I'm working on a 89 formy like mine but this one has issues. My buddy bought it cheap and it had some problems. One which, the last owner installed a hypertech fan switch to the passenger side head and that was for the gauge. It read 260 all the time. I took out the switch and plugged in an autozone replacement and now the gauge stays on 100. What is going on? How do I fix this. I also noticed that his car has the fan switch it the front of the block and it is a two wire hookup. How can I make the switch work there? Can I convert a two wire to a one wire hypertech switch? How do I make the gauge work? How come his car isn't like mine? Both cars are dual fans. He has a 305 but mine's a 350. Help.

One thing that I forgot to mention is that we both have are stock PROMs in place. I thought that he may have a hypertech whatever but he doesn't and I still can't figure what is up with this car. Please help.

That switch is for the fan only.
The sender for the gauge is in the drivers side head. It should have a single green wire going to it.

An open circuit, the gauge will read zero (or 100 on the gauge). A closed circuit (to ground) and the gauge will peg as high as it can go (as it does when you start the engine).

The fan switch needs to be put back in place, too. The second fan (and the engine) will appreciate it.

yeah, I get that but now I found out the clown also put a hypertech switch on the gauge, not the fan switch. His fan switch is in the front of the engine and has a two wire connection. On the passenger side, the switch isn't connected to anything, almost like it is used as a plug. The fan still kicks on at around 220 but this car has me confused. We put a stock replacement in the drivers head but I'm thinking that now the gauge doesn't work because of the pigtail. The hypertech came with a pigtail and that one is still on the car. Can that pigtail with a factory sensor cause the gauge to always read 100? The fans still kick on when the radiator is around 210-220 but my temp reader might be inaccurate cause the battery is low. Please help, I'm out of ideas.

That is a bit hard to understand but, a fan switch will not harm the gauge.

If the wires are yellow and black, that is the coolant temp sensor for the ECM. The ECM controls the primary fan switching (on at 220°F).

This is for the secondary fan and should have a single gray wire attached to it.

Stock what ? Sensor, sender or, fan switch ?

The drivers side head needs a coolant temperature sender[. It has a single wire (usually green) and most replacements I've seen, have a spade terminal on it. Regardless, it's a single connection.

The gauge sitting on 100 indicates it isn't hooked up to anything or there is a break in the circuit somewhere.

As you crank to start the engine, the gauge should do a full sweep to show that it's working. If the gauge is sweeping, it's fine, the problem lies between it and the sender.

It's a very simple circuit. One wire from sender to the bulkhead, inside the car, to the gauge.