You have the wrong sender for VDO. You probably have mercruise senders. They will do exacty what you describe. I just switched from vdo guages to gaffriges. Didn't change the senders and they didn't work. I also have the vdo senders that are yours for the cost of freight. I now have the gaffrig senders and all is good. vdo has the voltage reference backward to Merc and most senders we see.
BT