2005 Chevy Venture Van. Interior blower motor comes on once ignition is on. All heat and a/c switches are off. Can't stop fan motor.

radiator fan may not be coming on make sure you can hear it running. Fan motor or relay is probably bad

You replace parts as part of your preventative
maintenance program,not to solve a problem
When working on a problem only component testing
will find the issues
You need to check for fuel pressure at the motor with a gauge
You check spark with a test plug you buy,not ground a plug
or wire on the vehicle
You can check for compression or cylinder leak down
to test some mechanical issues
I would assume ALL Fuse & links have been checked,as
well as battery voltage 12.6 v or more & cables etc
The ignition switch could be bad,turns starter but not all
the internal contacts are good

check and test motor under dash behind the glove box test resistor [fan speed ] and fan both connections are access able

Both "blower" fuses are fed by the ignition switch. If there is no power to those fuses than the switch is the likely problem.

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

is it the fan knob or hot/cold knob that doesn't turn well?

Look under the dash near the center and going to the passenger side of the vehicle. You will find a box that houses the heating coil and the AC coil on the interior of the vehicle. The blower is located in that same box. To open it you will have to remove the screws or bolt nuts that hold the cover to the main case and then remove the mounting screws for the blower itself. Typically to test the blower motor I run a direct connection to the motor and see if it will activate. If it does not then the blower motor is bad. If it does activate with a direct connection to power then the problem is normally a switch or fuse, the vehicle as you know has both, what you may not have seen is an in-line diode or fuse on the main power wire. the location of this varies on the wiring harness so it may take some time to locate. If that link is broken there is no power getting to the blower. The best way to find that is a hand search of the harness starting at the case and tracing the wire back to the fuse box where it connects. There are two fuses for this system. One is on the interior of the vehicle and should be a 15 or 20 amp fuse, the other is located in the power distribution box under the hood. This is normally a 30 to 40 amp fuse because it also protects your compressor circuit.

Check the fuse and then the switch it self, if the fuse and switch is good. Then check the blower resistor and blower motor it self. I'm leaning towards the switch it self since you mentioned that it was sporadically and on the highest setting would by pass the blower resistors. Good luck and if it's not the switch then it's going to be the blower motor it self. Auto Zone has them in stock and can give you the step by step instructions for you to get the job done with common tools.

I had this problem in a Chevy Astro once. It ended up being the switch. Not sure if that's your fix, but it almost sounds like the switch.

It could be a bad blower motor - tap on the blower motor with the end of a hammer when you have the switch in the on position - if it comes on then it is a bad blower motor. If it doesn't come on I would check to make sure you are getting power and ground to the HVAC control head.

it sounds to me like your blower moter resister or your blower moter switch are going bad both are right under the hood next to the blower moter one has 2 witers on it the other has 4 it usuley on the pass side right near the window