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working on a 2007 peterbilt with a isx 871. the fan is on all the time. at the OEM ecm connector its only putting 9 volts to the fan solenoid. all the A/C stuff tests out good. no breaks in the wires. Could it be a bad ECM ?

does the ecm say the fan state is on in the Insite software?

9 volts sounds a bit odd too though.

Also, have you checked to ensure there is full air pressure in the fan-hub air supply line?

Hook up insite and see what is controlling the fan. Will it shut off if you manually put air to it?

Could be the valve or a plugged line somewhere going to the fan.

It's not gonna be a bad ecm. Is this thing stock or demandated ?

i have full air pressure going to the solenoid but only a little pressure going out due to the low voltage from the ecm. i can put 12 volts to the solenoid with a power prob and the fan spins freely take it off and plug it back in and the fan locks up. insite says that the fan demand is 0. everything looks good on insite it is set to on/off fan type and 0v to turn on. it is stock.

Maybe a bad ground?

ground is good at the battery's and the ecm.

Read insite for ecm voltage... If its showing over 12 and only 9 going to fan Id swop ecm to test and see what happens.. Maybe bad circuit in ecm