On the 88 iroc I already replaced the radiator fan motor, the temp sensor under the throttle body, had the radiator cleaned, put a 160 thermostat on it and temp reaches 240 and then the fan turns on.

I believe this car has two temp sensors. one for the gauge and one for the computer. the fan might be coming on when it should and the gauge is reading wrong.

also the low speed fan should come on first thenhigh may have bad low speed relay

A friend of mine installed this on his 87 IROC and it triggers the fans at a lower temp. http://www.hawksthirdgenparts.com/83-92camarofirebirdfanswitch.aspx If you turn on the AC, will the fans come on immediately?

I assume you have two fans if the low speed fan is coming on like its supposed which should be above 200* and then the high speed fan comes on like its supposed to at 240* then I fail to see the problem. Also a T-stat does nothing to lower the operating temp of your car. If your temp continues to raise after it reaches 240* then you are probably looking at a head gasket failure, check for bubbles in your overflow tank.

the temp switch for the fans is on the block by the starter. It works on a grounding system. When it reaches temp, it closes and grounds the fans. I just ran a switch off that ground, located it behind my hatch switch. Leave it on in city, turn it off on highway. Works beautifully.