MGL Avionics User Forum

MGL Guys,
On my VW engine I have Swift sensors and probes. Oil Press is 0-100 PSI 0.5 - 4.5V. Oil Temp is 1/8" NBT @ 1/4" tip. EGT is K type clamp @ 1/2" tip. CHT is 14mm J type. What selection should I make in the engine setup menu to get real temperatures and pressures to show on the Challenger iEFIS ?
As it is, with a cold engine, (I haven't started the engine yet as the EFIS is not functioning) I get about 10 PSI of oil press where I should be getting zero, 32* F oil temp where it should read 70*F, the OAT (Yes it's that warm now in California), and the EGTs and CHTs are reading 135* and 139* respectively when they should be ambient air temp. I did the CHT probe in a hot mug of water and that one cylinder showed an almost 200* increase in temp.
This is all on page 1. I'd really like to get this EFIS squared away so I can start my engine.
P-40 Pete

I have similar concerns with my Jabiru install.

Guys,
After reading other related posts here I found I had other ground wires going to the same engine stud ground as the RDAC-XG. I removed all extraneous ground wires and now the CHTs and EGTs read ambient air temp. Still working on oil temp and press reading high.
P-40 Pete

Guys,
While researching oil temp probes I found this tidbit of information on the Aircraft Spruce site for the MGL-Westach oil temp sender. It read, "Note that EFIS will read 32 deg F (0 deg C) until you reach the operating temp of 70 deg F". Since I haven't started the engine yet I presume my Swift oil temp and my EFIS are acting similarly, i.e. the oil temp reads 23 deg F and not the ambient air/ engine block temp. I now wonder if my oil press reading of about 10 psi is because the engine isn't running.
P-40 Pete

NTC resistive probes often used for automotive oil and coolant temperature measurements are not designed or intended to read correctly at ambient temperatures. They may - but that is luck. They tend to be qualified only at a single temperature - for oil this is usually at 120 degrees C, 100 degrees C for water. In addition the error band tends to be specified at +/-5% at that temperature. That's not very accurate.
This is the primary reason we don't bother displaying anything at ambient or below at least a "warmish" kind of temperature. Displaying something that is patently wrong is worse than displaying nothing...

These probes tend to get useful around say 50 or 60 degrees C - but should always be taken with a pinch of salt. If the EFIS reads 100 degrees, given the error band - the probe could be at 95 or 105 degrees. That does not include any tolerances the RDAC measurements will have (these tend to be smaller and are not normally of concern).

Bad grounding methods of the RDAC are the usual culprit when grounded probes are not reading correct. In order to measure the probes resistance the RDAC sends a current through the probe - this causes a voltage to drop on the probe. The current needs to be small to avoid the probe from heating by the current itself. That means at operating temperature the voltage we need to measure at the probe itself can be as little as 0.1V. Let's say that would be equal to 100 degrees. If we measure 0.11V or 0.9V we will be 10 degrees off (not actually since the curve is not linear but round about that).
So that is just 0.01V contributing to the error.

Now 0.01V is very easy to drop on a ground cable. It's a resistor, even if it hopefully has a low resistance. So lets assume the RDAC is grounded to some ground supply point in your avionics panel and the engine is, as usual, grounded via a fat cable going to the battery negative.
Your avionics ground point in turn connects to the battery negative usually via a longer and thinner cable. Your combined avionics current flowing could be 2A to 5A or more depending what you have connected. Much more if you have things like landing lights etc connected here as well.
Possibly even your ground run from your alternator or regulator/rectifier goes here. That can contribute 20A or more if your battery needs charging.
So - it's easy to see given V=R*I that we can get several tens of millivolt drop on our ground cable very easily. Polarity depends on the direction of the current.
So now we have the situation that the RDAC sees the voltage drop of the actual probe PLUS the voltage drop on your ground cable. Obviously this can result in a very wrong reading. Resistive oil pressure senders will also give a wrong reading (and can be quite a lot - that caught me out once as well so you are in good company).

Now if the ONLY ground connected to the RDAC is a direct run to the engine block - we would only have the RDAC supply current on that wire. That's pretty small so we can usually ignore it. In this case the RDAC sees the true voltage on the probes.

Rainier,
Thank you so much for the explanation of what is going on. The content of your reply should be in the RDAC manual. This will save a lot of people a lot of time and frustration.
P-40 Pete