[DML] RE: The Flickering Tach, High Hunting Idle Challenge

[doctorDHD@xxxxxxx]

John,
 
I'm not exactly sure what is wrong but I don't think it's the idle ECU any  
more.
 
 With a 2700 ohm in series with the center conductor of the shielded  wire 
going to the pickup coil the cars runs great.  No more tach  flickering, no more 
fast idle, no more idle hunting.  The question is  why?  I have two ignition 
ECUs.  The original one and one I bought  from Josh H.  There is no difference 
between them.  Could they both  have the same fault? Should I try a third? 
The resistor seems like a nice fix  for less than 10 cents but it bugs me as to 
what is exactly wrong.
 
The pickup coil reads 579 ohms cold and there is infinite resistance from  it 
to ground, ie. no shorts to ground.  When I look at the pickup coil  signal 
on a scope it looks distorted, ie not a very smooth sine wave.  When  I put the 
resistor in series the wave gets a bit smaller in amplitude as you  would 
expect but it also cleans up a bit.   Is it this distortion that  is throwing off 
the ignition ECU to produce spurious signals that are messing  with the tach 
and the idle ECU?
 
I have uploaded two relatively small videos of the scope pictures.   You can 
see the difference in the signal when the resistor is in place. (You can  also 
hear the difference in how the car sounds.)
 
Can anyone put a scope on this signal and see how it compares to  mine?
(While there can you measure the amplitude as well. )
 
 At idle my signal was 10 volts peak to peak and it went to 40 volts  peak to 
peak at 5000 rpm.  Page 22 of the Technical Information Manual says  it 
should be 0.3V at idle and up to 100Volts "depending on engine speed." 
Something is amiss!
 
D² & 6530

Dave,  What makes you think the pick up coil is bad that it has to be
replaced. It  will normally read about 600 ohms cold +/- 5% then up to 700
ohms hot. It's  just a coil of wire on a plastic bobbin. If it's bad the car
won't  crank.
John Hervey







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