My 1989 Nissan Sentra kept having battery problems. I had it replaced. Then the temperature gauge kept moving up towards Hot. I had the radiator flushed. Now the new battery is dead again, and I have been told to get a thermostat, a new battery cable for the positive post (this is definitely bad), and a temperature sending unit, not the one for the gauge, but the one for the computer. The auto parts place says the temperature sending unit for the computer is also called a temperature switch. They have one in stock and are holding it for me. Before I drive this distance, can you tell me if the temperature switch is in fact the temperature sending unit for the computer? I appreciate any help you might give since I know knowing about car repairs.
From what I've read, I think there's something wrong with the logic. If the engine really is overheating, that is what needs to be addressed, not the sending unit that is reporting that overheating. It sounds like they want you to change the sending unit for the computer because you aren't happy with what the gauge is showing. That doesn't make sense and won't change anything.
First lets address why the temperature gauge is reading too high. Ideally the best place to start is with a scanner that can read live data so you can see exactly how hot the Engine Computer thinks the engine is. If it is running around 195 to 210 degrees, the gauge is reading wrong. That is very uncommon. I suspect you're going to find the computer is seeing an engine temperature of 230 degrees or higher. That's about the temperature at which you'll find the gauge reading higher than you're used to seeing.
If the engine runs hotter in the city and normal on the highway, suspect the electric fan isn't turning on. Normal airflow at highway speeds is sufficient to keep the engine cool without needing the fan to turn on. If the engine runs hotter on the highway, the heat is not being given up to the air by the radiator. The most common cause, especially on an '89 model, is corroded cooling fins between the tubes of the radiator. That happened to my '88 Grand Caravan many years ago. The clue is the temperature does not shoot up to severe overheating such as when there is a major failure. It just goes up a little as you drive faster, and it gets hotter on hot days and stays cooler on cool days or evenings. The cooling fins will often crumble when you touch them.
Next, check that the plastic shroud is in place behind the radiator. That is less of a problem now because the fan is built into it. Look under the front edge of the hood to see if the rubber seal is there that runs from corner to corner. It it's missing, air can bypass the radiator.
If your battery is dieing while you're driving, that is not due to the battery. That means the generator is not recharging it after starting the engine. That is easy to identify with an inexpensive digital voltmeter. Measure the battery voltage while the engine is running. It must be between 13.75 and 14.75 volts. If it is low, first check the generator's belt to be sure it is tight. If you find between 12.0 and 12.6 volts, the generator is not working. Further testing will be needed to determine if the problem is with the generator itself or the wiring going to it.