Tech Stuff

Ignition diagnostics you will actually use: Part 1

Truglia is the owner of Car Clinic, a state-of-the-art repair facility in Mahopac, N.Y. He is ASE A6 certified with a M.A. from Columbia University. (Vehicles diagnosed and waveforms captured by G. “Jerry” Truglia, Kevin Quinlan and Adam Varney.)

Years ago, vehicles required annual tune-ups. Because there was so much money to be made, shops invested tens of thousands of dollars in “big box scopes” to diagnose all of the ignition problems that were common in the day.

Now, let’s fast forward 20 years. Tune-ups are once every 120,000 miles on many vehicles. Many repair shops don’t even use a lab scope. Suffice it to say, ignition diagnosis is becoming a lost art.

Ignition diagnostics is not that complicated. However, we want to give you techniques that you can use now to diagnose cars quickly and make money in the process. We’ll start simple and work our way up. This month we will cover ignition diagnostics without getting into interpreting the waveforms. We’ll cover that next time.

Figure 1. The “rubber hose” cylinder ignition cut-out test.
<p>Figure 1. The “rubber hose” cylinder ignition cut-out test.</p>

Quick ignition diagnosis strategies

Trick #1: The rubber hose trick [Figure 1]

What does it do? Identifies which cylinder has the ignition-related misfire.

How do I do it? Place a short piece of rubber vacuum hose between the coil pack tower and the spark plug wire. This allows for easy cylinder shorting to identify a weak or dead cylinder. Use a grounded probe or test light to attract the spark through the hose. Never divert the spark for more than three seconds at a time to avoid damaging the catalytic converter.

CAUTION: Never touch the hose (if you do, it will be a very shocking experience).

Figure 2. Secondary ignition waveforms tend to show a slight ramping up of voltage before the spark jumps the gap unlike a primary ignition waveform.
<p>Figure 2. Secondary ignition waveforms tend to show a slight ramping up  of voltage before the spark jumps the gap unlike a primary ignition  waveform.</p>

Trick #2: Getting coil-on-plug (COP) secondary waveforms [Figure 2]

What does it do? Allows you to diagnose if a misfire is plug or coil related.

How do I do it? Install an ignition jumper wire that you can make up using a spare spark plug wire. Simply hook up to the spark plug wire you put in series between the COP and the plug like the way you are used to, and you have your secondary waveform!

If you find out it is the spark plug to blame, make sure that the OE spark plug was installed. Do not get brainwashed by the parts store voodoo that you can use different metal or brand plugs. Just stick with what works. Also keep an eye on whether the plugs were torqued properly and the right amount of anti-seize is used, since globs of it can cause misfires. If you want to get really picky, seemingly good spark plugs might have different internal resistances. If you do an ohms check on each plug, do the comparison game. The one that sticks out like a sore thumb is causing the problem. 

Trick #3: Quick ICM check

What does it do? Checks if the ignition module is to blame.

How do I do it? Try removing an ignition coil (one coil, two coils or all three), and connect a headlight. Use the alligator clips from the headlight and connect them to the coil pins (- and +).

This will load the circuit and make sure there is enough power and ground to fire a coil. Do not use a test light since there is not enough of a load. The test light may flash but that does not mean the coil will fire. The headlight requires 6-plus amps, while the test light only requires 3 to 4 milliamps to light.

This makeshift headlight tool has multiple uses. Use the headlight in the picture with a flasher to find a short in a vehicle. Just this time connect the alligator clips to the blown fuse terminals and get yourself a low current inductive amp meter or a compass to locate the short.

Trick #4: The ol’ spray bottle with baking soda

What does it do? Finds out if old spark plug wires are bad.

How do I do it? Try using some baking soda and water mixture (not corrosive like salt) to locate voltage leaks to ground. You can use a grounded screwdriver or a test light while spraying the mixture to be extra thorough, running the screwdriver or test light down the wire.       

Make sure that both the screwdriver and test light have a good ground. Don’t forget to keep your fingers off the metal of the test tools.

Trick #5: Know the capabilities of your scan tool

Did you know that you can use a scan tool to locate a misfire?

The GM Tech 2 utilizes a misfire graph.

The Ford IDS has a Cylinder Power Balance Test that helps isolate misfires.

The Ford IDS and most scan tools fitted with generic OBD II displays misfire data using Mode 6. Mode 6 often has misfire count for each specific cylinder.

In this example, the OTC Genisys scan tool displays Mode 6 Ignition data from a Chrysler vehicle.

The Mode 6 example below is from a Generic/Global scan tool from ATS (Automotive Test Solutions). It highlights misfires in red and translates the Mode 6 into language we understand.

The ATS scan tool decodes the hexadecimal data and displays Mode 6 data into useful information that we can read and understand. Not all scan tools do. If the data is not decoded you will need to access the OE’s website to look it up.

Trick #6: Be acquainted with the common pattern failures

A couple examples of this stick out in my mind. Ford ignition coils up into the late 2000s are problematic. If one is bad, you legitimately have a good reason to just go ahead and change all of them. Use quality products. I have found BWD and Wells ignition package coils are robust. I prefer using these over the OE Ford ignition coils.

Another example: In the mid-2000s, four-cylinder Nissan Altimas’ sporadic no starts are caused by defective CKP and CMP sensors. The OEM made them out of plastic instead of metal, and they just crack and take oil in.

CMP and CKP diagnosis

These days it’s not as simple as the CKP controlling just spark and the CMP controlling just fuel. On some vehicles one or both sensors are responsible to the PCM to determine spark timing. So, if we want to diagnose ignition problems on today’s vehicles, we are going to need to know how to diagnose CMP and CKP sensors.

Whether you’re working on a distributor or distributorless ignition, camshaft and crankshaft sensors fall into one of three general types, depending on how they create engine speed and position sensor signals: Magnetic Inductive (AC voltage generators), Hall Effect, and Photo-Optic.

That’s it. Three different types. If you can learn how to test these three you can test all crankshaft and camshaft sensors.

You do not need to learn a new procedure for each make and model on the road. Keep a couple basics in mind.

The crankshaft rotates exactly two times when the camshaft rotates once. However, this will not always be reflected when comparing the two waveforms. Let’s take the three different sensors in order.

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