The SDS design is based on dual ECUs with dedicated sensors for each ECU. Consequently, loss of a single ECU or sensor does not prevent continued safe flight provided the pilot responds appropriately. This is as simple as switching to the “good ECU”. For 6 cylinder systems, each ECU had redundant sensors except TPS which is shared.
If you believe the programmer is displaying a spurious error message, you can clear the error pressing the + button on the programmer. If the error quickly reappears, it’s probably a legitimate sensor failure rather than something spurious.
Unfortunately sensors can and do fail. However, the SDS MAP values are set so that in the event of a sensor failure the system will continue to operate although perhaps not as smoothly as desired and advanced features will not be available.
Inflight Response to Sensor Failure
Typically 6 cylinder engines (i.e. IO-540) are run in dual ECU mode – the ECU switch is set to “Both”. The info below was written with this in mind.
ECU 1 controls CYLs 1,2 & 3 while ECU 2 controls ECU 4,5 & 6. If a sensor fails on either ECU, the engine will likely run rough as there can be an imbalance between injector fuel flows based on the “failure mode” settings for the ECU with the bad sensor. See table below.
My standard operating procedure for when the engine runs rough is to:
- If there is an engine failure, fly the plane ALWAYS!
- Turn on aux fuel pump
- Switch tanks
- If the problem persists switch to ECU 1 and if necessary try ECU 2
- If time permits, check to see if an engine circuit breaker has popped and attempt reset only if absolutely necessary
- After landing, repair / replace the defective sensor / correct the problem before further flight.
The SDS programmer display can provide info on sensor errors. If it is safe to do so, you can scroll through the display “gauge modes” to see if a sensor problem has been detected.
The engine will run on a single ECU – even if using the ECU with a failed sensor (except for the Hall Sensor – see below). However, it is advisable not to run in “both ECU mode” if a sensor fails as the cylinders controlled by ECU with a sensor failure may have significantly different fuel flows compared to the other ECU. This could cause a rough engine. Simply select one ECU or the other and manually adjust mixture as required.
Hall Sensor Failure
The Hall Sensors are critical for proper engine operation as these sensors control engine timing. If either Hall Sensor fails, the cylinders controlled by the failed sensor will cease to operate and the engine will become very rough. If this happens, throttle back (to reduce roughness) and switch to ECU 1. If the problem persists, immediately try ECU 2. As long as 1 of the 2 Hall Sensors is still operating, the engine will operate normally using the “good” Hall sensor .
More so than any other sensor, be sure that these sensors are terminated correctly, not subject to vibration and excess heat. A bad installation can cause significant issues.
Diagnostics – Non Critical Sensors
The EM5 ECUs will display a “check engine” warning if there is a sensor issue AND the programmer is set to display data for the ECU that has the bad sensor. If you suspect a bad sensor, check both ECUs on the programmer display and see if a “check engine” warning is displayed. You can see which sensor is faulty by scrolling through the “gauge modes” and seeing which sensor has an error message. The programmer will display ERR in the Gauge modes for the failed MAP, Air Temp, CHT, TPS or crank sensor (RPM).
The newer EM6 ECUs will display a “check engine” warning if a sensor on either ECU fails. You will still have to scroll the gauge modes on both ECUs to see which sensor on which ECU has failed.
As most sensor failures are due to wire fatigue rather than failure of the actual sensor, be sure the sensor leads are adequately supported. Careful wiring is essential.
The table below shows what happens when one of the various types of sensors fails.
| Sensor | Purpose | Gauge # | Failure Impact |
| TPS | Throttle Position Sensor: Used for acceleration enrichment | 1 | Non critical, disabled if does not align with MAP within a specified range. |
| MAP | Manifold Absolute Pressure: Adjusts mixture based on pressure in throttle body. | The ECU will pull the input high (to 30 inches) giving fueling for max power on one ECU. Would lead to rich running if not at sea level and wide open throttle. Switch to other ECU and it’s negated. | |
| CHT | Cylinder Head Temp: Measure CHT for ECUs. These sensors are separate from EFIS CHT. | CHT sensor failure open or shorted, ECU pulls high and fuels for warm engine (0 at both hot and cold end of scale) from defaults entered in map. This will not cause any change in running on a warm engine but would change mixture on a cold engine as CHT is used for starting and warmup enrichment. | |
| OAT | Outside Air Temperature measured neat throttle body | Air temp failure open or shorted- defaulted values at hot and cold ends to 70F. Will cause a slight mixture change if OAT isn’t 70F. | |
| RPM | RPM | RPM data is sent from the ECU to the EFIS. If this connection fails there will be no RPM shown on EFIs. However, this has no effect on the engine operation. RPM can still be displayed on the programming head . Also, RPM reading in SDS programmer will be replaced by ERR if crank sensor error on ECU with failed sensor. If tach output from each SDS ECU goes to twin EFIS tach inputs, you will still have RPM available on EFIS from other crank sensor. |