Coolant Fan Will Not Run On High!!!

[weeicemon]

My 2000 Venture driver side fan will not run on high speed. When the van is completely cool, the fan will run on low with the AC on. As soon as it heats up, the other fan starts to run on high, but the driver side fan turns off completly. I have been to the mechanic multiple times. Changed the relays and fan motor. No luck! Could this be the PCM? I do have a bad secondary air pump which makes the check engine light show all the time but from my understanding, this should not effect the fans.

[Sparky1349]

Hey Weeicemon,

There are actually 3 relays involved here cool fan relay 1, cool fan relay 2 and cool fan relay (in the haynes manual it isn't numbered so for clearity I will call it cool fan relay 0) and 2 fuses Cool Fan Maxifuse1 and Cool Fan Maxifuse2, all of which are in the underhood accessory junction block.

Cool fan relay 1 has to be good if both fans run at low speed as well as both motors have to be good. In low speed the right side fan is power by 12 volts from cool fan relay 1, but it connected in series to the left side fan via cool fan relay 0 and then to left side cooling fan which is connected ground. So in this mode the fans are connected in series, each motor get about half the voltage so they run at about half the speed. So cooling fan 1 maxi fuse and cool fan relay 1 works (known because low speed mode works).

In high speed mode the cool fan relay 0 activates which grounds the right side fan and isolates the left and right fans from each other. Cool fan 2 maxi fuse is good because the right side cooling fan goes to high speed so we know that cool fan relay 0 activated(cool fan 2 maxi fuse feeds the cool fan relay 0 and cool fan relay 2 coils so that proves it is good).

That leaves only cool fan relay 2, my recommendation is swap cool fan relay 1 and cool fan relay 2, if the system continues to work as before (both work on low but only the right works on high) then both relays are good and the problem is a bad connection to the relay, look for corrosion on the sockets that the cool fan relay 2 plugs into. If there is not a bad connection then there is a fault internally to the underhood accessory junction block, which is not easily fixed you may have to replace this underhood accessory junction block (think used part here, from GM I bet it is expensive).

If after the relay swap neither fan works then it is a bad cooling fan 2 relay (now in the cooling fan relay 1 slot because you swapped them remember.)

Good Luck,
Sparky