1994 C280, when the car is running none of the gaues work, speedo, tach, temp, nothing. And a few warning lights remain on. I tried a junkyard OVP with no success.

What else should I check?

The car has a very bad battery, could that be the issue? The car has to be jumped, but seems to run fine once it is running.

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68 280SL - 70 280SL - 70 300SEL 3.5 - 72 350SL - 72 280SEL 4.5 - 72 220 - 72 220D - 73 450SL - 84 230GE - 87 200TD - 90 190E 2.0 - 03 G500

Nissan GTR - Nissan Skyline GTS25T - Toyota GTFour - Rover Mini - Toyota Land Cruiser HJ60 - Cadillac Eldorado - BMW E30 - BMW 135i

Yes, Benz had a lot more electronics introduced in the early years of the W202, and W210 etc. I'd blame it on the battery since this sounds like a similar situation I had with my 1997 C230 [very similar car].

Something also to remember is that this is a Mercedes of the eco-junk degrading wiring harness era and the crappy design caused all kinds of electrical flaws as age and time went by when the harnesses degraded.

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Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
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2004 Chevy 2500HD Duramax 220k