Is my Coil Bad?

96 Chevrolet C1500 1/2 ton truck, 4.3l 6 cyl

Hi, people

I am slowly trying to run down the cause of an intermittent misfire on
the truck above. It gets warm, then the MIL comes on and it starts
running bad. I have replaced plugs, wires, cap & rotor. The ignition
module checked out OK. Can't find any vacumn leak by physically
observing. Now I am checking the coil. According to the Wells manual at
Auto Zone, the secondary should read 5000 to 25000 ohms and the primary
should read .1 ohm.

When the coil is cold it reads as follows:
Primary: .1 @ 200 meter setting
Secondary: 5.86 @ 20k setting

Engine at operating temperatu
Primary: 2.9 @ 200 meter setting
Secondary: 6.33 @ 20k setting

The primary resistance is 3 times what is expected when the coil is
hot. Does this mean it is really good or really bad?

Thanks,
Libby

If the MIL came on, it set codes. The easiest thing to do and usually
the cheapest thing is to read the codes to find out just what the
computer 'thinks' is wrong. A miss can be caused by a loose gas cap or
a loose dip stick on some vehicles and no amount of money or parts will
fix that....

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's

Libby Chantel wrote:
>
> 96 Chevrolet C1500 1/2 ton truck, 4.3l 6 cyl
>
> Hi, people
>
> I am slowly trying to run down the cause of an intermittent misfire on
> the truck above. It gets warm, then the MIL comes on and it starts
> running bad. I have replaced plugs, wires, cap & rotor. The ignition
> module checked out OK. Can't find any vacumn leak by physically
> observing. Now I am checking the coil. According to the Wells manual at
> Auto Zone, the secondary should read 5000 to 25000 ohms and the primary
> should read .1 ohm.
>
> When the coil is cold it reads as follows:
> Primary: .1 @ 200 meter setting
> Secondary: 5.86 @ 20k setting
>
> Engine at operating temperatu
> Primary: 2.9 @ 200 meter setting
> Secondary: 6.33 @ 20k setting
>
> The primary resistance is 3 times what is expected when the coil is
> hot. Does this mean it is really good or really bad?
>
> Thanks,
> Libby

The coil doesn't set a code of it's own. The code is P0300 - multiple
misfire, which a bad coil could cause.

There should be a hot and a cold reading on the coil. For instance mine
calls for 1.13-1.23 ohms cold on the primary and 1.5 ohms hot. This is
not a large jump. Yours has a large jump so I would be suspecting it
unless the book calls for that much jump?

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's

Libby Chantel wrote:
>
> The coil doesn't set a code of it's own. The code is P0300 - multiple
> misfire, which a bad coil could cause.

So far I can't find the actual spec on that particular coil. guess I
will go to AZ and read their Haynes manual. What they had in the Wells
book just gave one value each for Primary / secondary, hence my
confusion.

Libby Chantel wrote:

> 96 Chevrolet C1500 1/2 ton truck, 4.3l 6 cyl

> > intermittent misfire
> > I am checking the coil. According to the Wells
> > manual at Auto Zone, the secondary should read
> 5000 to 25000 ohms and the primary should read .1 ohm.
>
> When the coil is cold it reads as follows:
> Primary: .1 @ Secondary: 5.86K

> Engine at operating temperatu
> Primary: 2.9 Secondary: 6.33K

Probably, but if this is an expensive coil, consider getting it tested
with a coil ringer, which auto electrical shops and TV shops should
have, because it can detect even slight shorts in the windings that an
ohmmeter will miss.

As far as I understand, the primary coil should be anywhere from .5 to 1.5
ohms while the secondary should have anywhere from 6-10 thousand ohms
resistance. Resistance should increase with temperature but I can't imagine
you'd get a 29X change in resistance on the primary.

-Bruce


"Libby Chantel" > wrote in message
oups.com...
> 96 Chevrolet C1500 1/2 ton truck, 4.3l 6 cyl
>
> Hi, people
>
> I am slowly trying to run down the cause of an intermittent misfire on
> the truck above. It gets warm, then the MIL comes on and it starts
> running bad. I have replaced plugs, wires, cap & rotor. The ignition
> module checked out OK. Can't find any vacumn leak by physically
> observing. Now I am checking the coil. According to the Wells manual at
> Auto Zone, the secondary should read 5000 to 25000 ohms and the primary
> should read .1 ohm.
>
> When the coil is cold it reads as follows:
> Primary: .1 @ 200 meter setting
> Secondary: 5.86 @ 20k setting
>
> Engine at operating temperatu
> Primary: 2.9 @ 200 meter setting
> Secondary: 6.33 @ 20k setting
>
> The primary resistance is 3 times what is expected when the coil is
> hot. Does this mean it is really good or really bad?
>
> Thanks,
> Libby
>

Dude, it's way out of spec, right? Replace it. What's it going to be,
50 bucks or so?

The coil turned out to be ok, just FYI everyone.
I appreciate all your help.
Libby

My info says the p300 code is usually caused by a lean condition. Too
much air, maybe a vacuum leak or not enough gas. If you have the cfi
fuel system, they are noted for problems.

If you think it is an ignition issue, I have heard of using a
timing-lite to see the misfire, I've never tried it, I guess the timing
pulse will act-up.