A bad relay, resistor, or wiring can also keep the fan from coming on. It sounds like your low speed fan is not coming on at all, and when you reach the red on the gauge you are getting hot enough to kick on the high speed fan. The low speed fan should have kicked on before you ever reach the red on the gauge.

95 Contour Zetec, ATX (wrecked, sitting in the barn) 95 Mystique Zetec, MTX 2000 Taurus SEL 3.0L Duratec 1994 Crown Victoria LX 4.6 V8 1993 Chevy Silverado 4.3 V6