Distributor electrical testing

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Scott in NM
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Distributor electrical testing

Post by Scott in NM » Sat Jul 24, 2021 8:21 pm

Jeep was missing a bit under load and had terrible acceleration so decided to check ignition components. Spark plug wires and plugs tested just fine, suspected condenser in distributor. Replaced condenser, engine would crank but not fire. Put old condenser back in, same result. No spark at spark plugs. What the ??????

Jeep refuses to start (cranks but no spark) although it did not have this problem prior to this. So how do I spark chase throughout the distributor to see where I'm losing electricity? All I did was replace the condenser. I have 24V to the + side of the coil, but unsure the best way to chase continuity throughout the distributor either with or without power. Anyone have a suggestion as far as tests to run?

Scott


Scott in NM
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Re: Distributor electrical testing

Post by Scott in NM » Sun Jul 25, 2021 7:48 am

Disregard previous post. Found something on coil testing that should put the distributor through its paces. Still confused why replacing a condenser would screw things up lik this though.

Scott

Scott in NM
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Re: Distributor electrical testing

Post by Scott in NM » Sun Jul 25, 2021 9:29 am

OK, here are my coil/distributor test results in all caps. Not sure what to think, would like to get some input but afraid of the answer. Entire jeep is as it came off the assembly line.

1. Remove distributor cap.

2. Disconnect the leads to the screw-terminals of the coil. These are the primary terminals.

3. Measure low tension side
a. Get a test-meter and set the meter to ohms (resistance).
b. Measure the resistance between the two primary terminals (the screw terminals).
c. Readings should show almost ZERO, just a few ohms at most (usually between 1.5 to 6 Ohms).
NO RESISTANCE.
d. If you get a higher reading, then there is an internal break in the coil windings and coil is bad.
e. Reconnect the leads to the screw-terminals

4. Remove a spark plug wire with spark plug assembly (wire plus plug).

5. Check the High Tension
a. Insert spark-plug with wire assembly lead in the HT outlet (the high-voltage central terminal of the coil) with the coil connected up and installed in the distributor, but with distributor cap removed.
b. Hold the spark-plug against a good clean metal part of the engine block (paint-free). Oil cap works well. Make sure you use insulated pliers or flight gloves to hold the plug against the block or your hair will curl.
c. Get a friend to turn the ignition switch to "on" and crank the engine.
d. You should see a good spark jumping the plug-gap. If not, then the HT -side of the coil is failing.
NO SPARK.
e. Disconnect the leads to the screw-terminals. Now test the coil HT-side by connecting one lead of the ohmmeter to EITHER of the primary terminals (the screw-terminals of the coil), and connect the OTHER lead to the secondary terminal (the high-voltage central terminal of the coil). The reading should be less than 20,000 ohms. Higher than this indicates an open circuit, a very low reading indicates a shorted internal winding. A reading of around 14,000 ohms is normal.
READS 6 OHMS.

6. If you get a spark with one spark plug wire and spark plug assembly, it is also a good test of all the wire and plugs. If none work, then the issue is still with the HT side of the coil and the coil needs to be replaced.

7. If the coil readings are within the specs but vehicle won't start after running, or cuts out when running, test the coil immediately..... an internal-wire breakage is most likely when coil is warm or hot.

My conclusion - Coil is bad. However, brand new coil with at most 40 miles on it.
My question if bad coil - How does replacing the condenser burn out a coil? This can't just be coincidence!

Anyone with any thoughts?

Scott

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Re: Distributor electrical testing

Post by jnissen » Tue Aug 03, 2021 10:41 pm

Some better digital multimeters will have a capacitor test mode. Remove the condenser and measure the capacity. If it’s really bad this test should be way off or unable to run.

Capacitors typically fail by shorting out internally. A dead short should result in high coil current with no spark. The spark is generated when you interrupt the coils current. If the condenser is continuing to allow current to flow then the coil never generates the high voltage necessary for a spark. It bypasses the points in this case.

By installing a good condenser then as the points open up the capacitor minimizes the erosion across the points from the higher voltage as the primary field collapses.


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1955 M38A1
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