[TR] TR250 Windshield Wiper Motor and switch

Randall TR3driver at ca.rr.com
Sat Jan 10 22:17:32 MST 2015

> On page 133, he mentions that for high speed operation "Both the 
> terminal 'S' and Terminal 'F' of the dash switch have been shorted to 
> ground."  But the diagram on page 134 for high speed 
> operation seems to 
> show current through the ground and one of the other terminals on the 
> switch.  I would have though with both terminals shorted to 
> ground there 
> would be current through both wires to the switch.

In effect, there are two coils inside the wiper motor.  For low speed, both coils are in series and there is only one ground through
the wire marked R/LG on Dan's diagram.

For high speed, the junction between the two coils gets grounded.  So, one coil gets full battery voltage, and the other gets
nothing.  In effect, it no longer matters whether the R/LG wire is grounded or not, there will be no current through it either way.
It just so happens that the switch does ground the R/LG terminal, but it doesn't need to.

> So, I was wondering, do I need to connect both the R/LG and N/LG to 
> ground to test if the fast speed is working?

No.  As above, when the N/LG is grounded, it doesn't matter if R/LG is grounded or not.

> One more question.  The three wires going into the motor all 
> seem to be 
> the same color.  How do I know which should be hooked to 
> which wire in 
> the harness?  

Run the motor until it is partway through a cycle, then disconnect the power and use your ohmmeter (DMM on ohms X1 scale) to
determine which contact is grounded to the housing.  They will all show some continuity, but one should have a much lower reading
than the others (roughly the same reading you get if you short the probes together).  That is the R/LG terminal.

Now measure the resistance from that terminal to each of the other two.  The one that shows the highest resistance will be the G
terminal.  The remaining terminal is N/LG.

I don't have one handy to try, but it is possible that the speed difference does not show up unless the motor is loaded.  And the
difference is not particularly large anyway, only about 20%.  If your motor runs when either terminal is grounded, you will probably
see the speed difference when you get the wipers connected.  If not, there must be a short between the two terminals inside the

It's a shame Dan didn't provide resistance measurements.  Perhaps someone else can.  (I only have the earlier style 2 speed motor,
which may not be the same as the TR250.  And it's buried in a box somewhere.)


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