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GELM6000 IGV Difference Alarm
GE LM6000 IGV Difference Alarm while running. But both A and B sensor match perfectly in calibration.

GE LM6000 IGV Difference Alarm while running. But both A and B sensor match perfectly in calibration. Can run in manual in calibration mode and walk IGV up by 10% all the way to 100%. Both sensors match perfectly. Run engine and we have difference alarm. We've replaced one of the IGV regulators and no change. Our IGV bushings have a little movement but nothing crazy. We tried to adjust the null bias on the servo card during a calibration, no luck.

Any ideas?

I think it is important to determine if the difference is real or if the problem is an incorrect sensor reading. Under what conditions does the sensor difference occur? Is it only during a significant transient or is the engine operating relatively steady state? Is it more or less likely to occur at high power or is there no apparent relationship between the engine operating condition and the presence of the fault? What is the trend of the signals IGVA and IGVB when the fault occurs? Do the signals suddenly jump and that is when the error occurs or do the signals gradually separate? How do the secondary voltages of the IGV LVDT behave? Do they show a smooth response as the position changes or do they jump?

I would trend the IGVDMD, IGVA, IGVB, the LVDT secondary voltages, IGV torque motor current and typical engine parameters when the fault occurs. Erratic behavior of the IGV signals including the LVDT voltages would point toward electrical issues (Loose or bad connector?) If the error occurs during a transient and the signals gradually separate the difference is more likely real and more likely to be a mechanical issue. If it seems to occur at higher powers then vibration could be a contributor.