jumpy tachometer

Josh, the oil pressure gauge works on a different principle from the tach. The oil pressure gauge is more similar to a temp gauge.

Basically, the OP gauge gets positive voltage via the purple, switched at the key. Then, the Oil Pressure sender, provides variable resistance to ground (the engine block) depending on how much pressure it is registering. If your OP guage is jumpy, you could have the same type of dirty or loose connections mentioned above or a bad sender.

A good test is to remove the sender wire from the OP Sender. Then, take the wire and ground it directly to the block. If it pegs the gauge consistently, either the sender is bad or the connection to the sender was loose or dirty.

GT40 ?

If so the tach signal has a 22K ohm resistor in line. (Grey wire from ecoil)

Ryan, is the tach your only gauge that's misbehaving?

Thanks for the suggestions. I took off and brushed with a tooth brush the connections on the back of the tach. And I rotated the engine size knob, also on the back of the gauge back and forth for a bit. All the connections appeared tight and clean, though I went ahead and cleaned and tightened again. There must be a connection on the engine side of things to check also. Can somebody tell me where to look for this? Would I repeat the process on this end? Or is there a part (sender?) that could be faulty? Thanks again for helping this rookie. I haven't put it back in the water to test yet, thought I'd try all I could before dragging it to the lake. -Ryan

I have heard / read elsewhere that jump starting may also lead to a "jumpy" tachometer. I had such an isntance on my '96 Sport but may have been a coincidence.....

A good start is checking with a DMM but removing, cleaning and reinstalling all connections never hurts either.