Then your volt meter still has power to it when the main switch is off. This will cause a drain on your battery. The gage should be wired to a switched power source. The wire # 27 is usually used for this purpose. You'll have to disconnect the battery ground cable that goes to the frame. Then unfasten the instrument cluster and move it outward and tilt it down so you can see the back of the volt meter. Find the + 24 volt lead for the volt meter (this will be the terminal on the gage with a quick connector) and follow it till you come upon a metal wire number tag. The number should be 27. Some folks wire it to the #85 wire on the newer 4 wire main switch which will also work ok. You do not want to connect it wires 8, 9, 10 or 11.
54 M37, 66 M101, 45MB, 51 M38, 60 CJ5, 46 T3-C
Disclaimer: Any data posted is for general info only and may not be M37 specific or meet with the approval of some esteemed gurus.