What is diagnostic procedure for detecting a fault on blower motor circuit?

Answer 1

Start with the simple stuff first.

Check your fuses or circut breakers. It's a fairly simple circut to diagnose. All you need is a test light with a globe-not an led. Start with your fuse or circut breaker by making sure you have power coming out of the fuse supplying the fan speed switch. If the fuse is repeatedly blowing then there is a short to ground in the circuit or there's a high current draw caused by a faulty blower motor or fan speed resistor.

An easy way to find a short is to hook a light bulb up in series across the fuse. The bulb will light up if a short is completing the circut.

Now you dissconect circut components (blower motor, switch, etc) and plugs/connections one at a time until the light goes out. This tells you the short is past that part of the circut. Look for melted/burnt wiring, wires rubbed through, smoke from fan motor or resistor.

A poor earth can be found with a test light by conecting to positive and probing earth points (a good earth will be indicated by the test light glowing brightly, a poor earth point will give dim or no light). I recomend using a multimeter in place of a test light when diagnosing more modern systems.

Check the operation of the fan speed switch. Does the fault occur on a particular speed? With your test light, probe the wires coming out of the switch to the resistors (don't probe the wire itself, probe at connections) to make sure the low medium high terminals power up when switched. If not, the fault is in the contacts of the switch.

Next check the operation of the fan speed resistors. If the fan does not operate on a certain speed but fine on others its likely the resistor for that speed setting is faulty. When the low speed is selected, the power from the switch goes to the low speed resistor where it then flows through the medium and high resistors and onto the fan motor. When the next speed is selected power is turned off the low resistor and directed to med resistor then flows through the high then fan motor. High speed directs power to high resistor or connects directly to fan motor (providing full system voltage for max speed). The low speed setting creates the highest resistance because the power has to flow through all resistors first. The higher the resistance, the greater the reduction in voltage across the motor, causing a slower fan speed. With your test light, probe the output connection of the high speed resistor and connect the test light to ground. If the circuit is ok the bulb will glow dimly on low speed and get progressively brighter as the next higher speed is selected( reducing resistance increases voltage). If the light goes out or becomes very dim when the next speed is selected the problem is with that resistor.

If fuse, switch, resistor, wiring tests ok and has a good ground connection, fault is in blower motor its self. A distinct burnt electrical smell is a good sign the motor is burned out. Check for mechanical obstructions like a rag or pen jammed into fan blades. That would stop or slow the fan. The motor tries to work harder to overcome the obstruction causing a higher current draw which can cause wiring to melt, fuses to blow etc.

Excessive noise is another sign of obstruction or poor fan motor condition. Check for coolant leaks or stains under the dash/motor area/ vent ducts. Also check the a/c evaporater drain isn't blocked. The fan motor will short/burn out if exposed or immersed with water or coolant. A multimeter can be used to test the motor by probing the positive and negative wire and measuring resistance(ohms) an open circut indicates the motor windings have failed. A test light is the quickest and easiest method but if in doubt I recommend fault finding with a multi meter for accuaracy and to reduce the chance of damage to the electrical system.

There are many different variations of this system which can be more complex (like climate control) but the principals of operation are the same. Thebest advice is to keep it simple, look for the easy simple causes first because most of the time the fault is very simple and can be overlooked.