Grand Rapids Engine Information System and CHT hookup

I have the Grand Rapids EIS. It came with wiring for the CHT but included in the package were two CHT probes that go under the spark plugs ala' ContinentalLycoming style. I'm using a Rotax 912 that already has probes in the #2 & #3 cylinders. I am not sure which wire would attach to the probe and then what to do with the remaining wire. I've read everything I can find from Grand Rapids regarding this and can't find anything that explains how to use the Rotax probes and I'd really rather not use the "under the spark plug" sensors. I wrote to GR and their explanation was so far off topic that it made no sense to me. I tried to call but they are in Michigan and are not at work. I've included a picture here (not sure why it turned sideways when I inserted it):
Attachment 24709
I'm looking for someone that has a GR-EIS and has successfully accomplished my quest.
Thanks in advance,
Fred

The Rotax CHT probes are resistive temperature sensors like a water temperature probe on a car. The resistance of these type probes varies with temperature. So in this sense they look like an oil pressure sender, or coolant temperature sender to the engine monitor. Since the heads are water cooled this isn't really a CHT and is really more measuring water temperature than anything else. The only time it is really a CHT in my opinion is if you lose coolant and

The typical CHT probes are K type thermocouples. These work by creating a small voltage that varies with the temperature. The Rotax probes produce absolutely no voltage.

Bottom line, the thermocouple and the Rotax CHT probe work totally different.

GRT has an addendum for 912/914 installation that covers their recommended CHT installation. http://grtavionics.com/media/912-M20...0-Addendum.pdf

In it they recommend using a thermocouple style probe, but not under the spark plug. They say to replace the Rotax probe and install the thermocouple in that location with a 10mm bolt.

If you really want to use the Rotax Probes, they won't work with the CHT inputs on the EIS unless the inputs can be reconfigured. You would have to use a couple of the unused aux inputs to use the Rotax probes. They would wire just like the diagram in the manual for the coolant sensor since that is essentially what they are.

Happened to see this also:

http://grtavionics.com/media/Fluid-Temp-Aux.pdf

Maverick - If your EIS came configured as mine did, you'll have to send it in to GRT for a modification - then hook it up per the drawing John provided. Call GRT (again), if they don't answer, leave a VM. They've always been really good at returning calls, even if working from home. They'll need your serial number to see if the re-configuration is needed for your unit. They did the mod with a quick turn around and no charge for me. BTW - They supplied the resistor; if memory serves, it was with the original shipment.

Eric Barlett of GR said, "there is a dedicated coolant temp sensor input on the EIS you may connect one of the sensors to. Connect the other one to an auxiliary input." Since the documentation and the feedback from so many others with long term experience with Rotax, the water temp and CHT are so similar that most don't monitor one or the other so, I'll monitor this way. Thanks all.

This issue came up a few years back for me also. I welded a bung on the spider tank.....tapped it...then installed a coolant probe for coolant temps. I used the cylinder head temp probe to hold the sensor for cylinder head temp and now I have both. BTW...the cylinder head temp and coolant temps are always about the same. My motto...trust but verify.:cool:

Also, the spider tank on the 912 is very thin....be carful welding it. It will also need to be grounded since it just sits there....only attached to the coolant lines.