I'm thinking of pulling the temp gauge out of the cluster and replacing it with a vdo 52mm jobbie.

If there's an air gap in the factory cluster circuit will that flip the ecu out?

I can't say for sure, but it shouldn't. The water Temp gauge runs off a seperate sensor to the ecu for Temp.

The ecu doesnt get anyfeed back from the dash cluster for anything i can think off(top of my head) that would cause it to "flip"

Can always unplug the sender to test, but I doubt it will cause a problem. Its the single wire plug for the gauge on the coolent neck

i'd say it wont have any effect, my gauge wasn't working for weeks in mine (b0rked sensor) when i bought it and it ran fine :) 

As Ace says the stock temp gauge has it's own separate sender in the coolant neck, and from the manual diagram it looks independent from the ECU sender circuit.
 
Do you intend to mount it behind the fascia like the OEM one?
One potential problem I can spot with mounting  in there is that although the VDO unit is close to the diameter of the OEM one, the face would sit back further in the fascia than the stock unit (maybe 10 mm or so) because of the glass on the front. Not sure how that would look. 
 

Reddtarga

Do you intend to mount it behind the fascia like the OEM one?
One potential problem I can spot with mounting  in there is that although the VDO unit is close to the diameter of the OEM one, the face would sit back further in the fascia than the stock unit (maybe 10 mm or so) because of the glass on the front. Not sure how that would look. 

 
 
Thanks for the replies. Good point RT, I hadn't considered the vdo being recessed further the factory one. I have a cluster I don't mind sacrificing but if works i'll be happy.
 
While I have your ears, can you foresee any problems using the n/a cluster in my turbo? (Omori gauge going in place of the voltmeter)

nope. turbo-NA only difference is volt or boost guage(or speedo adm/jdm), obv from the same revision.

^ yep agree. I installed the Omori boost gauge and TRD 300kph into an NA cluster in place of the turbo one on my old car.

After searching the globe without success for a mechanical TRD speedo I wear going to try for a cable driven ADM 260kph clock - no luck finding one of those either. May go for a brand new one. That or find a VDO speedo.