Ok, so I just got my T coils to work (there was a wrong resistor on the board) and for some reason the tach jumps around a lot, at idle (1100RPM) it shows 550rpm (half) then it jumps up to nearly accurate but is very weird and jerky.
And this is not a CAS issue, it does not jerk on MT and I have dataloged it many times. This had to be some kind of T coil issue, not sure though.
If a video would help, I could get one up.
Thanks

 

Most likely, the wiring for your trailing coils is a little flaky. I've seen things like this when eiither the IGt-T or IGs-T wire are having a problem. One of them connected correctly and one of them flakey.

I've also seen this when the trailing ignitor is bad.

Ken

 

if you have another tach you can test with or want to mount inside the car use the yellow/blue wire on the trailing coil plug. It could be bad wiring in the car/cluster. Or in my case got all of the coil pack trigger wires backwards.... that will have the same affect....

 

when the wires were backwards, did the T coil still fire?