Many mechanics are cautious when it comes to dashboard work on 240s, often with good reason:
Abundant NLA plastic brittle and ready to go kaput as well as 240 spaghetti mess of 1960s era wiring stand-alone harnesses layered one over the other that could short out or be plugged in incorrectly as well as previous mechanics hacking/molesting heater fan housings or 'spontaneous-lightening' stereo-install (or similar) wiring under there 30+ years later doesn't help.
Difficult to estimate labor times, car down-times, no fun to be on the hook to replace some broken piece of unobtainum plastic that turns to **** in your hands or have the customer calling constantly for their car back or probability to cause harm/electrical fire with the best of intentions high.
Story of a 27+ year old car generally, but 240 plastic/HVAC boxes/electrical mess under the late model car dashboard doesn't help.
Inland SoCal out of the salt fog or AZ is nice for rust/as long as greased, heater fan motors seem to last well there.
But plastics maybe not so much, though garage & dash blankets help.
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Have a spare 1992-1993 245/wagon tire size cluster w/working temp gauge.
The halogen main-illumination bulbs on the 1989+ clusters are costly new, but probably good time to replace them with the cluster out...or convert to dimmable/durable L.E.D. if that's a good option/viable.
cleanflametrap.com has good pictorial on all the dash parts & 240 electronics generally.
Wouldn't buy parts until you can nail it down/diagnose more definitely, wouldn't authorize replacing the temp gauge sender on a whim either, would just get the car back for now?
You can probably install a generic VDO 52mm gauge (used from a VW or otherwise) beside the main cluster for water temp using the existing VDO sender I'd guess...or connect one for testing purposes/temporarily without risking much harm.
Easier to read/use the existing 240 water temp gauge in the combined instrument though...188F is about middle of the late model gauge even if it doesn't have #s on it.
Changing out an instrument cluster or even bypassing/removing the temp board or temp gauge is fairly straight forward for even the novice mechanic/ DIYer.
One little note/caveat on the SRS 240s (and basically every OBDI/early-late 90s airbag & newer car ever) & cluster R&R:
ALWAYS disconnect the battery - before removing the combined instrument cluster & verify the airbag warning lamp works before cluster reinstall, else you'll have a perpetually glowing SRS/airbag light that may take more in-depth electrical knowledge/tools to reset...
...for whatever good you think that 27-year-old airbag is going to do for you....maybe something...scared to find out/imagine the situation where that'd be relevant, but doesn't hurt to try to do no harm/put the car back together the way it was as general practice.
MUCH more that's electrical heaped onto the last couple years of 240 on a chassis that's still based on a '67 140.
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Wish you luck!
You have a very odd writing style. (like I'm one to talk).
*Think* I can get the gist of it, but it makes me a little cross-eyed with the block paragraphs, odd capitalizations & abbreviations.
English appears to be the 1st language from the look of it, but taught/acquired and implemented how mysterious.
Maybe there's some not-totally-creepy harmless/awkward credible & qualified TB nerd/dork that has spare under-dash parts on hand in a large box local to you that can per persuaded to come over with all that junk and diagnose the cluster with the car in your garage (best place for a car to be dead/have downtime).
Believe Lord_Athlon moved from Tucson, AZ to CO though.
Dash work is sort of conducive to qualified mobile mech/electronics/shade-tree guy coming to you more than taking the car somewhere; no toxic fluids to dispose of really, no special tools, no heavy lifting (engine cranes/large parts), just spare parts that don't take up much space on hand, some wire crimpers & DVOM & experience w/diagnostic ability & caution, as well as honesty/reputation to stick to the job without assuming liability to keep costs down (pay cash/car's on your property/your liability.)
Heavy-lifting/shop environment not necessarily required, strictly speaking.
Or just zoom-call/video chat/couple of quality how-to pictorials like cleanflametrap or quaified youtube videos (hard to sort the hokum from the expert) & DIY?
The look on many peoples faces when a 240 dashboard is removed in ~10 minutes late on a friday night w/reassurances to "come back" to finish the job is often pretty priceless.
"sure sure, trust me, it'll all go back in properly the way it was from the factory and get other things tested/addressed/inspected while it's apart with spare NLA parts on hand."
Articulating this or not doesn't seem to change the general mood/owner's facial expression much.
Poor 240 owning sods.
Condolences.
Late is the date for the 240, many jobs that once resembled repairs are more like restorations or mini-restorations now;
That was then, this is now.