I've got an OE voltmeter in my '87 D250 BUT...it came with a complete cluster from a 1992 D250 and I think all the gauges are driven by the computer. It was all part of the conversion to 5.2 Magnum, A518, and the complete cooling, heat/AC and wiring systems. It looks stock, using the original gauge bezel but now it has a 'Check Engine' light and other such stuff. I don't know if the voltmeter could be installed in the earlier cluster or used without the computer.