[s-cars] coil diagnosing

Paul Gailus gailus at mindspring.com
Fri Jul 15 00:53:12 EDT 2005


A problem with using a Megger on our coils is that the
other side of the secondary winding is internally grounded.
So you'll be measuring the resistance of this winding
(plus that of the internal blocking diode that's in series with it),
instead of the insulation resistance.
However, it should work fine for testing "waste spark" coils
like those used on the 2.8L V6's since both ends of the secondary
winding are floating.


----- Original Message -----
From: Rich Assarabowski Sent: Thursday, July 14, 2005 9:29 AM
Subject: RE: [s-cars] coil diagnosing


> Paul:
>
> The only way to test them is with an insulation resistance tester often
> called a "Megger" (brand name).  Basically you're looking for a low
> resistance path to ground (insulation breakdown) which occurs when you
apply
> high voltage.  You can't do it with an plain old ohmmeter since an
ohmmeter
> only applies 9V from it's internal battery instead of the 1000's of volts
> you need for insulation to break down.  The dielectric testers apply a
high
> voltage and they measure resistance to ground at these conditions.
>
> Shops which repair electric motors or alternators use these.  Too bad, I
had
> one of them available in my previous work place...  You might try taking
the
> cover with all 5 coil packs to a shop (there's an automotive electrical
> place on Main St. in E. Hartford) and see if they're willing to do it.
You
> have to be careful in how they test it, as you can also ***damage***
> insulation if you don't do it right :(   Without knowing the voltage
rating
> of these coils it's a bit of a risk...
>
> Only other way is trial and error one by one (ugh!).  The bad one I pulled
> out in mine looked no different than all the others, the shorts are all
> internal.  Oh, and it checks out fine with an ohmmeter.  FWIW mine was the
> #5 coil.  It was easy to find since it was missing at idle (I used the
"pull
> the injector" trick).
>
> -- Rich A.
>    '92 & '93 S4
>




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