cooling fans

the primary fan is supposed to come on at 1/2 and the secondary at 3/4 right???...because when i do the diagnostic on the ecm both fans comes on....but when im driving theres only one fan that comes on at 3/4 when its too late....primary fan doesnt come on...does anybody know what it could be?..thx.....i have a 92 3.4

one fan should come on when the temp is a little over half way the other comes on when you turn the ac on and well stay on until you turn the ac off