Sound like a bad temp sensor?

If you're not sure, can you post a picture of your oil pressure sending unit which probably has a switch tee'd into it.

What color wires did you disconnect for each?

I disconnected the oil and water temp sending units on the motor. The water temp send has a green wire and the oil press send has a blue wire.  Just found what I guess is the oil press switch t-ed in to the fitting for the press sending unit and it has a green wire.  Didn't see that that other day.

So, next step is disconnecting the oil press switch?  I should still have oil pressure indicating?

Why do you suppose I still had water temp indication when I disconnected the sending unit?

UPDATE:  I disconnected the oil press switch and the Check Engine light is now out. And to answer my own earlier question, yes I did have oil pressure indicated.

So help me out, what does the switch do but turn on the check engine light?  Seems like it is just an idiot light.

You also have a temperature switch that does an equal amount of nothing except turn on the same light.

It'll have a green wire and a tan/black wire and should be across the intake manifold from the temperature sender.

So, I think you already know your problem is that the oil pressure switch is bad.

You can disconnect both and the engine will run just fine with no "idiot light" warning for the driver.

The oil pressure sending unit has a light blue wire and it feeds the gauge on the dash.

The temperature sending unit has a tan wire and it feeds the temperature gauge on the dash.

A gt40 since it's computer controlled also has an ECT (engine coolant temperature sensor) that is one of the handful of signals to the computer for engine control. It has 2 wires going to it and strictly feeds the computer. The manual says a green/orange and a gray/red wire. I think they're in a wire loom with a 2 prong connector to the sensor

Should be on the opposite side of the intake from the temperature sending unit on your gt40..

The temp switch on the GT40 is right where you say it is Marty, and maybe Kenny's is in the same spot or it's on the manifold. He should be able to say where.

I think the discussions get tough because of terminology like switch and sender (sending unit) getting mixed up 

KennyO, my 95 had the same issue 5 years ago, new temp and oil pressure sending units fixed it.  I changed both not wanting to visit the issue again.  They are not expensive and take little time to change them out.

Maybe you or Marty or Gary S or any other gt40 owner can check and post what colors yours are.

I see them with 1 tan/black wire from the oil switch to the temp switch, then 2 wires from the temp switch. One to the ECM (tan/black) and the other to the dash light (green)

Looking at old threads, both tan/black and green have been mentioned for those wires.

Kenny O could post what color his switch wires are too for the later non gt40 engines with the dash light.

I think I'm losing my last remaining marble cuz I don't remember.