98 Neon Rad fans only work with A/C turned on; overheating

I had to replace my Radiator a week ago, because the plastic top of the radiator cracked for the 3rd time. It was hot, and I was driving long distance at highway speed.

Twice before I had good luck with fixing the crack using melted cable ties. I attempted to fix this way a 3rd time, but gave up and bought a new aftermarket radiator. Good thing too because the original radiator looked pretty bad when I pulled it out.

I had a lot of trouble getting the fans installed behind the new radiator. I did a lot of wiggling of the radiator. Finally I cut off the useless part of the battery tray to make more room.

Things were fine for a few days around town, but 1st time on the highway the Tranny cooling line blew off. Had it towed home, and I re-attached the hose. Topped off the tranny fluid, and took it for a very short test drive. The car really overheated, and the tranny was not happy. Then I realized I had put the tranny fluid in the engine oil dipstick (oops).

Today I put 5-6 qts of fluid in the correct hole. Car runs in 1st and reverse only. As I was backing into my driveway, the car seriously boiled over with a fountain of coolant coming out of the reservoir. Also, the temperature gauge at the maximum and the ding ding alarm went off. Because the hood was up at the time, I realized the fans were not running. If I turn on the A/C, the fans will run.

OK, I may have a cooked tranny, which is a separate issue. What I really want to know:

1) If the rad fans work when the A/C is on, then the solid state fan relay must be OK, right?
2) If the Temperature gauge shows max, and the overheating warning alarm is sounding away, then the temperature sensor must be working, right ?

What am I missing here?

The Temperature sender on the engine has 3 wires one of them to ground is for the gage the other two go to the computer which controls the fan relay

The Temperature sender on the engine has 3 wires one of them to ground is for the gage the other two go to the computer which controls the fan relay

So the Dashboard Gauge showing very hot (and alarm) does not guaranty that the fans will be told to turn on.

Can someone explain how the fans will turn on with the AC Compressor on, but not with the temperature sensor. A schematic would be helpful.

NOTE: The Fans did work correctly before I replaced the radiator, so I can believe that I broke a wire, or failed to reconnect a connector.

I also have a cheap bluetooth scan tool that works with the android torque app, if that will help. On my 2001 Chrysler van, that tool shows the coolant temperature in real time. I have never tried it with the neon, but I will give it a try tonight. I did not get a Check Engine Light, which I would have expected before the car boiled over, if this coolant temperature circuit was not working

Since the Fans come on with the AC Compressor, can I assume the solid state relay is working?

The temp that the scan tool reads will be temp that the computer sees Tan/Dark Blue and Black/Blue wires to the sensor Violet/Yellow to the engine ground is for the gage

The temp that the scan tool reads will be temp that the computer sees Tan/Dark Blue and Black/Blue wires to the sensor Violet/Yellow to the engine ground is for the gage

OK today I turned on the car and di not move it. I let it fully warm up in the driveway. I had my OBDII Bluetooth adapter plugged in, and I was monitoring the Coolant temperature with the Torque app. The coolant temperature reported by the Torque app seemed to track the dash gauge. I let the engine warm up to 219 degrees F, and still no fans came on.

I am wondering what else could be wrong. I recall from reading my FSM that if the car is moving at less that 40mph, and the temperature reaches 99C or 210F, the fans should come on full blast. Turning on the AC does work correctly (fans come on full blast).

I hope I have not cooked the tranny! I have solid 1st and reverse, but no 2nd or 3rd.

I am curious what the resistance should be at the 100 degrees F. I could pull the plug on the sending unit, and simulate temperatures.