Can't arm rear motor on vectored yaw tricopter - SOLVED

I’m using a Pixhawk 2.4.8 with Arduplane V3.9.6, and can’t seem to get the rear motor going. I’ve got it setup on SERVO7_FUNCTION,35. But in MP under the Servo Output tab, I only get PWM showing on positions 1-6, nothing on 7. Motor 3 shows as a function for position #7.

Once while connecting to MP I got the following message:
QuadPlane initialised
MotorsTri: unable to setup yaw channel
Motors: unable to setup motor 3

Not sure what to try next. I can arm with MP, and can get the front motors going, as there is a PWM going to outputs 5 & 6.

Parameter file:

Here’s a pic of my test bed to try out the hardware and firmware. I’m hoping it could fly in QSTAB mode.


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the rear motor is motor 4, so it would be function 36

That’s it, thanks Peter! As soon as I did that and moved the ESC to RC out8 all 3 motors started up. Now on to put on the props on for a quick hop flight test.