Step by Step Guide to Troubleshoot and Repair a VGT (2024)

What is a VGT?

Variable-geometry Turbochargers or VGTs are a type of turbo charger designed to allow the optimum amount of air flow to change depending on engine conditions. The optimum air flow or aspect ratio changes depending on engine RPM and engine load conditions. The aspect ratio at low speeds is very different than the aspect ratio at higher speeds. Most variable-geometry turbochargers are now equipped with electronically controlled actuators, that use a signal from the ECM to move either vanes or a sliding sleeve in the turbocharger, which increases or decreases the exhaust gasses driving the turbine wheel which in turn increases or decreases the air flow from the turbocharger.

What Causes Failure?

VGT actuators while very effective for creating optimal power under different engine conditions can also cause issues in the event of a failure. These actuators may be electronically controlled, but they actuate the turbo mechanically, meaning they are subject to mechanical failures, as well as electronic failures. This means these actuators are not only affected by wiring issues, but good old fashion wear and tear. Issues with the turbocharger vanes can also cause actuator failure. For Instance, if the turbo vanes are seized and will not move the actuator, trying to rotate to move the vanes open or closed may put a bind on the actuator gear and cause the motor that operates the gear to overheat and fail. It is very rare that you have a VGT Actuator gear break, but it is in the realm of possibility.

Along with mechanical failures VGT actuators are subject to electronic failures as well. It is important to note that most electronically controlled VGT actuators are considered “smart devices” which means that they send messages back and forth between the ECM. These “smart devices” also have the ability to perform diagnostics on themselves, meaning if for example the turbo vanes were seized and the gear could not reach the desired position, this information is relayed to the ECM and in most cases a fault code is set and an warning light is activated to alert the operator that there is an issue. A typical VGT actuator circuit will have a 12-volt power supply wire, a return or ground wire, and 2 CAN bus or datalink wires. A short to ground on the 12-volt power supply wire has the potential to short the actuator out causing a failure. If there are any issues with the CAN wires the ECM will not be able to send/receive messages to the actuator thus the ECM will not be able to correctly control the position of the actuator and in turn can cause performance issues.

Why is it Calibrated?

It is important to note that most electronically controlled VGT actuators will need to be calibrated once removed or replaced. The reason behind the calibration of these actuators is simply for the actuator to learn the range of movement that opens and closes the vanes or sliding sleeve of the turbocharger, so that it can move the vanes to the correct position when the ECM commands. Without calibration it would be impossible for the ECM and VGT actuator to position the vanes to optimize air flow to the engine and would cause performance issues and possibly premature failure of other components. For example, there is a potential to create too much boost a low engine speeds and too little at high engine speeds. This will cause high exhaust manifold pressures and affect the air-fuel ratio ultimately causing low power issues.

Troubleshooting a VGT

Proper troubleshooting of turbo components is an area that cannot be talked about enough. When dealing with Turbo and Actuator failures you are looking at several thousand dollars for the cost to repair. Replacing an actuator is much cheaper than buying a turbo meaning it is very important to separate the 2 components to determine failure. During any troubleshooting of a VGT actuator you should always verify mechanical turbo parts are functioning properly as well.

In the video below, we used a Diesel Laptops Cojali Commercial Vehicles Kit and determined we had a failed actuator on a 2014 Cummins ISX CM2350 Engine. We first found a fault code of 1894 that is a VGT Actuator Circuit Abnormal Update Rate fault. This fault is set when the ECM does not detect any messages from the VGT Actuator for 1 second. At this point the Check engine, or warning light will be illuminated, and the ECM will no longer attempt to Actuate the VGT and will also no longer perform EGR Valve actuations. Eventually the ECM will reduce power if the fault is active for an extend period.

There are a several things that can cause this fault code to come active. The VGT Actuator not receiving power from the ECM, or an open circuit in the datalink between the ECM and VGT Actuator are just a few. The only way to find out is to begin the troubleshooting procedure:

1. The first step is to determine if there are any associated or related fault codes that could be causing the issue. In this case there were not.
2. The next thing to verify is battery voltage. Ideally, we would like to see 12.6-volts, but anything over 12-volts will be enough to consider our voltage check a pass and move on. Using our multimeter, we found 12.56-volts on the batteries indicating we had the correct voltage.
3. Once we have determined we have proper voltage the next step is to verify voltage at the VGT Actuator. Using our multimeter, we checked voltage between our power supply pin or pin 1, and our return pin or pin 2 of the VGT actuator connector and found 12.53 volts. When checking for voltage at the component we are looking for a reading within 1 volt of battery voltage, this means we were well within our specification.
4. Our next step is to determine any failures in the datalink circuits. In this case we should have had a few associated faults if there was an issue with the circuit, and we would need to correct those issues and recheck for our fault.
5. Our last step before determining a failure is to verify terminating resistance of the VGT actuator. Using our multimeter, we checked the resistance between pin 3 and 4 of the VGT Actuator and found an OL reading meaning an open. The resistance spec on the terminating resistance of the VGT Actuator is between 108 and 132 Ohms. With a reading of OL we know we are out of spec and that the VGT Actuator has failed. After determining the VGT actuator failure we would need to replace and calibrate the new VGT actuator.

In the video above we calibrated our original actuator, but the procedure is the same when using a new actuator. We were able to complete this diagnosis and repair using our Diesel Laptops Cojali Commercial Vehicles Kit

Filed in: Training