[Mgs] Oil Pressure question

Richard Ewald richard.ewald at gmail.com
Tue Jul 29 00:39:14 MDT 2008


Not just lighting circuits.  Back when I was teaching for Volvo, in my
electrical fault tracing class, I would disconnect the ground plate at one
front headlight. All the grounds were connected to the plate, but the plate
itself was not grounded.  Turning on the headlights caused the wipers to
start.  Turning on the high beams, caused the front wipers to go to high
speed, and if it was a wagon caused the rear wiper to turn on.
I made the students use a wiring diagram to figure it all out before I even
let them open the hood. (bonnet <-obligatory LBC reference)
You should have seen the looks on their faces when the wipers started up.
I will save you the reply.  The washer motor(s) grounded at the same plate
as the headlight.  Current from the headlight feed back though the washer
motor (not enough to start them) and hit the wiper motor that thought you
had activated the washers, so it turned on the wipers.

On Mon, Jul 28, 2008 at 11:15 PM, Simon Matthews <simon.d.matthews at gmail.com
> wrote:

> I would be willing to bet that the trailer problem was not with a
> bulb, but instead a bad earth. Bad earths can cause all kinds of
> strange behaviour in lighting systems -- what may happen is that there
> is a current path through the indicator bulb, to ground and (since
> ground is not connected to the vehicle ground) from ground  in the
> reverse direction through the brake light circuit (or through whatever
> actuates the trailer brakes).
>
> Regards,
> Simon
>
> On Mon, Jul 28, 2008 at 1:54 PM, Bob Howard <mgbob at juno.com> wrote:
> > Bill,
> >   This sounds related to the question someone asked Car Talk on
> > Saturday, the guy out west who asked why the electric brakes on his
> > trailer went on whenever he switched on his 4-way flashers.   The
> > suggestion was that he had a bad lightbulb in one of the flasher
> > fixtures. It will be interesting if he gets called back for Stump The
> > Chumps to learn what the cause was.
> >    What would be the equivalent for you---the OD lockout switch, the
> > reverse lamp switch?
> > Bob
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