In a message dated 97-12-07 16:13:53 EST, 105671.471@compuserve.com writes:
> There is a switch deep within the bowels of the wiper motor assembly
> that makes the circuit allowing the motor to continue to run until
> the blades reach the parked position. This switch is connected to
> black wire which should provide continuity to chassis ground. If you
> have an ohmmeter, unplug the connector and check for zero ohms from the
> black wire connector terminal to battery negative. If this connection
> is good then I would suspect the internal switch which will require
> disassembly of the wiper motor. Sorry.
Dave:
I hate to correct you, because 99% of the time you are correct, but this time,
you are only partially correct.
The black wire does indeed provide a ground connection, but it is required at
all times for the motor to run, not just to park. The switch you mentioned
(the parking switch) alternates between the green wire and the ground wire. It
is connected to the green wire in every position except park, at which time it
connects to the ground wire.
For John's benefit, the following is a description of the workings of the
parking switch:
The switch has two inputs and one output. The inputs are the green and the
black wires, and the output is the brown/light green wire. The brown/light
green wire goes to the dashboard switch. When the dashboard switch is in
either of the two operating positions, the brown/light green wire is isolated
from the circuit. When the dash switch is in the off position, power from the
brown/light green wire is fed through the dash switch to the wiper motor by
the red/light green wire (which is the low speed wire).
As long as the dash switch is on, the internal parking switch in the motor
housing has no effect. When the dash switch is off, the parking switch will
keep power applied to the motor till it reaches the park position, at which
time it disconnects from the green power lead and connects to the black ground
lead.
If you have followed through all this, it appears that the park switch
actually shorts out the motor winding - and indeed it does! The magnetic field
which has built up in the motor while power was applied now collapses, and the
motor stops! Right now! If you hold a wiper motor in your hand while it is
operating, and let it go to the park mode, hang on tight, because it will want
to jump out of your hands, so strong is the stopping force.
One of the possible failure modes of the internal switch is for the little
plunger that operates it to become stuck from the drying of the grease in the
switch housing. Either way, if there is continuity on all the wires going to
the switch, the wiper will have to be opened up. This is not at all hard, and
if a new switch is needed, they are available, and are not too expensive.
As for trouble shooting, if the wipers work at both speeds, the red/light
green and the blue/light green leads from the dash switch, and the black
ground wire are OK. If the Green wire has 12 volts (with the key on) then one
of the switches (dashboard or parking) is faulty. If any of the preceding is
not true, then there is the problem - whichever wire is not up to snuff needs
to be looked at for opens or bad connections.
If the preceding checks are OK, then connect a voltmeter to the brown/light
green wire at the wiper connection with the wiper motor working. If all is
well, it should read 12 volts most of the time, dropping to zero for just a
moment once each cycle (The wipers should be in the park position when the
voltage drops to zero). If not, the parking switch is faulty.
If the above test passes, put the voltmeter on the red/light green wire at the
wiper motor. Run the motor for a mpment, and then switch off the dash switch
when the wipers are far from the park position. There should be 12 volts on
this wire as long as the wipers are not in the park position. If not, then the
dash switch is faulty.
Hope this helps!
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition
'68 MGBGT---organ donor for the '74
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