Blower motor works intermittently

First we need to know when the problem occurs. If it is always when you first start the engine, and it never quits after running a while as you're driving, by far the most likely suspect is the fan motor itself. The intermittent operation is caused by worn brushes inside the motor. Once the motor starts running, normal vibration keeps the brushes working. The problem will get progressively worse over weeks and months. A potential clue is the motor will often start running when you bang on the heater box.

If the fan usually starts up right away, then quits after a few minutes, the cause is more likely to be overheated and burned connector terminals, usually on the control panel or speed switch. Replacing just the control panel for that will not totally solve the problem. The mating connector terminals must also be cut out and replaced individually, otherwise the resistance they cause will generator more heat, and that heat will migrate into the new controller and overheat its terminals. That will result in a repeat failure.

The diagrams below are for a heater system with air conditioning. If you do not have AC, I'll post a different diagram.

According to how the first diagram is drawn, it looks like you can't get directly on the two wires for the fan motor. If that is incorrect, and you can see the plug for the motor, it's those two wires where you need to take voltage readings. The goal now is to catch it when the problem is acting up, then try to not bump anything and cause the motor to start running. Also, these voltage readings are only valid when the connector is plugged in. You'll need to back-probe through the back of the connector, next to the wires.

If you can't get right on the motor's connector, look for the red / light green and the black / tan wires going to the heater box. Those readings will tell us where to go next.

That rules out the fan motor itself. Once it starts up, if the brushes are worn, it will keep on running for that drive cycle. When the motor quits while driving, there's usually going to be a burned connector terminal. The only proper repair for that is to replace the part with the overheated terminals, and to cut the mating terminals out of the connector, along with four inches of wire, then splice in four inches of new wire of the same gauge, and install a pair of crimp-on-style universal terminals. Solder those terminals too for the best connection, then plug them in individually into the new assembly.

The overheated terminal is not going to be at the resistor assembly because it is not in the circuit for the highest speed. The high speed would still work. One good possibility is the black/white ground wire for the control assembly. A clue for that is the illumination lights would also not work at the same time the fan was dead because both circuits share the same ground.

Take the voltage readings at the points I indicated, and at the wires on the back of the control assembly, but remember, they only have diagnostic value if they're taken when the fan is turned on but it's failing to run. Those readings will help us know where to go next.

I didn't mention the motor connector terminals because those are usually heavy enough and rarely overheat. The best suspect is the terminals on the back of the control assembly for the speed switch, especially if the fan is used on one of the higher speeds often. Look at those terminals on the old assembly you took out. Typically two will be dark or black, and the plastic around them will be melted. If you find that, I'll describe in more detail how I replace the mating terminals in the connector. This problem requires a two-step repair. If only one step is performed, the problem will recur in the near future.

The next suspect is the fan motor relay. Those contacts can be arced too and cause intermittent operation. The fastest thing to do is swap it with one of the other relays like it and see if the problem keeps on occurring.

Then we're going to have to rely on voltage readings to figure out where the break is in the circuit. Find the black / tan wire on the back of the control assembly, (black arrow in the first diagram I posted). That point kind of splits the circuit in half. Remember that all connectors have to be plugged in, the ignition switch must be on, the heater control must be on and we want to know what the voltages are when the fan fails to run. If you find 12 volts on that wire, the break has to be after that point, meaning the switches or the ground wire.

Take the readings in as many places as you can get to in that circuit, then post them in your next reply for my wondrous analysis.

Vacuum lines do not have to be plugged in. For a preliminary test, I'll let you unplug the control panel, then you can get to the terminal in the connector, but use a test light, not a digital voltmeter.

Where the problem comes in, ... Imagine standing on a garden hose and blocking 99 percent of it. With the faucet turned on and the nozzle closed, if you start with 50 pounds of pressure at the faucet, you'll have 50 pounds at the nozzle and at both sides of your foot. That would falsely imply lots of water will flow when you open the nozzle, but when you do, that blockage is going to result in the tiniest trickle of water.

Likewise, when there is a major, but not total blockage in an electrical circuit, as long as it's turned off further down the line, as happens when you unplug something, you'll find 12 volts all over the place. That is the voltage that falsely implies current will flow, and the fan will run, when you turn the switches on. Unplugging anything is the same as closing the nozzle on the end of the hose.

You need to open the nozzle and try to get water to flow, then you'll find 50 psi before your foot, and 0 psi after your foot. It's those pressure readings that show where the blockage is located. You need to try to get current to flow, then you'll find 12 volts everywhere up to the break / bad connection, and you'll find 0 or very low volts after that bad connection. The key point is water has to be trying to flow, and current has to be trying to flow, and that can only happen when there is no other break in the circuit, meaning everything is plugged in.

The reason I specified using a test light is when you unplug something, then use a test light, those require current flow to make the bulb light up. To say that a different way, the bulb "completes" the circuit and gives current a place to go. Voltmeters, for all practical purposes, do not complete the circuit, and current flow doesn't have to flow through the meter to let it give its reading. Think of a pressure gauge on a compressed air line. No air flows through the gauge for it to do its thing. Same with voltmeters. No current flows through them. They just measure electrical pressure, (voltage). Current does flow through a test light, so for this type of problem, test lights are more accurate.

Good evening,

Sorry, but we all work part-time at this as most of us have full-time jobs and it takes us away for a while at times.

Can you copy and paste the last response so I can help?

Roy

Let me check.

Roy

Trying to get the questions combined.

Roy