Yep I know where you are at. I have the 3126 on a 1997 Freightliner. I had to install an after market engine temp gauge and I did not have a reading to my transmission so I went ahead a installed a transmission temperature gauge also. You do not want to know the cost of fixing the OEM gauges.
Is your after market temp sensor installed in the bottom port of the thermostat or the top port?
Do you have a 3126B or 3126? I understood that the 3126B was built after 2000?
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1997 Triple E Empress, Freightliner Chassis, CAT 3126 w/ 275 horse's.
MD 3600 Tranny, 2011 Ford Escape Hybrid w/Blue Ox tow bar and Roadmaster Base Plate.
Amsoil Synthetic Oil only for my baby.