[Mgs] Overdrive does not work when it is hot. Checking for an open circuit

[Richard Ewald]

You still don't understand.  It is more likely a circuit with high
resistance hot, not an open circuit hot (those are pretty rare)
Assume you have a circuit drawing 8A @12V (assumed, I understand that
the actual voltage will vary, but this is unimportant).  Now further
assume that you have a Lucas bullet connector with .3 ohms resistance.
What is going to happen to the voltage across that connector?
Will it stay the same?
Will it go up?
Will it go down?
use Ohms law and solve for voltage E=IR  E= 8 X .3 = 2.4V
With an 8 am draw, a .3 ohm resistance will cause a 2.4 volt drop
across that connector.
If you put a voltmeter with the red lead on one side of that
connector, and the black lead on the other side of that connector with
the circuit on, it will register 2.4V.  This voltage drop demonstrates
that there is excessive resistance in that connection.

Pretty simple huh?



On Tue, Sep 7, 2010 at 8:35 AM, Paul Hunt <paul.hunt1 at blueyonder.co.uk>
wrote:
> Oh dear.  I've been working with this stuff for 40 years, playing with it
> for a dozen or more before that.  I'm going to try and explain what I mean,
> ignoring the offensive attitude and language, even though you may well not
> bother reading it.
>
> Draw out the circuit in a straight line, from earth, through the battery,
> manual and gearbox switches, the solenoid, to earth.
>
> The voltage all the way along circuit from the battery 12v terminal to the
> solenoid terminal in a working circuit with switches closed will be battery
> voltage.  It doesn't matter whether the solenoid is passing the correct
> current, is operated and doing its job, or if the solenoid has an
> open-circuit in the windings or to earth, you still have battery voltage at
> the solenoid terminal.  A voltmeter will only show a break in the circuit
up
> to the last point you can connect it to, which will be the wire leading in
> to the solenoid.  You can have a break inside the solenoid winding, or its
> earth connection, and the voltmeter won't show it.  You can't get in to the
> solenoid to test it without removing it, and this is an intermittent fault
> remember.  By the time you have taken it out the fault may have 'repaired'
> itself again, and in any case you have broken the earth connection to the
OD
> body so can't test that anyway.  It means getting under the car
> with tools to remove the solenoid, a waste of time if the fault proves to
be
> electrical elsewhere.   A resistance test will show if the solenoid winding
> and its earth connection are correct, but you can only do that with the
> power disconnected, when the OD will be disengaged anyway.  The only way
you
> can be sure that the circuit is *complete and drawing the correct current*
> is with an ammeter.
>
> I'll say again, simply connecting an ammeter to a parted connector in the
> engine bay, driving along waiting for the intermittent fault to occur
again,
> and looking at the ammeter when the OD stops working is the easiest way of
> proving whether the fault is electrical or not.
>
> It's a two-connection, real world monitor that allows the OD to operate
> normally, that you can leave in
> place until such time as the OD drops out again.  The only thing to
> bear in mind is that the connector you have parted may have been the source
> of the bad connection, in which case the OD probably won't fail again due
to
> the disturbance,
> ammeter connected or not.
>
> Only when you have proved current isn't flowing when it should be, would
you
> then test along the circuit with a voltmeter, *which needs the ammeter to
be
> left in circuit to be sure it hasn't repaired itself again*, to see where
> along the circuit you stop getting voltage.  If you can get all the way to
> the connector on the wire leading into the solenoid and still have 12v, but
> no current, then the problem is either with the solenoid or its earth, but
> you can't be sure which.
>
> PaulH.
>
>
>
> ----- Original Message ----- Your reply shows that you have no idea what
you
> talking about.