While trying to figure out the problem with the idle, I had seen somewhere that the water temperature had influence on how the engine handled things and so I tried to change out the sensor. The process at this time was trial and error because I still had not deciphered the correct procedure from the shop manual.
I went to my local NAPA and checked to see if they had the part available and not only did they have it available, they had one at the store. Later when I got home I located the sensor plug on the driver side of the engine and pulled out the old one. In the image on this posting you can see the old one on top and the new one on the bottom. Not only is the new one shorter but it is also a different diameter so it wouldn’t fit in the same threads.
The old sensor went back in and I went out online in search of the part that would fit. The parts man suggested that I get an adapter piece of brass to let the new one fit but I’m a little stubborn and wanted to get the real part instead. Eventually I found the correct part online at eBay and soon I was ready once again to replace the old sensor.
This is where my confusion really sets in and that confusion lead to a new discovery about how this car was made and how it operates. First, I found out that the reason the one new sensor is small is because there where two different production runs of this car model. My car was in the first run from January 1987 – July 1987 and they used the bigger sensor. After that Nissan retooled some items and built cars in the second half of the year with a smaller sensor among other things. Also with the new sensor in, my dash always showed that the engine temperature was zero and my fans would not kick on. On top of this the car ran just as bad as it did before. This is where I studied the shop manual a much as I could and found out the exact wiring to test to find out what was happening.
What I then found out was that this car actually had two coolant sensors in it. The wiring from the computer went to a completely different location, connecting to a different sensor right in the intake manifold. The other sensor I replaced goes straight to the instrument panel to show the engine temperature at the coolant thermostat and when it gets warm enough, that gauge turns on the fans.
So, I tested the ohms on the old sensor, the new sensor, and the new small sensor and found out that my old one was working just fine and the new one was reading the wrong temperature range. The old one was put back in after cleaning the threads and now it appears to read just a little better than before. I also replaced the sensor that connects to the computer but that really didn’t help either.
Since I had figured out more of the manual and the production year issues, I could see more how to get some diagnostic information out of the computer. What it told me was that there was nothing wrong with the temperature sensors … that’s what I get for not reading carefully.