[Vwdiesel] HELP! fried starter solenoid

[Sandy Cameron]

At 02:48 PM 17/10/2006 -0400, you wrote:
>93 Eurovan diesel
>not sure exactly what's going on
>couple weeks ago I had some kind of short from ignition switch to  
>starter relay in panel
>wire was getting hot, meltin insulation, smoking
>some guys suggested worn brushes in starter (starter was new Bosch  
>rebuild 1 yr ago)
>I decided to bypass ignition switch and relay - installed a manual  
>starter switch from battery directly to starter solenoid
>this worked great until today
>now no matter what I do I can't get anything to happen at starter  
>solenoid
>even direct line from battery - WITH battery charger attached - gives  
>nothing
>starter does not even lurch or try to turn over once
>I see very weak spark where wire touches solenoid contact, nothing more
>is this possibly the heavy braided postive lead from battery to  
>solenoid?


Here's your answer:

I remember you had this starter all apart a year or so ago. Perhaps
something inside has gon bad (brushes sticking or worn), or if the switch on
the back of the solenoid was bad, the solinoid might pull in but the motor
not crank, resulting in excessive current throught the ignition switch.

The "suck" coil is designed to get out out of the circuit in a fraction of a
second, leaving the "hold" coil to keep things engaged while cranking. If
the suck coil stayed energized, the current it draws would set the wiring on
fire and fry the ignition switch.

The only way I could see this happening is if the motor side of the solenoid
got grounded, but in that case, the starter would not crank.
Seems your problem is from the motor side of the solinoid, toward the motor,
and the brushes inside. one of them may have grounded out.

I think I already sent you my starter R&R doc last year, but can send again
if helpfull.

Sandy

*********************************

Driving the A3, stopped at the local coffee shop for a cuppa yesterday, and
when I came out it would not start. No solenoid click, no interior light
dim, very strange. Tried a hotwire from the battery to the solenoid control
terminal, no click, small spark.

Got a husky friend to push it down the parking lot and bump-started it
(always bump-start in 4th gear, 5th is not strong enough and may strip
teeth, and anything lower than 4th just results in a sudden stop) and went home.

Remembered that most, if not all solenoids have 2 coils in them, one very
strong to suck it in to mesh the pinion with the ring gear, and another
higher resistance to hold it there while starting. The solenoid core also
pushes a copper washer against the back of the 2 large terminals (trust me,
you can't get at them, it's swaged together) completing the circuit from the
battery to the motor.

The less obvious thing is that the "suck-in" coil is not grounded at one
end. Here's why.
It draws a lot of juice and is best switched off as soon as the core is far
enough in to mesh, and close the heavy switch.
So....... One end of both coils are connected to the actuator terminal
(comming from the ignition switch).

The other end of the "hold" coil is grounded.

BUT,  the other end of the "suck" coil is connected to the big terminal that
goes to the motor.

How it works:
When the key is turned to START, +12 volts is applied to the start terminal
of the solenoid.
current flows through the hold coil to ground as long as the key is held to
start.
Current also flows  through the "suck" coil, and this is a trick,  THROUGH
THE MOTOR  to ground.(a very low resistance path)

When the suck coil does it's job, and the solenoid core bottoms and the
copper disc applies +12 volts to the motor lead,  both ends of the suck coil
are at +12 volts so no more current passes through it, and only the hold
coil keeps the starter engaged untill the key is released.

Of course, as this is happening, +12 is applied to the motor and it is cranking.

How come +12v from the main battery cable does not go back through the suck
coil from the motor terminal, to power the hold coil when the starter switch
is released? 
It does, for an instant.
When the key is turned to start, both coils are powered, together, in the
same polarity, aiding each other, to pull the solenoid core in.
When the key is released, the current comming back in the opposite direction
from the motor terminal through the suck coil is now reversed, and the suck
coil magnetic polarity is reversed,  canceling the polarity of the hold
coil, neutralizing the magnetisim and releasing the solenoid, breaking the
circuit (this happens almost instantly). Very clever. That was a trick used
in the old electro-mechanical generator cut-outs before alternators and
solid state, that disconnected the generator from the battery when the
engine stopped.

Until yesterday.............

Now, the rest of the story.

When I got in to it today, I pulled the starter, thinking I had solenoid
trouble. (see starter R&R, archives)

However, it's a 94, Hecho en Mexico, Mexican wire, not used to the great
white north, cold, polar bears, snow and ice, and road salt.

The formerly thick piece of cable from the solenoid to the motor disolved in
my hand when I touched it. No current through the suck coil, (could not
reach ground through the motor) no click, no action, tiny spark at the start
terminal caused by the small current through the hold coil

Replace the wimpy wire with a chunk of a booster cable hanging on the  shop
wall, soldered connections made by propane torch. Gave it a quick test on
the bench (quick, because there is no front bearing when it is out of the
car) and it worked. 
Re-installed, and all is well.

Note that worn out brushes, an open armature segment, or other motor fault
that opens the circuit, can result in similar symptoms.

Don't I just love to stiff the dealer parts man the cost of a new starter,
for a simple stupid piece of too-small, NAKED AND UNPROTECTED wire!
It's in my genes.

Sandy