Bench test speedometer mercedes

Group,
I want to test or run a speedometer on a workbench. I am not trying to calibrate or test any thing other than the fact that the speedometer drive operates and the odometer gearing is in tack. I can
assume that the speedo drive is powered by a signal that originates at the tail shaft sensor but I can't find what the end electrical product is at the speedometer. Would it be possible to use a signal generator and stimulate the drive in the speedometer to turn? If so dose any one have specific knowledge about what frequency and shape the signal should be and what voltage and current involved? Also is a primary power source required at what connections? I know repair shops bench test these units. I have several of these 126 chassis VDO units that need tested and I feel there is a way but need help to get started. These speedos are all from the 126 and 107 Mercedes (alike)
Thanks Bruce Buchanan,
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I love how I can just take a drill to the back of mine and by simply adjusting how far I squeeze the trigger I can adjust the speed. So much simpler. However, the analogous non-electric method of delivering oil pressure to the dash is not as welcome. The first time I pulled the gauge cluster out to troubleshoot why the ignition switch was not turning the car off I started the car up. Unfortunately I wound up with oil all over my legs and carpet. Not Good. What really sucked was the I was wearing long johns, so it had to soak through quite a bit before I felt it. I probably lost about a pint of oil.
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