Swap an 898 c si for a helix 9. Plug and play or not?

Will the wiring harness and coax cable for my 898 c si work on a helix 9 upgrade? Will the helix 9 communicate with another 898 c si and a terrova

If it is a networkable helix 9 then yes it will connect to both granted you get the 5 port hub otherwise you can only connect 2 things together. The wiring will all work but check your power cord to see if it has a large oval thing on it right before it plugs into the FF. If not you might want to put the new cord there to help with possible interference. Might need an adapter for the ethernet cable to the back of the unit. The actual connecting harness if the same as the Helix 10 then it will be different and you'll have to use it but it is much better snap on no more screwing to connect.

It ain't that simple. WHICH HELIX 9 ???????????

The one with the same features as my 898c si. Side image down image, 2 d sonar

The same would be the first generation Helix 9 SI GPS (discontinued). The transducers are different. The Helix comes with the Compact SI transducer. Your 898 has the HDSI transducer.
Neither one has Down Imaging, they have a Down Imaging view that is created by blending the two SI pulses.
You can use the HDSI with the H9 and get shaper images due to longer transducer length.
The Helix has an internal GPS antenna, but can benefit from an external one placed at the transom if the display is at the console.
The power cords are the same--PC-11.

The current Helix 9 with Side Imaging is the Helix 9 CHIRP MEGA SI GPS G2N. It comes with the MEGA SI transducer that has a Down Imaging crystal in the transducer for true Down Imaging and has the extra Imaging frequency of 1200 kHz (1.2 MKz). It CHIRP's sonar and Imaging.