9 steps to easily diagnose ignition problems

If there is any ignition component that gets an unfairly bad rap, it’s the coil. Coils are very quickly blamed for ignition problems, yet almost universally they are not at fault. Coil manufacturers must love this, however, as it sells a lot of coils.

We discussed the theory behind an ignition system in our last issue, and now it’s time to troubleshoot common problems. Ignition problems generally fall into two categories: Things that cause the car to not run at all, and things that make the car run poorly.

A 12-volt test light is your best friend when diagnosing ignition problems that keep a car from running. If your car still has points, a tach/dwell meter and a timing light—preferably a dial-back type—are pretty important cousins to your test light when it comes to figuring out why it’s running poorly. 

A little common sense will also go a long way. Always keep your fingers and other appendages clear of a spinning cooling fan—if it’s thermostatically controlled, remember that it can turn on when the engine is off—and don’t forget that a coil can pack a wallop.

Let’s start with a diagnosis plan that should uncover most ignition problems found.

If the car won’t run...

Is There Spark at the Plugs?

When looking for troubles, it’s usually best to start at the spark plug-end of the ignition path and work backward. The first thing to do is to pull a spark plug wire while an assistant cranks the engine and check for spark. Before having the assistant crank the engine, insert a spare spark plug or screwdriver into the plug boot and then position the plug wire near a ground source.

If you see a strong spark, keep trying this with all the plug wires. If there’s a good spark at each wire, you’ve either got a fuel problem—remember that 5 percent of all carburetor problems are actually carburetor problems, after all—or you’ve got your timing set way off. There’s also a chance that you’ve got your plug wires on in the wrong order.

Is There Spark From the Coil?

If you don’t have spark at the plug lead, the next thing to check is whether you’re getting spark from the coil to the distributor. Pull the coil wire from the distributor and hold it near a ground. Again, have your assistant crank the engine and look for spark from the coil lead.

If you have spark, you likely have a problem with your distributor rotor or cap, so you may want to replace them. In recent years, rotor problems have been much more common than cap problems, so you probably want to start with a rotor.

Is There Power to the Coil?

If you don’t have spark from the coil lead, you’ll need your 12-volt test light for the next test. Attach the ground clip of the light to a good ground, and then touch the point to the positive side of the coil while the ignition switch is turned on. (This is assuming a negative-ground car.)

The light should glow. If not, you’ll need to trace this part of the ignition circuit to determine why the coil isn’t getting power. (Note: For positive-ground cars, switch the polarity on this test procedure.)

Does the Switching Mechanism Work?

If you have power to the coil, hold the pointer of your light to the negative side of the coil while your assistant cranks the engine. Your light should flash on and off as the engine spins over, telling you that the switching mechanism in the distributor is working. (This is true whether the car has points or an electronic ignition.)

If the light glows steady or not at all, it’s time to get into the distributor. If working with a positive-ground car, don’t forget to switch the polarity on this test procedure as well.

To determine why your distributor isn’t providing the switching for the coil, you’ll need to get out the manual for your car or ignition system (if it’s an aftermarket system) to go through the testing procedure for your points or electronic switching mechanism.

If the car runs poorly...

How Are the Plugs?

As obvious as this sounds, pull the plugs and make sure they’re the right ones for your engine. Also make sure they’re not wet, oil-soaked, or just plain gunked-up.

If they look bad, don’t worry about it just yet, but either replace them or thoroughly clean them. Rarely will plugs be bad, but they need to be the correct ones and in good condition for many of our subsequent tests.

Is There a Strong Spark?

If your car is hard to start or runs poorly when it is starting, the first thing to check is spark quality. Using an assistant, repeat the previous test for spark at the plug wire. Do you see a sharp spark that will jump a gap of at least a quarter inch? Can you see it in broad daylight? Can you hear it crackle? If not, it’s probably too weak as the result of electrical losses along the spark food chain.

The easiest way to start looking for these losses is to reconnect all the plug wires and start the engine. Ground your 12-volt test light and then use the pointer to follow each plug wire from the distributor cap to the plug—and don’t forget to check out the ends of the plug wires, too.

If at any time you get a spark from the plug wire to the test light, you have a bad or dirty plug wire that is giving the spark a second path to follow to ground. Make sure to test the coil lead as well. (If the test light briefly illuminates while following the plug wires, that’s also a sign that the wires aren’t doing their job to contain the electricity.) If the plug wires pass the test, move the pointer of the test light around the distributor cap and see if you can catch any stray sparks. Then repeat the test around the coil, especially at the top, where the terminals are located. If everything still passes this test, take a spray bottle filled with water and mist the plug wires, the distributor cap and the coil. Then repeat the test. The water may help bring out some stray sparks—in fact, you may not even need the test light once things are a bit damp.

Is the Distributor Shaft Worn?

Remove the distributor cap and rotor and wiggle the distributor shaft. If you can feel any slop at all, the shaft or bushings in the distributor are worn.

If you’re still using points, this will result in inconsistent point gaps and timing issues as a result. Worn distributor shafts cause fewer problems for electronic ignitions, but either way you should rebuild or replace the distributor if you want the best drivability and performance out of your car.

Good Condition and Set Correctly?

Points are the switching mechanism for the spark, and if they are worn, corroded or not set correctly, poor running will follow.

If your car is still using points, visually inspect them and look for a clean, smooth surface. If you see corrosion or pitting, replace them. Set them to the gap or dwell angle specified for your car and then set the timing again.

Is the Timing Properly Set and Consistent?

Make sure you’re properly setting the distributor’s timing and that the advance mechanism is working. When checking the timing at idle, are you disconnecting the vacuum advance as the manual says? Are you using the right timing marks on the timing tab? Is the advance going through its curve properly and achieving the proper amount of advance at high engine speeds?

Make sure you check all of these things before getting more drastic and blaming the carburetor. We discussed how to properly set a distributor’s timing in our last issue.

When you set the timing, whether at idle or higher engine speeds, you should see a very steady, consistent reading at the timing mark. On engines with worn distributors, you may see a variance of as much as 10 degrees. If the timing is bouncing all over the place, it’s time to rebuild or replace the distributor before the engine runs well.

Problem Fixed?

Even though there’s a lot going on regarding a car’s ignition system, some detective work can usually solve the problem. Before blaming the carburetor for a poor running engine or replacing a bunch of hardware, first check all of the basics. A methodical approach should uncover the problem.

While a stock ignition system may be fine for a car that’s still original, we know that some people crave more performance—plus some modern equipment can make a classic a bit more friendly to drive. Coming up soon, we’ll take a look at aftermarket ignition upgrades for both street and race.

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distributorguy New Reader
7/11/18 10:53 a.m.

You should also test the spark plug wires.  0- 5000 Ohms is a good range for the plug wires to operate within.  Anything above 5000 Ohms per wire will stress the coil and force other failures in the HT (10kV+)system.  They can even lead to misfires which can destroy an electronic ignition or a condenser in the LT (12V) triggering system.  

sir_mike New Reader
1/7/20 3:09 p.m.

You're correct about the coil and starting problems but if the car does start the coil can still be bad.I was having a problem with my 68 Cortina GT.Start fine and idle but driveability not right.Hard to pull out on hills or even slight grades.Would have a miss about 3500rpm's sometimes.Sluggish on hills et.Checked timing,plugs wires all fine.Tried some re jetting of the twin Webers...nothing changed.Then one day when it starting missed I looked at the tack and it was going crazy.Needle bouncing all over the face.Electrical I thought....but what.Then i remembered what a local drag racer told me about his cars issue.Was using a Pertronix coil about 3yrs old.Changed to a  very old...25yrs at least...Lucas Spot coil I had on the shelf...problem solved.All the issues mentioned above gone.Runs like she should now...So that Pertronix went in the trash.Had the same on the other Cortina so changed that to a new Lucas one just in case it failed sometime.So in my case a duff coil was the performance/driveabilty problem.Sorry for the long post...

1/7/20 5:34 p.m.

Cannot believe the Condenser wasn't mentioned! If you are still running points and a condenser, replace the condenser first.  Much cheaper than a coil!

sir_mike New Reader
1/7/20 7:10 p.m.

In reply to Vintageant :Used to carry a spare set of points/condenser back in the past.Especially since my Cortina had a Ford dist.that ate condensors.Now with Lucas dist in both I use Pertronix ignitor's.


britsportscar None
1/8/20 3:31 a.m.

Recently returning to UK from Florida with my trusty TR6 , wouldn't start after standing in damp/cold for a couple of weeks even though it had been running sweetly previously.

Before pulling everything apart I remembered previous UK experience and removed distributor cap, sprayed inside liberally with WD40, sprayed HT leads and plug caps.

Fired up straight away.

Condensation in engine compartment often an easy fix in cold damp climates !

1/8/20 1:48 p.m.

Good article, with simple tips.

I remember visiting Jamaica in the 2000's and on my travels one day, I saw a family standing around their car with hood open.  I pulled over to assist, they said car won't start, just quit. First, I  asked do you have fuel in the car and they said, yes.  Secondly, I did my spark test, no spark at plugs, then off to the distributer. Popped the distributer off and the rotor had somehow split, where it connects to the shaft, problem solved (rotor had fallen off, firts time I saw that issue).

1/28/21 1:22 p.m.

I had an interesting ignition failure several years ago. I was driving my car when the engine died with no warning. I coasted to a stop and took a look in the engine compartment. I found a spark plug lead had come off the distributor and put it back on. I didn't expect that to fix the problem and it didn't.

I had the car towed to a friend's house (I was 800 miles from home) and set about diagnosing the car. The engine was getting fuel. There was what looked like a good spark at all four plugs. The timing was correct.

On the third day of working on the car, a bunch of friends were over helping and it was starting to get dark. I was trying to start the car when someone noticed arc'ing on the side of the block. On this engine, the coil is at the front of the block and the distributor is driven by the rear of the intake cam, so the coil lead runs along the block between the two.

I took a look at the coil lead and there was a pin hole on its side where the arcing was observed. I put a piece of electrical tape over the pin hole, tried to start the engine, and it fired right up.

californiamilleghia SuperDork
1/28/21 2:56 p.m.

how do you bench test a coil ?


stu67tiger Reader
1/28/21 4:05 p.m.

I had a new very early first year Jetta, 1980 I think.  After not very many miles it startred running rough. A bit of troubleshooting found that the points were burned.  Filed them a bit to get home.  Replaced by the dealer, they burned again.  OK, so what's going on?  I opened the distributor and poked around,  wanting to find something to blame.  The cap and rotor looked OK,  plug wires ok, the wiring to the coil was ok, but suddenly I realized what it might be.  No internet in those days, so at the VW dealer, I asked the service manager what the square thing on the side of the distributor was.  The condensor for the points, he replied.  OK, so what's the cylindrical thing inside the distributor?  He popped the hood, opened the distributor.  Oh my, or words to that effect.  Somehow the early Jettas ended up with two condensors of the proper value, both wired into the circuit.  That can damage the points just as bad as no condensor.  He clipped the lead on the external condensor, no more problems.

frenchyd UltimaDork
1/28/21 5:25 p.m.

In reply to Carl Heideman :

My favorite item is a pen like tester. Lay it on a plug wire and if the window glows the plug is firing. Fastest way to check all 12 plugs/ wires. 

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