In Front of the Flywheel - May - 2017

Crank and Cam Part III… The Horror and the Good

Have you ever replaced a major component, such as a cylinder head or an engine, and then, after you thought your repair was complete, the vehicle wouldn’t start? Maybe you installed a remanufactured engine or one from a salvage yard; it doesn’t matter. Maybe you bought a used cylinder head to save the customer money. Or maybe an engine remanufacturer installed the timing chain incorrectly.

Whatever the case, it happens. What we’ve covered over the past two months is the precursor to the oddball situations we’ll address today. These repair situations are usually man made… unintentionally caused during another repair.

In the last couple issues, we’ve covered a variety of crankshaft and camshaft sensor tests. These tests pertained to the sensors themselves, as well as crankshaft-to-camshaft timing issues.

In this issue, we’ll expand even further and address the nightmare issues that usually occur after a major repair. These problems are the horror that’s usually man made, and the good you can access to alleviate the horror. These issues are more common than you may realize.

For our first example, we have a 2002 Dodge Durango with a 4.7-liter engine. A used-car dealer purchased the vehicle at auction, with plans to repair and sell it. Many used car lots do this and sometimes these situations provide the most interesting diagnostic dilemmas.

According to the dealer, the vehicle had an engine that had “come apart,” so someone installed a salvage yard unit. The first salvage yard engine was also bad, so they replaced it with a second one. Now the truck, on its second replacement engine, cranks but won’t start.

The first step is to verify the customer’s complaint and make some observations. The engine cranks, and it seems to try to fire once in a while; when it does, the engine stops cranking momentarily, almost as though it has over-advanced ignition timing.

Of course, the battery is connected to a charger because the vehicle has been cranked to death. As a result, the MIL is off and there are no DTCs stored. Where do we go now?

With a salvage yard engine, mechanical issues are always possible. But we can’t rule out fuel or ignition problems. In this case, we perform a relative compression test to determine whether the engine has a mechanical problem. If the engine passes a relative compression test, then we can take a mechanical fault off the table. We’ll also use an ignition sync to get an idea of spark timing, or to see if it even has a spark (figure 1).

The blue waveform shows starter motor current. The starter current is slowly moving up and down, due to the erratic cranking speed, but no single cylinder seems to stand out as being lower than the rest.

The red waveform shows primary ignition voltage. This is the waveform that raises everyone’s eyebrow: First, it appears to multi-strike the ignition coil, which is possible on a Ford, but a Chrysler?

Second, if we count the compression humps between the ignition firing events, we come up with about eight engine revolutions between those events. On an eight-cylinder engine, there are four compression strokes per crankshaft revolution. Eight compression strokes, or two revolutions of the engine, would require only one ignition firing event. Why is this happening?

Ignition timing on most modern vehicles is based on the crankshaft position sensor’s input to the PCM. Since the ignition coil is firing we have — at the wrong time, but it’s firing — we’ll change the scope connections to view the crankshaft and camshaft position sensors’ signals.

While consulting service information, we discover that there are two sets of wiring diagrams for this vehicle: One is for a Durango equipped with a Jeep truck engine controller (JTEC) while the second is for a next generation controller (NGC) Both diagrams are for the 4.7-liter engine (figure 2). There must have been a mid-year change on this vehicle.

Our Durango is equipped with a JTEC engine controller, which we confirmed with a quick visual inspection. We make the proper connections and then crank the engine again (figure 3).

Channel C (green) is the camshaft (CMP) sensor and channel D (gold) is the crankshaft (CKP) sensor. As you can see, both sensors are outputting clean digital signals and the voltages are appropriate. But are the signals correct? Here is where we need a known good capture to compare with our test results.

Known good captures can be a challenge. There are many ways to obtain them. The three most common methods are:

  1. your own database
  2.  a web site
  3. a buddy

Your own database requires time, testing, and good organizational skills. A professional web site can be an option, if you can find one with the information you need. Finally, a fellow technician may have what you’re looking for. In this case, we consulted the International Automotive Technicians Network, or, to find a known good waveform (figure 4).

This waveform came from a 4.7-liter engine equipped with a JTEC. Upon initial observation, the crankshaft position sensor (blue) doesn’t match the signal we captured. And, even though the camshaft waveforms look very similar, they don’t match either.

Count the pulses from the camshaft sensor signal. The pattern from the engine installed in our vehicle is 1-2-3-2-2-1-3-1. If we repeat the same count on the JTEC capture, we come up with 1-2-3-3-2-1-3-1.

A little more research and we came up with a capture from a 4.7-liter engine equipped with an NGC. The crankshaft and camshaft sensor signals from the NGC application matched our vehicle’s signals exactly.

So what happened? The engine from the salvage yard was wrong for the vehicle. The used car lot wasn’t happy to hear that, since it meant that we were going to have to replace the engine with the right one to solve the problem.

It’s hard to point a finger at the salvage yard in this case. Think about it… they pulled a 4.7-liter engine from a 2002 Durango and labeled it accordingly. How were they to know there was a difference?

The owner of the shop asked if the flywheel could be replaced to solve the problem, like on an older Chrysler product. Of course it couldn’t: The crankshaft reluctor is inside the engine and the camshaft sensor signal is different, too.

The next question was a bit frightening: “Can we just replace the computer?” Basically, swapping modules on a Chrysler product probably isn’t the best idea. Second, the connectors are different. Third, we’d need to swap out the entire vehicle harness to accomplish this task… and no one wants to open that can of worms.

There are a handful of tests you can use for a variety of different situations. Scoping the crankshaft and camshaft sensors is one of those tests. The horror of the situation is often due to man-made failures. The good is the knowledge and tooling to identify the issue quickly in the future.

Adding this test to your arsenal, and performing it early in your diagnostic process, can greatly increase you efficiency and alleviate a lot of diagnostic pain.

Do you have engine or electrical diagnostic issues you’d like to see addressed in the future? Let Scott know. Email and you just may have your question covered in a future issue of GEARS.