I have a 2005 GMC sierra with 4.8 v-8 I can not get the radiator cooling fans to cut off. they run until they kill the battery.they started coming on when the truck was not being driven and completely cold engine. please tell me what I can look for
Relay 1 may well be the culprit, always hots, if the contacts are sticking close, both left and right fans are connected in series running a lot speed. When high speed is called for, relays 2 and 3 close with relay one already closed, this puts the two fans in parallel.
So in low speed, each fan sees 6 volts, in high speed each fan sees 12 V.
Usually when a relay sticks, just a very tiny arc holding the contacts together, being as frugal as I am reburnished the contacts, have a tool for this for a nice mirror finish.
Normally that pull in at 7 volts, a bit low, armature has a spring on it to hold the contacts open, other end goes to a tab, adjust that for more like 9 volts pull in. By decreasing the voltage, relay may not even drop out to about a volt. Since the armature is closed, completes the magnetic circuit, so requires a lot less magnetomotive force to keep it closed. The reason for the very low dropout voltage. By increasing the pull in voltage, the dropout voltage also increases for a more positive release of the closed contacts.
Another reason this relay made be sticking closed, is because it was Made in China.