I've been chasing a strange oil pressure problem for 2 years on one of my motors. During the course of me trying to fix the problem, I replaced the sending unit in my 500 EFI with the correct one from Mercury. My boat is a 2000 model Fountain with Gaffrig gauges. I have since discovered that Gaffrig has their own senders, 0-80 PSI, and 0-100 PSI. Will a 0-80PSI Gaffrig gauge operate properly with a Mercury sender, or do I need a Gaffrig sender?

The gauges on my 03 said Gaffrig on the face but the back said gaffrig / livorsi. I bought all new Gaffrig monster gauges last year and all the gauges that ran on senders were out of wack with the merc senders, had to run a separate set of senders for them and still keep the merc ones to work with the computer and they still didn't work right. I went round and round with Gaffrig and got little customer service, I bought livorsi gauges and now everything works great. The old Gaffrigs ran on livorsi senders but not the new ones, my suggestion is buy livorsi and be trouble free.

man now I'm more confused...

I don't want to replace all my gauges to match... just looking to try and figure out why this one is acting weird, and I'm worried that buying a Mercury sending unit has me chasing a different problem.

As of now I only replaced the gauges that operate on senders. They are a little different but you can't really tell much difference, the color is exact match but they are all new monster gauges not old ones. Not sure if the older gauges will match as well. I am happy now and I have confidence that my gauges are working properly.

Merc senders work with Gaffrig Precision Instrument(aka Livorsi) gauges.

Exactly what is the gauge doing???? Chances are the gauge is bad.