This might be a stupid question but after reading the FSM a couple times I just don't see the answer there and I don't see it mentioned in Aaron Cake's writeup either so I have to ask.

There are 2 temperature sensors on an N/A S4 right? One for the gauge and one for the ECU? Where are they each mounted?

I am putting together the parts order for my megasquirt project (MS1 V3 on my N/A 87 RX-7 GX) and I am considering swapping to a newer GM coolant temperature sensor. Aaron Cake's writeup shows him drilling the front of the waterpump housing to add a second sensor but as I am keeping the stock air pump and it bolts right overtop of where he drilled I can't do that. I was thinking it would be nice to just remove the housing and modify it to accept the GM sensor in the stock location, but it got me thinking about whether I'd then lose the temperature gauge. I assume not or else it would probably be mentioned in the writeup but I just wanted a sanity check here

 

Correct, there are two.

The gauge sender is located on the rear iron below the oil filter pedestal. It's a single wire sensor, with a yellow/white wire on a bullet connector.

The CLT (ECU sensor) is located on the rear of the water pump housing.

You can remove the water pump housing and then drill and tap to the rear without causing problems. I just used the front because it means not having to remove anything and is more convenient for most people who may not want to remove the housing.

I actually show drilling and tapping the water pump housing in this video:

 

Awesome, thanks. I'll look at what is left to remove the water pump housing once I get everything taken apart. I have to do a lot of disassembly already for the megasquirt install and the water pump gasket is leaking so I have to pull the water pump too - maybe it's easier to just pull the housing and drill the back

 

Be careful when you go to place it as the the sensor tip can be deceiving on how close it will come to the thermostat..

I ended up just tapping the stock ECU sensor boss for the new sensor on the back. I also have a couple of nice ntp plugs on the way to plug up my mistakes...