Relay

Relays in the central distribution panel switch power circuits using a low control current. Depending on the electrical architecture, they control the power supply to lighting, fans, starter enable circuits and other loads. Intermittent failures, chattering contacts or a load that remains switched on call for an inspection of the switching element as well as the relay socket, power supply circuit and control circuit.

Always identify relays using the vehicle's chassis number, wiring diagram and original part number. Visual comparison or selection based solely on the model name is never sufficient. Trucks and buses commonly use 24 V systems; vans such as MAN TGE and Mercedes Sprinter generally use 12 V. The coil voltage must match the specified version.

In cab-mounted distribution panels, the version and equipment level can affect the relay layout. For example, take account of MAN TGX cabs XL, XLX and XXL versus TG3 TGX cabs GN, GM and GX, or Mercedes Actros Lang StreamSpace and Lang GigaSpace. Always use the vehicle-specific relay layout diagram; an identical cab shape does not guarantee identical circuitry.

Distinguish between normally open relays, changeover relays, time-delay relays and electronic versions. On common electromechanical types, 85 and 86 are the coil terminals, 30 is the common contact, 87 is the normally open contact and 87a is the normally closed contact. Two terminals numbered 87 are not equivalent to a changeover contact arrangement with 87a. Also check blade width, socket fit, locking mechanism, temperature range and sealing.

• Normally open relays: close a circuit when the coil is energised, for example to supply power to lighting or a fan.
• Normally closed relays: open a circuit that is closed at rest when the coil is energised, switching off a load or function.
• Changeover relays: transfer the common contact from the normally closed contact to the normally open contact when energised, allowing switching between two current paths.
• Time-delay relays: switch a circuit after a set delay or for a specified period, for example for a run-on function.
• Flasher relays: switch the direction indicators and hazard warning lights on and off periodically, unless this function is integrated into an electronic control unit.
• Electronic relays: use electronics to control or switch a load. Depending on the version, they also provide timing, monitoring or protection functions; fully electronic versions switch without mechanical contacts.

Select the contact rating based on both continuous current and inrush current. A fan motor places different demands on the contacts than a purely resistive load. Contact materials, such as silver tin oxide, and the design help determine resistance to contact erosion and welding. Built-in suppression resistors and flyback diodes are not directly interchangeable: versions with a diode require the correct polarity according to the diagram printed on the relay.

Burnt contacts cause voltage drop and heat build-up. Welded contacts can keep a load permanently powered and drain the battery. Chattering often indicates insufficient coil voltage, a poor earth connection or an unstable control signal. Corrosion and reduced terminal contact pressure in the relay socket often cause the same symptoms as an internal fault.

1. Read any available fault codes and check the switching conditions. A missing control signal may be an intentional inhibit from the ECU (electronic control unit), not necessarily a relay fault.
2. Use a multimeter to measure the supply voltage at terminal 30 and the voltage between 85 and 86 during the switching command. For brief interruptions, use an oscilloscope to reveal supply voltage dips or a pulsed control signal.
3. With the load switched on, measure the voltage drop across the closed contact and separately across the power supply and earth connections. Compare the results with the manufacturer's limits. As an illustration: a 0.5 V drop at 20 A generates 10 W of heat locally.
4. Measure the load current with a DC clamp meter and check for overloading or a stalled electric motor. Only measure coil resistance with the power off and the relay disconnected; compare the value with the component specifications.

Isolate the circuit according to the vehicle procedure before removing relays. Do not bridge contacts at random: this can bypass protection devices or damage electronic outputs. An audible click does not prove that the power contacts are conducting correctly.

Replace discoloured or deformed relay sockets and damaged connection terminals using the specified repair method. Simply fitting new relays into heat-damaged contacts will lead to high contact resistance again. Rectify moisture ingress, cable damage and overloading before installation. Afterwards, check several switching cycles under operating load, including voltage drop and temperature rise. Braem offers technical support with removal and installation to help carry out these repairs effectively.