Dependant on which type of gauge is fitted, if you switch the ign on, then pull the connection off the sender unit, the needle may read max pressure. I am guessing the sender may be located at the location of the oil pump/filter alloy casing the wire colour might be light green with a red tracer. check the connection for damage or crud that might be acting as a resistance.. I left my hat in the glove cubby hole and a mouse chewed lumps out of it, how it got in there, I have no idea must have been a gymnast, so not about to give up on rodent damage to the wiring yet... Do you have a volt/ohmmeter and can you use it..?