Starter Relay Guide: Wiring, Testing and Replacement

The starter relay is the small, inexpensive component that decides whether a generator starts. When the operator turns the key or the controller closes the start contact, the relay carries the current that energises the starter solenoid and pulls the starter pinion into mesh with the flywheel ring gear. If the relay fails, the engine does not crank, and on a standby generator a failed starter relay means the set cannot start during a power failure. Because the relay is cheap, it is often overlooked in maintenance, yet it is one of the most common causes of a no-crank complaint.

This guide covers the function of the starter relay, its position in the starting circuit, the terminal identification and the wiring, the internal contacts and the coil, the current ratings, the voltage drop testing, the no-crank diagnosis, the replacement procedure, and the selection of an equivalent relay when the original is not available.

The Starting Circuit and the Role of the Relay

A diesel generator starting circuit is a chain of components: the battery, the battery isolator or master switch, the start contact (the key switch or the controller output), the starter relay, the starter solenoid, and the starter motor. The relay exists because the start contact and the control wiring cannot carry the current required by the starter solenoid, which for a large diesel engine can be 20 to 50 amperes. The relay is a current amplifier: a small current through the coil closes heavy contacts that carry the larger current to the solenoid.

The starter solenoid performs a second function: it engages the drive pinion with the flywheel ring gear and then closes the main contacts that feed the starter motor, which then draws several hundred amperes. The relay therefore controls the solenoid, and the solenoid controls the motor. The starter motor guide covers the motor, the starter solenoid guide covers the solenoid, the flywheel ring gear guide covers the gear that the pinion engages, and the battery charger guide covers the battery that supplies the current.

Relay Construction and Terminals

The standard five-pin automotive relay has two coil terminals, a common contact, a normally open contact, and a normally closed contact. A four-pin relay omits the normally closed contact. The relay may be wired so that the coil is earthed through the case, which means the mounting must be clean and tight, or so that the coil has an independent earth wire. The contact circuit carries the solenoid current and must be wired with a conductor sized for that current, not with the thin gauge used for the coil circuit.

Most starter relays are of the simple electromagnetic type: a coil of fine wire around an iron core, an armature, a moving contact bar, and two heavy fixed contacts. When the coil is energised, the magnetic field pulls the armature and closes the contacts. A spring returns the contacts to the open position when the coil is de-energised. Some modern generators use a solid-state relay or a controller output module instead of the electromagnetic relay; these are more reliable but they must be matched to the load and protected against the inductive kick of the solenoid coil.

Relay Types and Current Ratings

The contact rating must exceed the solenoid inrush current, which is the current the solenoid draws at the moment the contacts close, before the magnetic circuit is complete. The inrush current can be three to five times the holding current, and a relay with a marginal rating will weld its contacts after a few operations. A relay with a suppression diode across the coil is preferred in a modern installation because the collapsing field of the coil generates a high voltage spike that can damage the controller output. The starter solenoid guide covers the load that the relay drives, and the fuel solenoid guide covers another continuous-duty circuit on the engine.

Starter Relay Failure Modes

The welded contact failure is the most serious because it can destroy the starter motor and the flywheel ring gear: if the contacts weld closed, the starter remains engaged after the engine starts and the pinion is driven by the ring gear at engine speed. The engine should be stopped immediately if the starter does not disengage. The high-resistance failure is the most insidious: the relay appears to work, because it clicks, but the voltage drop across the contacts reduces the current to the solenoid and the starter cranks slowly or not at all. This failure is often misdiagnosed as a weak battery or a faulty starter. The starter motor guide covers the motor damage, and the ring gear guide covers the gear damage.

Testing a Starter Relay

1. Check the battery state of charge and the battery connections first, because a discharged battery or a corroded terminal produces the same symptom as a failed relay.
2. Identify the relay terminals from the wiring diagram; do not assume a layout, because different relays have different pin arrangements.
3. Check the coil circuit: with the start contact closed, measure the voltage at the coil positive terminal. A reading close to the battery voltage indicates that the control side is working.
4. Check the coil resistance with the relay removed, and compare with the specification. An open circuit or a very low resistance indicates a failed coil.
5. Check the contact circuit: with the relay energised, measure the voltage drop between the battery positive and the solenoid terminal. A drop above about 0.5 volts indicates high contact resistance.
6. Listen for the click of the relay when the start contact is closed; no click indicates a coil or control circuit fault, and a click without cranking indicates a contact or downstream fault.
7. Bypass the relay with a fused jumper to confirm that the downstream circuit works; if the engine cranks with the bypass, the relay or its control circuit is the fault.
8. Replace the relay if the coil is open, the contacts are burned or welded, or the voltage drop is excessive.

The voltage drop test is the definitive test for a relay that clicks but does not pass current. A relay with a 1 volt drop across its contacts will not deliver enough current to the solenoid to engage the pinion and close the main starter contacts, and the result is a no-crank condition with a functioning relay. A relay should not be tested by holding it energised for a long period, because the coil is designed for intermittent duty in a starting application. The installation guide covers the general electrical practice, and the battery charger guide covers the battery condition.

No-Crank Diagnosis

The no-crank diagnosis should follow the circuit from the battery to the starter, checking each connection and each component in turn. The most common faults in practice are a discharged or failed battery, a corroded battery terminal, a faulty earth strap, a failed relay, a faulty start contact, and a faulty neutral or safety switch. On a generator with an automatic transfer switch and a remote start, the start signal comes from the controller, and the controller output and the interface relay must also be checked. The ATS transfer switch guide covers the control signal source, and the speed sensor guide covers the crank termination signal.

Replacing a Starter Relay

1. Disconnect the battery earth terminal before working on the starting circuit.
2. Note the terminal layout and the wire colours, and take a photograph before removing the relay.
3. Remove the relay from its socket or its mounting, and check the socket or the terminals for corrosion, heat damage, and looseness.
4. Check the new relay for the correct pin configuration, coil voltage, contact rating, and suppression diode. A relay with a different pin layout can be fitted physically but will not work correctly.
5. Fit the new relay, and tighten the mounting so that a case-earthed coil has a good earth.
6. Reconnect the battery, and test the starting circuit through several complete start cycles.
7. Check the voltage drop across the new relay contacts, and check that the contacts open when the start contact is released.
8. Record the relay type and the part number in the engine file for future reference.

A relay that is replaced should be examined to determine the cause of the failure. A relay with burned contacts usually indicates that the contact rating was too low for the solenoid current, that the solenoid is drawing excessive current, or that the contacts were pitted by arcing. A relay with a burned coil usually indicates that the coil was energised continuously, perhaps because the start contact was held closed or because the controller output failed. Replacing the relay without addressing the cause will result in a repeat failure. The starter solenoid guide covers the load side, and the after-sales guide covers the parts support available.

Selecting a Replacement Relay

When ordering, specify the engine model and the starting system voltage, the relay type and the pin configuration, the contact rating, and whether the original had a suppression diode. Order the relay socket and the terminal kit in the same order if the original socket is corroded. The sourcing guide explains the verification process, and the Cummins and Perkins cross-reference guide covers the interchange data.

Keeping the Starting System Reliable

The starting system is the part of a generator that must work first and works the least, which is exactly the combination that leads to failures. A standby set may sit for months and then be required to start within seconds. The practices that keep the starting system reliable are: keep the battery fully charged and test it regularly; keep the battery terminals, the earth straps, and the relay connections clean and tight; test the complete starting circuit, including the relay, under load at regular intervals; keep the connection between the relay and the solenoid protected from water and vibration; and, on a standby set, exercise the set under load periodically so that the whole starting system is proved.

A simple annual test that finds most starting faults is to crank the engine with the fuel shut off, or with the controller in a crank test mode, and measure the voltage at the battery, at the relay, and at the solenoid during cranking. A voltage drop of more than about 0.5 volts between any two points indicates a resistance that should be corrected. The maintenance schedule guide covers the intervals, the battery charger guide covers the charger that keeps the battery ready, and the glow plug guide covers the cold starting aid.

Battery, Cables and the Earth Return Path

A starter relay can only pass the current that the rest of the circuit can deliver. The battery, the cables, and the earth return path are all part of the starting circuit, and a fault in any of them produces a symptom that looks like a relay or a starter failure. The battery must be of the correct capacity and state of charge, the cables must be of the correct size for the cranking current, and the earth return must be as good as the positive path. A poor earth strap is one of the most common and most overlooked causes of a slow or a failed crank.

The voltage drop test is the way to find the fault. With the circuit energised and the starter cranking or attempting to crank, measure the voltage between the battery positive and the starter positive terminal, and between the battery negative and the starter case. Each reading above about 0.5 volts indicates a resistance that must be corrected. The test locates the fault without dismantling the circuit, and it distinguishes a cable or connection fault from a component fault.

The battery maintenance is also part of the starting system. A battery that is left discharged sulphates and loses capacity permanently, so a standby set must have a charger that keeps the battery at full charge, with the charge voltage and the temperature compensation set correctly. The battery should be load tested at the scheduled interval, and the terminals should be cleaned and protected against corrosion. The battery charger guide covers the charger, and the starter motor guide covers the component that the circuit drives.

Cold Starting, Block Heaters and Auxiliary Circuits

In a cold climate the starting system works against a higher load and a lower battery capacity. The engine oil is thicker, the cranking speed is lower, and the compression temperature is more difficult to reach, which means that the starting circuit has to deliver its maximum performance at the moment when the battery is weakest. The measures that improve cold starting are a larger battery, a block heater or a coolant heater that warms the engine, an oil pan heater, a fuel heater, and, on some engines, a glow plug or an intake air heater.

The starter relay is the component that distributes the control current to these circuits. On an engine with a glow plug system, the relay or a separate glow relay energises the glow plugs for a preheat period before the crank, and the controller must sequence the preheat and the crank correctly. The relay must be rated for the glow plug current, which can be significant, and the circuit must be protected by the correct fuse. A relay with an inadequate rating will fail after a few cold starts, which is a common fault on generators in cold regions.

The auxiliary circuits that support the starting system include the coolant heater, the oil heater, the battery heater, and the charger. Each of these has its own control and its own protection, and each can affect the starting performance. A systematic check of the whole starting system, including the battery, the cables, the earth, the relay, the solenoid, the heaters, and the charger, is the best way to ensure that the set will start when it is needed. The glow plug guide covers the cold start aid, the starter solenoid guide covers the solenoid, and the maintenance schedule guide covers the seasonal preparation.

Frequently Asked Questions