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Diagnostic diversion in a 2004 Jeep Wrangler

“New” does not always mean “good,” as this shop learned.
Monday, May 1, 2017 - 07:00
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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.

Figure 1
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.

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.

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