SU Electric Fuel Pumps

SU Electric Fuel Pumps

SU electric fuel pumps are one of the most distinctive pieces of equipment on classic British cars. The characteristic ticking sound of an SU pump priming up before starting is immediately recognisable to any enthusiast. Fitted as standard equipment on a vast range of British cars from the 1930s through to the 1980s, SU electric pumps are reliable, serviceable, and still fully supported with replacement parts.

How SU Electric Fuel Pumps Work

SU electric pumps are electromagnetic diaphragm pumps. A solenoid coil attracts an armature, which is connected to a diaphragm. When energised, the solenoid pulls the armature and diaphragm backwards, creating a low-pressure zone that draws fuel from the tank through an inlet valve. When the magnetic force overcomes a set-point, a pair of contact points open, the solenoid de-energises, and a return spring pushes the diaphragm forward — expelling fuel through an outlet valve to the carburetter.

As the diaphragm moves forward, the contacts close again, re-energising the solenoid, and the cycle repeats. The characteristic tick is the sound of the contact points opening. As the carburetter float chamber fills and reduces demand, the pump slows its ticking rate and eventually stops — resuming only when fuel is consumed. This self-regulating behaviour is a key feature of the design.

Single and Dual Type SU Electric Pumps

Single Type pumps have a single set of contact points and a single outlet. They are suitable for most single-carburetter applications and produce flow rates appropriate for engine outputs up to approximately 80–100 bhp.

Dual Type pumps have two sets of contact points and two outlets, arranged so that one diaphragm is on the intake stroke while the other is on the expulsion stroke. This gives smoother, more continuous flow and is better suited to high-performance applications, twin or triple carburetter setups, and larger-displacement engines.

Pump models are identified by the AUA (single) or AZX/AUF (dual) prefix. Common examples:

  • AUA 25: Low-pressure single type, suitable for most British classics up to the late 1950s
  • AZX 1307: Standard single type, one of the most widely used SU pump models
  • AUF 700 / AUF 800: Dual-type high-flow pumps used on high-performance and multi-carburetter applications

SU Electric Pump Routine Servicing

SU electric pumps are extremely long-lived but benefit from occasional attention. Key service items:

  • Contact points: The points are the most frequently worn component. Inspect for pitting, burning, or oxidation. Clean with fine abrasive paper or replace. Severely worn points cause erratic or slow ticking and inconsistent fuel delivery.
  • Diaphragm: Inspect for hardening, cracking, or perforations. A failed diaphragm results in complete pump failure or very low output. Replace as part of any full service.
  • Filter: Most SU pumps have a sediment bowl or filter at the inlet. Clean or replace annually.
  • Valves: The inlet and outlet valves are small discs of synthetic rubber or neoprene. Inspect for deformation or contamination. Replace if there is any sign of damage or if the pump is losing prime.

Dismantling an SU Electric Fuel Pump

  1. Disconnect the pump from the electrical supply and fuel lines. Plug the fuel lines to prevent spillage.
  2. Remove the end cover to access the contact points. Note the orientation of the rocker mechanism before removing any components.
  3. Inspect and clean the contact points. If replacement is needed, note the exact position and orientation of all components for reassembly.
  4. Remove the pump body screws and separate the two halves. The diaphragm is clamped between them — note which way round it is fitted.
  5. Remove the diaphragm, inspecting both sides for damage.
  6. Remove the inlet and outlet valves for inspection. These are small and easily lost — work over a clean tray.

SU Electric Pump Inspection

With the pump dismantled, check:

  • Diaphragm — hold up to light and look for pinholes. Check for hardening or brittleness.
  • Contact points — look for deep pitting or burning. Minor oxidation can be cleaned; significant wear requires replacement.
  • Valve discs — check they are flat and undistorted. They must seat completely flat to seal correctly.
  • Body — check for cracks, particularly around the valve ports.
  • Filter/sediment bowl — clean thoroughly and check the bowl is undamaged.

SU Electric Pump Reassembly

  1. Fit new valve discs if required. Ensure they sit flat in their recesses.
  2. Fit the new or serviceable diaphragm in the correct orientation.
  3. Reassemble the two halves and tighten the body screws evenly in a crossing pattern.
  4. Refit the contact point assembly in the correct orientation. Set the point gap if applicable — check the specification for your pump type.
  5. Refit the end cover.
  6. Before refitting to the vehicle, test the pump by connecting it temporarily to a supply and placing the inlet in a small container of fuel. It should tick steadily and produce a firm flow from the outlet.

Electrical Connections

SU electric pumps run on 12V DC (or 6V on pre-war vehicles). Correct wiring is important:

  • The pump should be wired through the ignition switch so it only runs when the ignition is on. This is a safety requirement — an unguarded pump that runs when the ignition is off will continue to pump fuel in the event of a crash.
  • Earth the pump body securely. Poor earthing causes slow ticking, low output, and rapid point wear.
  • Ensure the supply wire is adequately fused — typically 5 amps for a single-type pump.
  • Check the wiring connectors are tight and corrosion-free. High resistance in the supply circuit is a common cause of underperformance on older vehicles.

SU Electric Pump Fault Diagnosis

Pump doesn't tick at all: Check power supply to the pump (should be 12V with ignition on). Check earth connection. If power is present but the pump doesn't tick, suspect seized contact points or a failed coil.

Pump ticks but doesn't pump fuel: Check for a blocked filter. Check that both valves are seating correctly. A diaphragm with a large perforation can also produce ticking without fuel delivery.

Pump ticks very slowly: Low voltage supply (check wiring and earth connections), worn contact points with high resistance, or a stiff diaphragm.

Pump ticks rapidly and continuously even when engine is not running: Fuel is leaking from the system — check all fuel lines, carburetter float valve, and connections. A pump that ticks non-stop is trying to maintain pressure against a leak somewhere.

Intermittent fuel delivery / cutting out under load: The filter is partially blocked, the diaphragm is hardening, or a valve is partially seating. A full service usually resolves this.

SU Electric Pump Replacement Parts

We stock a comprehensive range of SU electric pump service parts including diaphragm kits, contact point sets, valve discs, sediment bowls, and filter elements. Full service kits are available for most common AUA, AZX, and AUF pump types. Complete replacement pumps are also available for most applications.