I was called to a shop with a complaint of a check engine light on a 2004 Jeep Wrangler with a 4.0L having about 61,000 miles on it (Figure 1). This vehicle belonged to one of the mechanics at the shop, and he had recently experienced an engine shutdown with a no-start condition. A well-known problem on this Jeep engine is the crankshaft sensor fails, resulting in an engine stall. The shop mechanic found this to be the problem on his engine as well and replaced it. About a week later he experienced a check engine light on with a Code P0340. This code directed him to a possible failure of the cam sensor. He then purchased a new camshaft sensor from the dealer, along with a new camshaft trigger wheel and shaft assembly (Figure 2). He made sure to index the shaft housing and position the trigger wheel in the same position that the old one was in. It will be important to properly align the camshaft sensor to the crankshaft sensor. If the camshaft trigger wheel is not properly orientated, it could result in a cam sensor code being set.
The shop mechanic took the Jeep for a test ride; however, the check engine light came back on. It's frustrating enough when you’re working on a customer's car, but when it's your own vehicle it always becomes a personal challenge. When the mechanic returned to the shop, he proceeded to check the wiring from the cam sensor to the ECM. He did not see any issues such as a short or open circuit in the cam sensor harness. Upon not finding a wiring problem with the camshaft sensor circuit, the only alternative was the ECU, but this is expensive so he was reluctant to throw this in the mix. At this point, he decided to call me in to get a second opinion.
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Figure 1 |
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Figure 2 |
2, 4 or more?
When I arrived at the shop I hooked up a generic scan tool to verify the trouble code the Jeep had set. The ECM had stored a DTC P0340, thus indicating that the ECM was not happy with the cam signal (Figure 3). This code has many different scenarios that can set the fault. The sensor could be inoperative, the signal could be corrupt by noise, or the crankshaft to camshaft timing could be out of synchronization. The only way to accurately tell what is occurring is to display all the signals involved on a multi-trace scope.
The shop did not have a scope, but the mechanic was willing to look over my shoulder to get a crash course on how to set up a scope and put together a game plan of what to view. About 20 years ago I started out with a 2-Trace Fluke 98 scope. This scope was complicated and had a long uphill learning curve. I learned how to use the scope by connecting to know good components. This allowed me to become familiar with the scope settings such as time, voltage and triggers. I then learned how to stabilize the voltage waveform on the scope display with triggers. I also learned the hard way that using triggers can hide an intermittent problem. When using the trigger, the last trigger event will be displayed on the scope. If the trigger is not there and the display shows a waveform, it looks like the waveform is still present even though it is not. In order to become better at using a scope, I took a few scope classes. Once I felt comfortable using a two-trace scope, I needed to move to the next level. There were many four-trace scopes hitting the market back then, and I needed to view more signals at one time on the same screen to enhance my diagnostics.
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Figure 3 |
I acquired a few different four-trace scopes and mastered them along the way, but as time moved forward there was an increasing need to watch more than four signals at once. Vehicles were getting more complex and engine management systems were moving to multiple ignition coils and multiple cam sensors. I needed to go to the next level and purchased an eight-trace scope. It seems like the levels of moving forward never end, but it is what you need to do in order to keep on the cutting edge of the automotive technology. You don't want to stand still as the advances in the industry surpass you. There are many technicians that are happy where they are but you need to embrace the technology and the equipment that handles it better.
There is a need for an eight-trace scope, especially if you didn't want to waste time constantly moving your leads around. And once you move your leads around, you always wonder what you may have missed. You don't want to have limitations to what you can view on a screen and an eight-trace scope will help you to build visual associations with many computer signals within the controller you are testing. This in turn will visually teach you how the ECU driver circuits react to input sensors. This is how you learn system strategies to help build your diagnostic routines. You want to have the ability to look at eight coil primary voltage patterns or even eight injector voltage patterns parading on a screen. This cannot be accomplished with a four-trace scope.
Time to test
The cam sensor on this jeep is a three-wire Hall Effect sensor that has a power reference, a ground reference, and a 5 volt signal line that's toggled to ground. The configuration and speed of the trigger wheel will determine the pattern and frequency of the signal you view on the screen. As part of my game plan I decided to use five traces of my eight-trace scope to view the cam and crank sensors and the three ignition coil drivers. This would allow me to check the integrity of the sensors and make sure the coils were being triggered correctly. It is always best to test the circuits directly at the ECM to ensure that you are testing the complete circuits involved. Testing any circuit at the component does not always guarantee that the circuit is not open from the ECM to the component.
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Figure 4 |
I went to my information system and pulled up a wiring diagram of the vehicle. I then located the circuits I wanted to test and found their pin locations at the ECM to attach my test leads (Figure 4). I hooked up my yellow lead to the crank sensor, red lead to the cam sensor and my green/blue/white leads to the coil drivers. When I started the engine I could see six primary patterns within one cam signal as it transitioned from high to low state (Figure 5). It is important to check the frequency of the coil primary to determine how many coil firings occur in one camshaft revolution. If all coils are fired correctly it is a direct result of whether the vehicle has proper cam sensor to crank sensor correlation. There are some systems that may drop out coil primary and fuel injector commands if cam-to-crank correlation deviates more than a few degrees and in other cases there could be a hard or no start condition.
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Figure 5 |
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Figure 6 |
The coil primary saturation times were about 5.4 mS. and the primary patterns seemed very consistent. What I had to do now was to home in on the cam/crank patterns. I deselected the coil primary patterns and focused on the cam and crank signal waveforms (Figure 6). The crank sensor patterns were in packs of four paraded transitions from high to low and three sets of the packs were equidistant within one transition of the cam sensor. The patterns were in proper orientation with one another based on my experience of viewing many good patterns on these 4.0 liter Jeep engines. The one thing that did catch my eye was that the cam signal was not meeting its low to high transition properly. The cam sensor was being feed a 5 volt reference voltage so the signal line should have been toggling from 0 to 5 volts keeping within its 10 percent threshold. If this signal never went above 4.5 volts the ECM would still see this as a low transition. The cam signal was not going above 2.1 volts but yet it was producing a frequency pattern. This was an indication that there was a current path to ground. When the camshaft sensor is at 5 volts it is in an open circuit condition. Source voltage is always present to the point of the open circuit, which in this case is the camshaft sensor. You need to first understand that when a trigger wheel travels through the Hall Effect sensor and blocks the magnet from acting on the Hall Effect sensing device the signal line will be high or in an open circuit condition. When the trigger wheel travels out of the Hall Effect sensor and exposes the magnet to act on the Hall Effect sensor the signal line will be low or pulled to ground which produces a current path so a voltage drop can occur. I simply created this effect by removing the cam sensor and passing a razor blade through the Hall Effect assembly (Figure 7). I set my scope up to only view the cam sensor waveform pattern and watched the scope pattern as the cam sensor signal line transitioned from 0 volts to 2.1 volts. I then left the razor blade in place to block the magnet from acting on the Hall Effect sensing device. While the razor blade was in place I proceeded to unplug the cam sensor connector. The scope pattern of the cam signal transitioned from 2.1 Volts to 5 volts (Figure 8). This vehicle had a defective brand new cam sensor from the dealer. The sensor was not allowing the signal to transition above 2.1 volts due to a current path to ground, but yet did allow it to transition low near 0.1 volts when the resistance of the circuit changed.
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Figure 7 |
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Figure 8 |
New doesn’t always mean good
The shop at this point ran to get another cam sensor from the dealer up the street. To my surprise the dealer took the old sensor back and gave him another sensor without any questions. There has always been a policy with most parts counters to not accept returns on electrical parts but I guess this depends on the relationship you have with the guy at the parts counter. When the mechanic returned he installed the second new sensor. Luckily it was easy to install and I proceeded to take another look at the scope patterns of the cam and crank sensors (Figure 9). The signal patterns were now running true, the cam sensor was toggling between 0 and 5 volts, the check engine light was off and there was no pending code P0340 in memory.
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Figure 9 |
The garage mechanic was in total denial and disbelief. He could not accept the fact that he went the sure route to buy a genuine Jeep part only to have it create another problem that would need to be solved. This is not uncommon in the field and I see brand new defective parts all the time. It's hard enough to go through the diagnostic process just to pinpoint a problem, but it is worse when you purchase a brand new defective part that has you second guess yourself. When this occurs you can end up with the same or similar problem that may take you down another path. This can be quite confusing because you feel that you have still not resolved the initial problem. It is important to have the equipment necessary to properly diagnose your customer’s vehicles and possess an understanding of how to use this equipment. A picture is worth a thousand words but a scope displaying a waveform to properly diagnose the vehicle in your service bay is priceless. I service over a thousand shops and I recommend to each and every one of them in order to repair modern vehicles you will need a scope.
It is so important that you test with accurate methods so you know that your test results are valid. This will allow you to never second guess yourself. If you know how to properly test a component and understand its strategy of operation then it will allow you to validate whether your findings were correct or whether you’re dealing with a defective or incorrect part for the vehicle. We now live in a world where the repair technician is not only responsible for accurately diagnosing the problem, but is also responsible for the quality of the parts they install. There is no guarantee that a manufacturer has quality control of parts they manufacture The true technician is the one who will take nothing for granted and is keen enough to weed out the bad items that he purchases to do a job with. My only hopes that this story hits home with many of my fellow technicians and you take the proper steps not to fall in the trap of "Diagnostic Diversion."