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Ignition timing: The forgotten test

Monday, April 1, 2019 - 07:00
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Do you own a timing light? When was the last time you used it? Old guys like me know what it is, but do younger technicians have a clue? To be fair, the answers to these questions vary based on geographic location. I am in the Chicago area and I cannot remember the last time I used a timing light. Salted roads, rust, emissions testing and “cash for clunkers” eliminated 99 percent of the vehicles that required ignition timing adjustments in my area. I own a fancy timing light but it probably has a thick layer of dust on it… if I can even find it. If I lived in an area like Phoenix or San Diego, this story might be completely different. Environmental/geographic issues such as this can often result in a 2018 vehicle in one bay of a shop while a 1976 vehicle could be right next to it. Not in Chicago! I rarely see something older than 1996, but I still own a blue wrench just in case. Regardless, ignition timing is a very important aspect of engine performance.

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The demise of timing marks

Today’s engine applications usually do not offer technicians a method of checking ignition timing because it is no longer adjustable. Most frequently there are no timing marks on the crankshaft pulley and no spark plug wires to connect our timing lights to if we even wanted to perform such a task. To muddy the waters even further, manufacturers no longer provide base ignition timing specifications. Does this mean that ignition timing is any less important? Of course not. My point: ignition timing can be incorrect, even though it is not adjustable by a technician, and subsequently cause driveability issues.

Figure 1 - A distributor, or an “octopus,” on a 1970 Challenger R/T.

For those of you who are green, or entry level technicians, let me paint this picture. There used to be a device on the engine that looked like an octopus (Figure 1). This “octopus” was actually called the distributor. It had ignition cables that plugged into the spark plug for each cylinder on the engine and often had an additional wire that connected to a single ignition coil. Yes, believe it or not, only one ignition coil. The octopus’ job was to distribute spark at the appropriate time to each individual cylinder. In order to do so the distributor, or the head of the octopus, needed to be installed correctly. It could be turned, in one direction or another, to establish base ignition timing. From there the engine computer would take over and advance or retard ignition timing, or when the spark fired, based on operating conditions at that moment.

A side note that should be addressed: For a few years in the mid-1990s and early 2000s, distributors existed on vehicles, but ignition timing adjustments were not possible. Even though it was possible to turn the distributor, this only effected camshaft sensor timing. This adjustment affected injection timing but not ignition timing. Ignition timing was now based on the crankshaft position sensor input to the PCM.  An example of this would be a General Motors 5.7- and 5.0-liter engine as recently as the 2000 model year and the General Motors 4.3-liter engine all the way up to the 2004 model year.

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With the introduction of DIS (Direct Ignition Systems) in the early 1980s and COP (Coil over Plug) ignition systems shortly after that, the octopus became obsolete. As a result, ignition timing adjustments became obsolete as well. These changes did not mean that ignition timing was any less important, it just became a non-adjustable part of the technician’s service procedure because mechanical components were eliminated and computerized ignition controls took charge of all ignition functions.

Gone but not forgotten

Now that the history lesson is complete, we come to how ignition timing is controlled on a modern vehicle. Ignition timing on almost all modern vehicles is based on the crankshaft position sensor input. The aspects for the operation of a four-stroke engine are still the same as it always has been, including ignition timing, and service information has kept up pertaining to most areas as engines have changed and advanced. However, service information lacks when it comes to the important variable of ignition timing. Because ignition timing is non-adjustable on modern vehicles the engineers designing the vehicles, and the individuals writing the service information, do not give us technicians all of the information we may need because ignition timing is something “we should no longer mess with.”  Allow me to share a story that illustrates the need for ignition timing specifications.

An early 2000s Ford with a 4.2-liter V-6 engine is in the shop for a low-power issue. The shop had already used the usual shotgun approach and replaced the fuel pump, fuel filter, mass airflow sensor, entire exhaust system (everything except the exhaust manifolds), camshaft position sensor, spark plugs, ignition wires and coil pack. In a very inefficient and costly way, the shop covered most of the bases for a low power issue. Upon my arrival at the shop, a test drive of the vehicle confirmed that the low power issue remained. A double check of the parts/components that were replaced was performed and no faults were found. What was missed?  Was ignition timing checked? Us old guys know retarded ignition timing can cause a very similar feeling drivability result but, as stated before, there were no timing marks or specifications for the checking of ignition timing. What do we do next?

Figure 2 - A worn keyway allowed a skewed CKP signal that results in retarded ignition timing.

A quick test of ignition timing, using some modern techniques (to be addressed shortly,) revealed that the ignition timing was in fact retarded. Because the ignition timing is based on the crankshaft position sensor signal the CKP reluctor was the next thing on the list to check. In this case the CKP reluctor was mounted on the crankshaft pulley. Removing the crankshaft pulley revealed a worn keyway that allowed the crankshaft pulley to shift (Figure 2). This shift resulted in a CKP signal that was late. The late CKP input signal to the PCM resulted in a late, or retarded, ignition timing trigger signal to the ignition coils. The only thing that was required to resolve the low power issue on the vehicle in question was a crankshaft pulley. The new pulley resulted in an accurate CKP signal to the PCM and consequently a correct ignition timing command.

My point of this whole story is that technicians nowadays, seasoned techs and green techs alike, overlook ignition timing because it is “non-adjustable.” Technically it is not adjustable, but it can change… if something is broken.

Checking ignition timing without a timing light

So how do we check ignition timing you may ask? A few paragraphs ago I referred to a “quick test” to check ignition timing on a modern vehicle. With the appropriate equipment, and knowledge of how engines work, this is actually an easy task. There are two methods that I am aware of that can be used to check ignition timing. Both of these tests require an oscilloscope. In addition, a high current probe and/or a pressure transducer will be needed. The current probe or the pressure transducer will provide a top dead center reference. Another channel of the scope will be used as an ignition reference and can be accomplished in a variety of ways depending on vehicle application and available scope probes. The first technique is a “ballpark” test and the second technique is much more accurate than the first.

Method #1:  Relative compression in relation to sync

Relative compression involves connecting a current probe around a battery cable, disabling the fuel system to force a crank no start condition and using some type of ignition sync. The engine is then cranked over and the starter motor’s current peaks can be observed. The current peaks equate to the higher effort required by the starter motor to compress the contents of each cylinder. Equal current peaks indicate that all cylinders have equal compression. For our discussion today, the ignition sync should fall near the apex of one of current peaks in the capture. This technique is not exact, but can give us a pretty good idea if ignition timing is close. Think about it — during cranking, most engine applications use base ignition timing. If we use what we have learned from older vehicles, calling on you seasoned technicians, the base timing should be (most likely) somewhere between O degrees to 10 degrees BTDC (Before Top Dead Center). This means that the ignition sync should occur very close to one of the current peaks or slightly to the left of the relative capture. If the ignition sync falls too far to the right of the current peak then the ignition timing is retarded. Conversely, if it falls too far to the left, the ignition timing is advanced.

Figure 3 - The ignition sync falls well to the right indicating retarded ignition timing.

The following relative compression capture (Figure 3) is from a 2002 Ford Mustang with a 3.8 liter engine. The vehicle barely ran and the relative compression capture explains why — Ignition timing is severely retarded.

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