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Re: Spending Quality Time with a 75 Midget

To: mgt mailing list <mg-t@autox.team.net>
Subject: Re: Spending Quality Time with a 75 Midget
From: Bullwinkle <yd3@nvc.net>
Date: Tue, 09 May 2000 00:10:23 -0700
> Does it sound like I replaced the coil with a new one that is bad? Or does 
> the Ballast resister sound bad?

I haven't worked on the 75 midget, or any car with a separate
ballist resistor and electronic distributor, but from what I
understand of the way many of these systems work, you could check
the following.
 
Normally, a ballast resistor is used to cut the voltage to the
coil during running.  When the engine is started, the resistor is
by passed to provide more voltage to the coil for starting.  That
means if the ballast resistor is bad, the car will start, but
when you release the key to the run position, it will die.  That
is unless something else is wrong in the starting ciruit.

If the ballast resister is in the circuit between the distributor
and the coil, you could temporatily place a jumper across its
terminals and see if the car runs.  If it does, then the resistor
is bad.

> Oddly enough, I have 12v on both sides of the coil.  Not sure why, other then 
> the negative side of the coil has two leads.  One lead is from the TACH which 
> has 12v going to it from the fuse block.

You most likely will have if you're checking for voltage and the
key is on.  If you want to check for an open circuit you need to
disconnect the wires.  Then set a mulitmeter to resistance, and
check between the non tach lead and a good ground, you should get
about an infinite resistance reading.  If not then the coil has
lost its ground.

You should also be able to test the distributor to see if it is
sending the proper signal to the coil.  Connect the positive lead
of a multimeter to the wire going from the distributor to the
coil.  Attach it first at the distributor and the black lead of
the meter to a good ground. Set the meter to the 15 volt scale,
and have someone crank the engine while you watch the meter.  If
the needle doesn't swing to 12V and back to zero every time a
plug should fire , then the electronic timing sensor in the
distributor is bad or for some reason it isn't sending the proper
signal to the coil.

The needle might not have enough time to go completely to zero,
but I think you should be able to see it flicker.

If you still can't tell, construct a small, or narrow, spark gap
(0.005"). Disconnect the possitive lead at the coil and place the
spark gap between the wire and a good ground and crank the
engine. You should get a series of small snapping sparks as the
distributor triggers.

You can check the coil tby connecting the meter between the
negative terminal of the coil and ground.  Cranking should result
in the same fluctuating needle if you were able to notice it. The
voltage here will probably be less.

You can also do the test between the positive terminal of the
coil and ground or any place between between the positive leads
going to the distributor and ground.  As the engine is cranked
over, the needle should swing up to 12 volts and back to zero
when the plugs are to fire.  If it stays at a constant twelve
volts, then the distributor isn't triggering the coil properly.

If everything I suggested checks out okay, then I would also
disconnect the lead from the tachometer to the coil at the coil
and see what happens.

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