Car Blower Motor Diagnostics
Blower Relay Replacement
Blow, bugle, blow, set the wild echoes flying
Just about all cars have a basic setup for blower motors. When the blower does not work
it can be for many reasons
some of which are:
1) dead battery
2) bad fuse
3) bad relay
4) bad blower motor
5) bad rheostat
6) faulty wiring
Power runs backwards with a blower motor. By
this I mean that the hot side starts out with the battery to a fuse to a
relay to the motor then on to the rheostat and finally to ground as seen
So when the motor doesn't work start in the middle
of the circuit and work whichever way you need to. Check battery
for 12 volts then apply direct
power and ground to the motor from the battery since
there are all kinds of other stuff on either side of the circuit.
If the motor does not run breath a huge sigh of relief because you have
what could otherwise be a very difficult problem. All you have to
do is replace the motor but first you should test both sides of the
circuit. One side should be 12 volts hot in run.
The cold side should offer some resistance depending on the fan speed
dial. You don't need the car in run to test
the cold side.
Now that the motor works and the hot side is hot
with the ignition key on run. If you do not
get 12 volts on the hot side it's either the fuse or relay. The fuse is easy to
check. If it's good then you have to suspect the relay which is
very hard to remove and replace as I have described
If you do get power to the hot side in the run
position then check the cold side. Run the cold side directly to
ground and if the fan works you may have a bad resister or rust.
ohm gage and test the different resister positions to ground.
Number 4 should be almost zero resistance. Further analysis of the
cold side of the circuitry can be found in other places on the net.
Testing and Replacing the blower relay
The blower relay is hard to get to. It is located on the drivers side
under the dashboard. There is a cluster of several relays mounted
in a plastic frame and secured by a 10mm nut and a plastic gripper on
the firewall. You can locate the blower relay because it has a heavy
blue wire. Blower motors carry a lot of current and so the wires are
easy to identify.
Above is a view of the bottom of blower relay assembly from
under the drivers side. You have to remove the hood release cable
first with a 17mm open.
Above I used my 10mm socket to remove the relay
assembly at right. Then I pried the plastic gripper connector away from the firewall.
This exposed the relay assembly module. The blower
relay is the big one at the back. Test the 5 pin relay to see if
it is failing. see "testing 5 pin relay".
My temporary solution for blower relay failure
Last winter I diagnosed this problem and decided
to run a fused hot wire to the hot side of the blower because I gave up
on trying to find and replace the relay. It was too hard and too
cold. The solution worked fine as long as you remember to turn off your fan. My son borrowed the car and got
stuck a few times by leaving the fan on so I decided to do it the job
the right way as described above.