Erratic oil temp readings

Hi Roger,

Firstly, since the erratic oil temp readings do not affect the CHT readings, the ground connection between the engine and the EMS can be ruled out as a cause. The grounds for the oil temp and the CHT sensors are shared and any significant fault here would show up on both measurements.

Next, I would try swapping the oil temp and CHT connections at the engine to rule out a faulty sender. As you've already done this, we can move on to isolating the fault to either the airframe wiring or the D120.

Since Dynon says that your D120 is serviceable, we can assume that they know what they're talking about and eliminate it as the cause for now. (This is worth revisiting if no fault can be found with the wiring)

Excluding EMI, (very tricky to pin down, but also highly unlikely) the only plausible ways that the airframe wiring could cause an erratic oil temp reading on the D120 are:

1. An intermittent or partial short of the signal wire to ground. This would cause momentary spikes in the temperature reading, although the relative magnitude of the spikes should reduce at higher oil temperatures.
2. Intermittently high resistance (or a partial break) in the signal wire. This would cause momentary drops in the temperature reading, though the effect would be less pronounced at lower oil temperatures.
3. An intermittent or partial short of the signal wire to some power source. This would probably also cause momentary dips in the displayed temperature, however if such voltage were lower than the D120's internal reference voltage the results would be more unpredictable.

I should add that more than one of these conditions could exist together at the same time, although any wiring which was that badly compromised would likely have become apparent when you replaced it.

Have you tried removing pin 7 (actually a female contact) from the Dynon's D37 connector and temporarily connecting an analogue temperature gauge to the removed contact? This could help by bypassing the D120's signal filtering and sampling rate limitations to give you an idea of whether the erratic readings are fluctuations upwards or downwards from a plausible base value.

While pin (contact) 7 is disconnected from the D120 (and anything else), it may also be worth checking the voltage at the contact. As the oil temperature sensor is electrically just a resistor to ground, any voltage present between (disconnected) pin 7 and ground indicates a fault somewhere in the signal wire.