I have a JD 4600 with same temp gauge problem. My fuel gauge went wacko years ago. Also had problem with thermistor. JD electronics suck Did you ever find a quick easy and cheap fix? I'm leaning toward an aftermarket gauge
/ JD 4600 temperature goes in the red within a minute or so running from cold.
The only thing you can do is to bolt on a cheap automotive temp gauge somewhere out of the way but still in view.
And just pull the wire from behind the dash and use that to wire up the new temp gauge.
Cheers,
Frits.