radiator cooling fan not working

here it is short and sweet. checked fuses replaced relay have continuity in harness. to fan out hooked to battery and it works any ideas thanks. its a 1990

probably something like the temp sending unit. I have a 1999 and first time it was a bad sending unit. Problem has returned and I am checking everything again. I haven't overheated YET but ^&%$ close. Seems the fan comes on only with the AC on or when the gauge reads near overheating. I suspect either a bad sending unit AGAIN in mine or a bad ground or temp gauge. Other than these annoyances, I do like my Cavalier. Just wish it was less annoying problems like my old Toyota was.... I sold my Toy with 270,000 miles and still running. I treated it badly. I treat my Cavalier like a baby because of the miles I drive, I change my oil every month, or 3000 miles....But these annoying issues like bad fuel gauge and fan switch or fan not turning on except with AC on...