WL106629 Compact Diesel SCR Solenoid for DEF Control

Advantages of SCR Solenoid

A SCR Solenoid provides a significant reduction of NOx emissions. These components have fast switching time and highly reliable performance. The current solenoids for SCR have a reduced size and energy efficiency which makes them suitable for both automotive and industrial usage.

High quality solenoids are durable due to their ability to resist various factors. Such solenoids can operate in extreme conditions for a considerable period of time. The other significant advantage of solenoids for SCR is that these devices help diesel engines to meet requirements of various regulations and environmental standards. Thus, solenoids for SCR improve fuel economy and prevent environment from pollution.

Cons of SCR Solenoid

One of the main disadvantages of the current SCR solenoid is their vulnerability to low quality DEF fluid and absence of routine maintenance. Excessive contamination, excessive heat, wrong voltage setting shorten their service life.

DEF liquid can crystalize under freezing temperature, which negatively influences the overall performance of the SCR system and solenoid itself. One of the main disadvantages is complicated electronics that is often required to control the operation of these devices. Finally, SCR solenoid may be costlier to manufacture and replace than basic solenoids.

General Troubleshooting Process

SCR solenoid problems typically include two types: inability to start and stop the solenoid. Start troubleshooting broadly, and then move on to details of components.

Step 1: Safety First & Visual Inspection

Power Down: Always isolate and turn off power supply to the system.

Visual inspection: Inspect SCR, solenoid, and the circuit board. Look for any burn marks, damaged wires or connectors. Pay attention to any signs of overheating.

Step 2 Basic Electrical Checks

Fuse check: Check if the circuit breaker is tripped. In this case, check the continuity of the circuit breaker and, if necessary, replace it with a new one.

Wire & Connectors Inspection: Look for damaged, burned out or loosely connected wires and connectors. Be especially attentive to the connectors as a single loosely placed pin causes an open circuit.

Resistance check: Use the multimeter to test for the resistance between the solenoid coil pins. Compare the result with the manufacturer’s specifications. If the reading shows an infinite resistance, it indicates a fault in the solenoid.

Step 3: Isolate the Error

Control Signal check: Inspect whether the control board sends the proper triggering voltage to the gate of the SCR.

SCR bypass (use with caution): This approach may be helpful to determine if the SCR or solenoid fails. Only bypass the SCR if you are aware of the circuit structure and take safety measures. If a test with bypass shows that the solenoid functions normally, the problem is the SCR or its trigger circuit.

⚠️ Typical SCR-Solenoid Failure Modes

With standard relays you don’t run into such failure modes that might occur in SCR solenoids.

Failure Mode Typical Symptoms Possible Root Cause

Solenoid Won’t De-energize (Stays On) The solenoid is energized, although the control signal is absent. The problem is often caused by leakage currents flowing through the SCR.

Solenoid Will Not Energize (Remains Off) The solenoid does not turn on. This is often a sign of an open circuit or lack of triggering signal.

Solenoid Intermittently Turns On & Off (Chattering) The solenoid chatters. The reason is “holding current” or “dI/dt” of SCR.

🔎 Detailed Diagnosis of each failure mode

Here’s how to troubleshoot a particular failure mode in SCR solenoid circuits.

1. Solenoid Does Not De-energize (Stays On)

In this case, the solenoid continues to stay on although the control signal is turned off. Usually, it means that the SCR doesn’t turn off. The common reason of the phenomenon is leakage current of internal “snubber” circuitry of the SCR.

Diagnosis: SCR solenoid has the snubber circuitry consisting of resistor and capacitor which protects the SCR from voltage spikes. At the same time, the snubber allows current (e.g. 4 mA) to pass even if the SCR turns off. Leakage currents may be enough to keep a sensitive and low power solenoid on.

Solution: Adding the ‘bleeding’ resistor to the circuit. In such case the current will pass through the bleeder resistor rather than through the coil, allowing the solenoid to turn off.

2. Solenoid Won’t Turn On (Remains Off)

This case is quite typical and often represents simple failure of one of the elements.

Diagnosis: Inspect the power path. You need to check the solenoid, wiring, fuse and signal of the trigger circuit to identify a place with a break.

Solution: Repair or replacement of a failing part (a fuse, broken wiring, faulty SCR).

3. Chattering or Intermittent solenoid operation

This is the most difficult issue related to the “holding current” requirement of the SCR.

Diagnosis: This problem occurs because of newer energy-saving solenoids that have less current than old ones. In this case, when the solenoid armature reaches its position of final stroke, the current drawn becomes too low to maintain latching state of SCR.

Solution: Install a resistor in parallel with SCR. Constant minimal current drawn prevents SCR from unlocking even if the current drops momentarily.

💡 A note on terminology

Watch out searching for a solution on the internet as the term “SCR” has two meanings when it comes to vehicle repairs.

Silicon Controlled Rectifier (SCR as used in this guide)

Selective Catalytic Reduction (part of diesel exhaust systems) For example, a Perkins engine diagnostic may mention “SCR Relay”, which controls coolant diverter valve. It’s talking about diesel exhaust system, not the silicon switch. Use full search phrases, “silicon controlled rectifier” or “SCR solenoid”.