Codes 1299 and 1285, CHT located location needed

E-Series Van E-350. Van will run fine at times but will act up randomly. When it does temperature gauge will peg out and oil temperature light will come on. The engine is not overheating. When I checked the live data a couple of days ago everything looked fine, but, it was not acting up. Today while it was acting up only thing on the live data was that the temperature was making radical change very fast (temperature reading changed anywhere from 30 to 60 degrees either direction in a second) One moment would read 258 next would read 290 then a second later read 230 the back up to 280 just within about 3 or 4 seconds. When I researched the codes found people have grounding issues which I did find a bad ground to which I fixed that and cleaned the rest of the grounds plus added a secondary engine to body ground plus another ground to tie the main body to engine ground into the battery to body ground. After seeing these reading I figure that the temperature sensor is the culprit and am finding that the ECT is no longer used and only a CHT is basically doing both jobs. Just need to know if this is the case and exactly where is the CHT located.