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Re: (ET) ok, not the fusible link


On Tue, 1 Aug 2006, David C Robie wrote:


ANSWERS WITHIN

On Tue, 1 Aug 2006 18:09:10 -0400 (EDT) Michael S Briggs
<msbriggs alberti unh edu> writes:


Dave,
        What do you mean by an open? An open circuit (which to me
means no
current would flow,

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Righto !  no current flow.  Another way of stating it, across an open
circuit in any complete circuit which includes a power supply, you will
find full power supply voltage.
  Across a 'patially open' circuit  (a high resistance) you will find
full power supply voltage
Across a closed circuit (such as closed relay contacts) you will find no
voltage.
Yup, I'm familiar with open and closed circuits, just wanted to make sure that's what you were referring to since you just called it an "open". 


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so you'd have no field, so the motor shouldn't

turn at
all).

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It will but erratically, lo power, and in a random direction.  The field
still will have it's tiny residual magnetism and some induced too even
though no 'strong' electromagnetism.  Not enough for the poor thing to
even determine which way to go.
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I suppose if there is some residual permanent magnetisation due to some ferrous material in the motor having been partially magnetized by the field previously, it might turn, but there doesn't seem to be enough residual magnetization for that (since it's not turning). 


The belt doesn't seem to be doing diddly when I try to go

forward.

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Not even a slight movement?  A slight one, 1/32"  would be enough to
prove 'open field'
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Nope, not seeing anything. I think though that that would still be a sign of no field, since the armature won't turn unless there's another B field there to exert a torque on it. 


It definitely seems like I'm getting armature current but no field
current

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Why not try 'downstairs' under the tractor first - at the motor's field
terminals? Sometime this is a plug and spcket sometimes it's a pair of
female terms on the motor,  males on the wires, and sometimes it's small
bolt on terms.  They used all kinds seemingly willy nilly.
With the field actuated the voltage there should be at or real near
battery voltage
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Since if the problem were at the motor, it shouldn't have field to go in reverse either. But it does. At any rate, I think I've found the problem - one of the moving contactors on the relay welded to one of the points it's only supposed to touch when going in reverse. The result being apparently that when I want to go forward, the current has a zero resistance path that bypasses the field coil, so I'm not getting any current through the coil, and no field. 




in forward. Hm, I suppose if the relay fails such that all the pins
are
connected, the current would just bypass the field coil entirely....

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If it did that you would have heard a powerful explosion with perhaps an
issue of blood.
:) It looks like that is what happened though - actually only half of that, one of the connections is permanently made. From looking at the relay, the leaf that moves between the two different possible connections is welded to one connection on one side. The result seems to be that when I want to go forward, one of the connections that's only closed when going in reverse is also made, and current can bypass the field coil through a very low resistance path. I'm not sure why I didn't get essentially infinite current and big problems - but, I was getting a *lot* of current, and that is presumably why after trying to go forward with the mower deck also running, everything else ended up shutting off, likely from tripping CB-1 or the other thermal circuit breaker. 



Only modes of relay failure;
*   open coil
*   top contact not making at rest
*   bottom contact not making when actuated
*   one of the contacts fused together, or not breaking due to being
bent.
Yes, the last one is what I'm saying happened. One of the contacts welded together. 


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I
need to get the pinouts of the relay figured out (see the email I
just
sent the list - the diagram in the service manual shows 6 pins on
the
relay, but it has 8).

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8 pins are normal.  2 coil, and 2 DPDT (3 ea) switches.  The diagram does
not show unused pins. It's a waste of ink.
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The diagram only shows 6, so it wasn't showing the two connections for powering the coil, which was confusing me, since for some reason it didn't occur to me that those need power. :) 

Mike