You must have lost vacuum at the back of the cluster. Is it stuck in the red(right)? Or a vacuum leak in the engine bay. Thats how I know I have a vacuum leak somewhere, when my gauge quits working. If its stuck half way, then who knows.

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1984 300SD Orient Red/ Palomino
1989 560SEC
2016 Mazda 6 6 speed manual
1995 Ford F-150 reg cab 4.9 5speed manual