Hi guys/gals

My rad fans in a 91 NA SW20 wont turn on. I checked the relays and they are fine. The temp sensor in the engine bay seems fine. I connected the fans directly to the battery and they turned on. I can go for a big drive and they wont turn on at all nor will they turn on when i let the car run for over 30mins on my drive way. But i found another sensor on the side of the radiator and unplugged it. Then the fans came on full force.

Any thoughts? My AC condenser has been removed if that helps

"But i found another sensor on the side of the radiator and unplugged it. Then the fans came on full force"
I believe that is normal, and it proves your fans work OK anyway. 
 
I have noticed my rad fans only seem to come on when the weather is very hot, and I have never noticed any
overheating problems, so maybe that's normal too.
 

I never noticed this issue till I took it for its first drive in Townsville when I moved here recently. Drove for 15 Minutes then the car overheated and fans hadn't turned on. Found the coolant was low. Filled her up then continued driving, the temp climbed for about an hour or 2 whenever I stopped at a set of lights but cooled down immediately when I began driving. Thats when I realised my fans aren't turning on. I haven't driven it since.

I found the part number for the Radiator Temp Switch:
89428-33010
89428-15030
One is oval shaped and one is a circle shaped connector. Going to rip mine off and buy an OEM one off of Ebay and see how I go. They go for about $15 including postage

The AC amplifier controls all the fan operation , the other sensor you unplugged would of been the AC high pressure switch.  By unplugging said switch it is the same effect as it operating/opening which causes both fans to turn on in parallel (High speed).
 
When fan relay no.3 is energised the fans should run slow speed, the coil of the relay has a permanent earth and the live is from the AC amp via "Gauge Fuse" 7.5A.  I would try finding the gauge fuse.  
 
Alternatively remove fan relay no.3 and check for a earth & live for the coil terminals (live would only be present when fans are meant to run) or jumper the supply/demand terminals and see if the fan runs.  If you jumper the terminals and they run then the issue is AC amplifier related.
 
If you jumper the terminals to test the control circuit just be sure to have the ignition off while you put the jumper wire in.  If it were me i'd just go straight for that to identify the problem area.