1997 SL2. Recently had water pump replaced, so I thought the overheating would correct itself. No such luck. The blower fan is not coming on unless I turn on the AC. Fuses have been checked and switched. Engine heats up mostly with the stop start in traffic.
All settings of AC activate the fan.

 

Overheats when? What does the scan tool show for engine temp?

 

Car will heat up while driving, especially if idling (or stopped at a light). Gage in car starts to rapidly climb. Discovered today that the fan will run if the AC button is on. Went for a 2 hrs test drive/errands, lots of starts and stops and had no heating issues as long as the AC was on.

 

There's a coolant temperature sensor that should be reporting to the computer. A scan tool will show you the ECT.

 

Replaced the ECT sensor and scan tool reading no errors, but fan still not turning. Was turning when AC on, but now not turning then either (AC also warm). Dis/reconnected the battery and went for a drive. Car heating up, fan not turning on. Discovered fan will run when ECT sensor disconnected. I will reinstall the sensor and see if that helps

 

What temperature is it getting to?

 

The AC system is low on refrigerant. This is totally coincidental.
When you turn the AC on, you should hear the AC compressor clutch click into place.

If you don't hear that and everything was working fine before, you likely are low on AC refrigerant. If the refrigerant low pressure switch is triggered due to low AC refrigerant, the fan also will not kick in, presumably because the people that designed it figured if the AC compressor is not loading the engine, then the fan doesn't need to be on.

Under normal conditions, with the fan kicking on when the coolant temperature gets too high, this won't matter. However, in your case, since the fan is not coming on with increasing coolant temperature, If you let the car just idle in place, meaning not moving, meaning no airflow across the radiator, it will overheat.

To test my theory: and this is a theory

If you want to verify that the low pressure cutoff switch for the AC refrigerant is opening the circuit that powers the AC clutch, find the switch, disconnect the pigtail, and put a wire between the two terminals of the pigtail. The switch is normally closed and when the pressure is too low it opens. Do this for only about 5 seconds, because you don't want to damage the AC sysstem.

The cutoff switch is located, when facing the car, all the way near the driver front fender in a metal refrigerant line that, when traced, leads towards the compressor. It is really really close to the fender so you can't really miss it. The sensor sticks straight up in the air.

You definitely need to get a scan tool that captures real-time data and see what temperature the ECTS is supplying to the PCM. Since you just put in a new water pump, it is possible that the impeller is not rotating correctly and you are getting cavitation which does not circulate coolant.

Thus when you measure the coolant temperature, it will be much lower than the 210 to 230 to whatever high temperature is actually present in the system. This is because the coolant is not actually flowing anywhere, so the ECTS is only reading the temperature of the coolant sitting in the cylinder head. Which, if there is no coolant circulation, takes much longer to heat up just sitting there.
-------
Another possibility would be that your thermostat is stuck mostly closed. This will greatly limit the flow of coolant in the engines cooling system, IE, very little flow, leading to the same general effect of the ECTS reading artificially low. If your thermostat was stuck completely closed, the car would probably overheat within 5 to 10 minutes or even sooner after the thermostat was supposed to open at 185, 190, 195°, whatever it is, with the car sitting still.

So getting that scantool and reading the ECTS temperature is absolutely critical to diagnosing this.

​​​​​​You'll need to hook it up after you start the car so it will not lose power when you turn the key to crank. Do this with an ice cold engine so you can watch the temperature reading as a function of time, especially noting when the car begins to overheat.