Heater blower work intermittently

That is the classic symptom of a defective fan motor, although your car may include computerized heater controls. Those also cause a lot of problems. When the brushes are worn in the motor, you can often get it started by banging on it while it is getting power.

One clue to worn brushes is once you get the motor started, it will continue to run as long as it is turned on. If it stops running while you are driving, it is more likely to be caused by an overheated connector terminal, a rusty ground wire connection, or a failing control module.

On most cars the fan motor is above the front passenger's feet. It usually does not look like a motor. It is covered with a plastic part of the heater box housing.

Turn on the ignition switch and turn on the heater fan to any speed. Now wrap on the housing with your knuckles. Sometimes the jarring from driving over bumps in the road will get a fan motor started.

This does not always work, especially if the bearings are getting tight. When it does work, your diagnosis is done and all that is left is to replace the motor.

When it does not work, we will need to step back and start with a proper diagnosis. The place to start is by measuring the voltage to the motor. The exact voltage values you find can vary and appear to be somewhat misleading. The important points are you must find something, (as opposed to 0 volts) on one wire, and 0 volts on the other wire, and to be valid, you must take those readings with the motor plugged in. Most of the time you can pull the plug partially apart just enough to stick the voltmeter's probe in there to take the readings. I really disapprove of poking through a wire's insulation to take readings. You can unplug the motor, then check for voltage on the wires feeding the motor. Most commonly, with the motor unplugged, you will find 12 volts on one wire, regardless of the speed you have selected, and 0 volts on the other wire. The problem is that doesn't check the ground wire. With the motor plugged in, there still must be 0 volts on the ground wire. If the ground wire is open, (bad / cut / corroded), voltage will appear on it, but only when it gets there through the motor.

A different way to approach this is to use a pair of jumper wires to apply twelve volts to one wire on the motor and ground the other wire. Polarity is not a concern. If the motor runs, it is okay. If you have the polarity wrong, the motor will just run backward. If it runs, we will need to work backward to the heater controls and resistor. If the resistor for speed selection is defective, on almost all cars the fan will still work on only the highest speed. That alone proves the motor is okay. If it comes to this, you will need to tell me if you have simple levers and switches for heater controls or a more complicated computerized control panel, then I will dig up a wiring diagram to figure out where to go next.

Here's the problem that blower motor isn't under the dash it's under the windshield wipers linkage under the hood area above the firewall. It's common for those motors to do that when they go bad work off and on. So that's where you want to smack when it's inop.

Thanks, Saturntech9. Yes, this is just a temporary solution. It is a method of verifying the wiring and controls are okay. The brushes in the motor are worn and are making intermittent contact on just a few sections of the "commutator" bars. (You don't need to know that technical term. I just threw that in to impress you). Whether or not there is a bad connection depends on the orientation of the spinning shaft after it coasts to a stop. If it's making a connection, it will start spinning the next time it is turned on. If it's not making a connection, you may be able to jar it enough to get it started, by banging on it. Once it starts spinning, it will continue to make enough good connections to keep going. The problem will become more pronounced as the brushes continue to wear down. Eventually you'll need a 20-pound hammer, or dynamite, (sarcasm) to get it going! How long this goes on depends on how long you're willing to put up with it.

It is not practical to repair the motor because the two halves of the housing are riveted together, and even if you could drill them out and get it apart, you wouldn't find replacement brushes anywhere. The proper repair is to replace the motor assembly. Those rarely cost more than $40.00 - $50.00, and when they're under the hood, they are generally easier to remove than when they're under the dash.

Those are harder to replace the wiper cowl has to come off and the wiper arms and wiper linkage. Plus the blower motor cover. Also it's tricky getting the new blower motor back in without the blower motor wheel rubbing when the bleed motor turns on. Don't put everything back together till your sure the blower wheel doesn't rub the blower cover when the blower motor is running.