Re: E34 Heater Question

["Brant M. Miller" <brant@xxxxxxxxxxxxxx>]

Sami -

The E34 heater blower motor power wiring is very straight-
forward:  from the front power distribution box it flows thru a =

blower relay that closes when the ignition is on, and back =

thru the front power distribution box  (all under the hood, =

driver's side), then thru the blower switch (in center panel)
and the blower resistors (left-hand side of the climate
regulation control module plenum) to the blower motor.

The fact that the blower works in speeds 3 and 4 suggests
that all components are satisfactory except the blower switch
and the blower resistor.  I say "suggests" because there is
some small risk that the motor is encountering high
mechanical resistance to start of rotation, requiring full or
near-full voltage to get started.  If you can reach the fan,
spin it by hand.  It should turn easily.  Then put a volt meter
between the power supply to the motor and ground.  If the voltage
advances steadily as you select switch positions 1, 2,
3, and 4 and is not a function of the direction of rotation
of the switch, you've got a motor problem.

If the motor is OK, the switch would be easier to test than the =

resistor.  That would be best
done by isolating the switch - but I do not know whether
or not there is a plug connector that will permit easy =

disconnect of the switch.  An ohm meter should show
near-zero resistance between the in-pin and, sequentially, =

out-pins 1, 2, 3, and 4 with the switch sequentially in =

positions 1, 2, 3, and 4.  That near-zero resistance should
not depend on the direction of rotation of the switch.

If you can test the isolated switch, and it tests OK, the problem
is the resistor.  The resistor is a simple device.  Pin 4 is
wired to the blower motor and to the speed 4 position
from the blower switch.  There is one resistor wired between
pin 3 and pin4, another resistor between pin 2 and pin 3,
and a third resistor between pin 1 and pin 2.  The three =

resistors are in series when the blower switch is set to =

position 1.  So, the resistance
should increase steadily as you work your way across
the blower resistor pins.  The resistance should hold steady,
variation indicating partial failure of an internal resistor.

Brant

>Date: 31 Mar 99 15:13:15 +0300
From: "Sarelius Sami" <sami.sarelius@domain.elided>
Subject: E34 Heater motor question
Where should I start troubleshooting the heater blower as it =

works intermittently. More precisely, sometimes the speeds =

1 and 2 wouldn't work unless I first turned the blower to =

speed 3 or 4 and then back to 2 or 1.
Sami =

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