symptom: hot-start failures (won't restart after a soak), intermittent dies. Pop the relay open, reflow all the solder joints, reassemble.
Brittle injector pigtails
Symptom: random misfire on one cylinder, sometimes weather-dependent. Wiggle the connector while running — if rpm changes, the pigtail is the issue. Cut and splice in a new pigtail.
Plugged cold start valve
Symptom: cold no-start, fine once warm. Pull the valve, confirm it sprays a fine cone when energized; clean or replace.
Bad ground at the EFI ground bus
The EFI harness has a multi-wire ground that bolts to the intake or thermostat housing. Corroded = floating reference, weird sensor readings, unstable idle. Pull the bolt, clean both surfaces, dielectric grease, reinstall.
Old fuel hoses leaking under pressure
Not strictly electrical, but worth noting because L-Jet runs ~36 psi (much higher than carb pressure). 50-year-old rubber hoses split. This is a fire risk; replace all fuel hoses with EFI-rated (SAE J30R9) hose during any EFI work.
Sources
- Nissan FSM — Engine Fuel (EF) chapter, 1975–1978 280Z
- Bruce Palmer — L-Jetronic guide (community-mirrored)
- atlanticz.ca — EFI section
- hybridz.org — for 280Z-related EFI rewires and conversions