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Spartan supplied the gauge package with the bare chassis , so Spartan would be the best source .
If the gauge has a manufacturers name and P/N on it you may be able to Google a replacement at a lower cost.
But , before you fault the gauge , you need to know if Spartan is taking info for the gauge from the Allison internal temp sensor ; or if they added there own and if Spartan added their own sensor ; Where ? So you can check the wiring .
A faulty Allison temp senor should set a code that can be retrieved using the shift pad.