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Blower Motor and Resistor Testing
 
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Pelican Technical Article:

Blower Motor and Resistor Testing

Nick Czerula

Time:

2 hours2 hrs

Tab:

$20

Talent:

*****

Tools:

Plastic prying tool, T25 Torx driver, DVOM, test light

Applicable Models:

Volvo V70 (1998-01)
Volvo V70 2.4T (2001)
Volvo V70 AWD (1998-99)
Volvo V70 GLT (1998-00)
Volvo V70 GLT SE (2000)
Volvo V70 R (1998)
Volvo V70 R AWD (1999-00)
Volvo V70 T5 (1998-01)
Volvo V70 X/C (2001)
Volvo V70 X/C AWD (1998-00)
Volvo V70 X/C AWD SE (2000)

Parts Required:

Blower motor, blower motor resistor

Hot Tip:

Use an incandescent test light to load the circuits

Performance Gain:

Proper working blower motor

Complementary Modification:

Replace cabin air filter

Volvo V70 vehicles are equipped with a climate control system with multiple sensors, controls, air temperature units and ducts. The blower motor that controls airflow into the passenger cabin, is among the most consequential components in the climate control system. The blower motor resistor (or final stage) varies voltage to the blower motor to achieve the desired speed. The blower motor is a D/C electric motor driven by varying current supplied by the blower motor final stage unit.

If your blower motor fails, replace the blower and resistor together. You don't want to have to go back in if one fails later. Over time, a blower motor may become noisy as the motor electrical contacts wear. The blower fan cages also fracture, creating a vibration when the blower is ON. If it fails or becomes noisy, you can use this procedure for the repairs. Once the blower is removed, inspect the heater housing for debris. It can accumulate leaves, dirt and rodent build nests, all of which cause noise or a vibrating blower motor.

Usually, when the blower motor resistor fails, the blower motor will work in only one speed, or fluctuate when a set speed is selected. Most times it defaults to high speed, leaving other speeds not functioning. To confirm the resistor is the root problem on your V70, test it first.

If you push the button to change the blower motor speed and nothing happens you may have a blown fuse, a bad blower motor relay or a bad blower motor resistor. There have been cases where the final stage has kept the blower motor running while the ignition key is off, therefore draining the battery and leading to a "no-start" problem the next morning. In this tech article we are going to review how to test each component in the system.

Remember that your car may have been serviced before and had parts replaced with different size fasteners used in the replacement. The sizes of the nuts and bolts we give may be different from what you have so be prepared with different size sockets and wrenches.

Protect your eyes, hands and body from fluids, dust and debris while working on your vehicle. If you're working with the electrical system, disconnect the battery before beginning. Always catch fluids in appropriate containers and properly dispose of any fluid waste. Recycle parts, packaging and fluids when possible. Never work on your vehicle if you feel the task is beyond your ability.

Our vehicle may vary slightly from yours as models do change and evolve, as they grow older. If something seems different, let us know and share your info to help other users. Do you have questions or want to add to the article? Leave a comment below. When leaving a comment, please leave your vehicle information.

I suggest using a current wiring diagram when testing your vehicle, as the wiring or circuit connectors may vary from my subject vehicle.

The blower motor (red arrow) that controls airflow into the passenger cabin is among the most consequential components in the climate control system.
Figure 1

The blower motor (red arrow) that controls airflow into the passenger cabin is among the most consequential components in the climate control system. The blower motor resistor (green arrow) (or final stage) varies voltage to the blower motor to achieve the desired speed. The blower motor is a D/C electric motor driven by varying current supplied by the blower motor final stage unit. Both of these components are located in the heater housing, behind the glove box. Working at the left corner of the engine compartment, open the fuse panel door. Check fuse #31 25 amps for continuity. Use the fuse sheet supplied with your vehicle to confirm the fuse location and assignment. Also, confirm with a current wiring diagram, as I find year to year there are changes.

Turn the key ON and set the blower speed to HIGH (green arrow).
Figure 2

Turn the key ON and set the blower speed to HIGH (green arrow).

Start by connecting your DVOM across the blower motor output connector.
Figure 3

Start by connecting your DVOM across the blower motor output connector. Connect your red lead to terminal 1. On my vehicle this is yellow and violet wire (red arrow). Connect your black lead to terminal 2. On my vehicle this is the blue wire (green arrow). The blower speed should be set to high. Your DVOM should read battery volts (yellow arrow). If the blower motor is running, it will be about 12 volts with a vehicle power supply on. If you're just testing using your battery, expect a two-volt drop from what open circuit battery voltage is at the time. If you get no voltage, try cycling the key off and waiting one minute, then cycling the key on with your DVOM already in place. If you have zero volts or lower than battery volts, the final stage unit may be faulty. In the next steps I will show you how to test it. If you have voltage but no blower motor operation, the blower motor is likely faulty. Keep in mind the blower resistor supplies ground to the motor. Earlier we checked the fuse and confirmed it to be good. You can also move the black lead of your DVOM to battery negative (ground) and keep the red lead on terminal 1. You should have battery volts. If you don't, there may be an open circuit between the fuse and the connector.

It's also a good idea to load the signal to see if the resistor can handle the load of the blower motor.
Figure 4

It's also a good idea to load the signal to see if the resistor can handle the load of the blower motor. I like to use an incandescent test light (green arrow). In this case, the test light is adding load to the blower motor circuit. Turn the blower speed to high. Your DVOM should read battery volts (yellow arrow). If the blower motor is running, it will be about 12 volts with a vehicle power supply on. If you're just testing using your battery, expect a two-volt drop from what the open circuit battery voltage is at the time.

Next, we will test the control signal to the final stage from the climate control panel.
Figure 5

Next, we will test the control signal to the final stage from the climate control panel. You will test this signal if the blower motor is not receiving the ground signal to run. This is the voltage shown during the last test. Unplug the four-pin connector at the resistor. Connect the black lead of your DVOM to battery negative (ground) and the red lead to terminal 1 (red arrow). On my vehicle this is the yellow / brown wire. Set the blower speed to off. The DVOM should read zero volts (yellow arrow).

Connect the black lead of your DVOM to battery negative (ground) and the red lead to terminal 1 (red arrow).
Figure 6

Connect the black lead of your DVOM to battery negative (ground) and the red lead to terminal 1 (red arrow). On my vehicle this is the yellow / brown wire. Set the blower speed to the middle speed. The DVOM should read about 8 volts (yellow arrow).

Connect the black lead of your DVOM to battery negative (ground) and the red lead to terminal 1 (red arrow).
Figure 7

Connect the black lead of your DVOM to battery negative (ground) and the red lead to terminal 1 (red arrow). On my vehicle this is the yellow / brown wire. Set the blower speed to high speed. The DVOM should read about 12 volts (yellow arrow). If you have the correct voltage on this wire throughout the blower speed range, the resistor is likely faulty. Use the following step to confirm the resistor is receiving the correct supply voltages to operate before condemning it.

Working at the four-pin resistor connector, you will be testing the remaining three wires.
Figure 8

Working at the four-pin resistor connector, you will be testing the remaining three wires. Terminal 1 is the yellow / brown we already tested. Terminal 2 is the white / violet wire and is the resistor feedback to the climate control panel. Unplugged or in manual mode this wire is usually close to zero. When plugged in and in auto mode, this wire will read from 1: 3 volts. The terminal 3 gray / red battery supply wire should be battery volts. Terminal 4, the yellow wire, is the ground.

This image shows terminal the 2 white / violet wire.
Figure 9

This image shows terminal the 2 white / violet wire. This is the resistor feedback to the climate control panel. Unplugged or in manual mode this wire is usually close to zero. When plugged in (red arrow) and in auto mode, this wire will read from 1: 3 volts (yellow arrow). The climate setting was set equal to the ambient temperature at the time of this test.



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Comments and Suggestions:
Dan Comments: I followed your instructions to check on my 1998 Volvo V70 T5. I set the blower speed to the middle speed. The DVOM reads about 4 volts yellow arrow. I set the blower to high speed. The DVOM reads about 7 volts. I check the fuse #31 and it looks fine. What could be the problem? Thanks
September 23, 2016
  Followup from the Pelican Staff: Yellow arrow in what photo? If the ground, in fig 9, the ground is faulty. - Nick at Pelican Parts  

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