I love my work. Each day is an opportunity, and today, I had the opportunity to proudly watch two of my advanced auto students turn a 162,000-mile pile of pentastar parts into a running Jeep Wagoneer, affirming the automotive training that they've received over the past two-and-a-half years.

Some Background

I teach in a certified training program where potential future technicians learn electrical/electronic systems, engine performance, brake systems, and steering and suspension systems. The program is certified by the National Automotive Technicians Education Foundation (NATEF).

As I write this, it's February, and my students should be able to cite Ohm's Law, apply it to a circuit, measure volts, amps and ohms with a Fluke 78, and perform voltage drop tests on live circuits. Further, they can hook up scan tools (Tech I, Tech II, Snap-on MT2500) to multiple vehicles, interpret inaccurate values and look up repair information, technical service bulletins (TSBs) and recalls on our electronic information system.

However, only a handful of students have the “right stuff” and can use all of these tools, couple them with prior knowledge and past experience, and methodically move through a no-start repair sequence without any help. Case in point: two of my high school seniors, Jesse and Aaron.

The Customer Interview

The story of our 1987 Jeep Wagoneer begins with Crystal, the sister of a former student, who came to me one afternoon with her “mechanically inclined” boyfriend, to tell me the tale of her Jeep that dies on occasion, only to restart and run for hours. She said it would happen regardless of engine or ambient temperature. The “M.I.” boyfriend had patronized our local discount auto parts house to the tune of $230 for an ignition coil, ignition module, spark plugs and injector cleaner.

I Hand Them the Keys ...

After handing Jesse and Aaron the keys, I ask, “OK, what should you do first?” Since I'm now an instructor/ service advisor, I've given them the customer interview information, but no more. They both know to verify the complaint first, followed by a thorough visual inspection. The picture of our Jeep reveals a smorgasbord of items to record, including:

• A big, fat, red accessory wire “attached” to the battery.
• Evidence of jumper cables being clamped to the battery (jumped backward?).
• A huge oil leak from the valve cover, coating the entire block.
• A new ignition coil, but the old module. (Hmmmm ... I wonder if he took it back for store credit?)
• Broken plastic vacuum lines, low coolant, low and dirty oil, bald tires and a cracked serpentine belt.

Jesse and Aaron carefully look over all of the wiring harnesses for signs of rubbing, connector integrity and “rabbit erosion” (which is my funny way of telling techs to look for signs that rabbits may have been nibbling on the wiring harness under the hood; it happens!).

Gathering Information, Finding Clues ...

After reconfirming that the Jeep won't start, Jesse heads into the classroom to find no-start information, while Aaron grabs a fuel pressure gauge, test spark plugs and our noid light kit.

I've taught no-starts before on engines without a fuel test port, so Aaron is relieved to find one on the rail. He installs the gauge, hooks two test plugs onto two plug wires, and a noid light onto an injector connector. While cranking, he finds fuel pressure (see Figure 1), but no spark or injector trigger (see Figure 2). Aaron also grabs the Snap-on scanner, hooks it up, and finds no codes. He tells Jesse his findings as Jesse prints out the no-start information from our electronic information system.

At this point, I urge them to check for applicable technical service bulletins (TSBs), where they find 12 that pertain to fuel/ignition/performance. One of them stands out. It's titled “No Start, Hard Re-Start, Or Engine Dies Out.” This one is 12 pages long, applies to our complaint and profiles my suspected reason for the no-start, which I've kept to myself. Sometimes the hardest part of being a teacher is keeping your mouth shut! I know that once these guys go through all of the motions and “own” the diagnosis, those lessons will stay with them for a long time.

Although fuel pressure is present at the gauge, Jesse and Aaron know that the fuel system isn't perfect; so they will check flow and volume after the engine is running. They may even pull out the digital storage oscilloscope (DSO) and look at the pump waveform and current amperage draw to predict its estimated time 'till death. For now, they know to make a note of the fuel system and focus all energy on the no spark/no injector situation.

Where's a “Chart A-3: Cranks, But Won't Run” When You Need One?

As an instructor, I enjoy the opportunity to give my advanced students a GM car, because if it's a no-start, and the student is methodical, the “A-3: Cranks, But Won't Run”' chart will lead to the cause of the no-start. No fuel? No problem. A-3 will send you on to “A-7: Fuel System Diagnosis.” No spark? A-3 sends you to chart “C-4B: Fuel Injection Ignition System Check,” where you carefully, step-by-step, test voltages, values and components until you find a defective rotor, ignition coil, module, pickup coil, crank sensor, or the reason for no B+ or ground to a particular place.

The “opportunity” that my students now face is the very real fact that no A-3 chart exists for this Jeep! Oh, no! What to do next? In my opinion, this is the place where only the best auto students - and technicians - earn their stripes: the “no A-3 zone.”

I asked Jesse and Aaron what they felt the next logical step was, and as they stared into the 4.0 liter abyss, they said, “Fuses.” I was so proud that they remembered the simple stuff! So, just like that, Aaron grabbed a test light and tested each fuse, one by one. That's when I said, “Nice job. Now you're finding out what is right - or, let's find out what is wrong by process of elimination.” At this point I felt that this was a critical moment, that teachable moment, when a student is ready to jump up to the next diagnostic level.

I asked them to begin thinking about Chart A-3. What comprises it? What critical steps do they include? Do they refer to any wiring diagrams? Without hesitation, they went to pull the computer control wiring diagram for the 4.0 liter engine. As we looked it over, we made note of which computer pins were inputs, which were outputs, and where the B+ and grounds were. The nice thing about information systems these days is the fact that the engine control system is all on one page, free from headlights, wiper motor, etc.

Now, the reason I'm giving Jesse and Aaron so much credit is the fact that they keep thinking about the no-start bulletin and relating it to our new “which-step-is-easiest” approach. Because the bulletin talks about a defective speed sensor (crankshaft position sensor, or CPS), they decide that this is an easy thing to test. Paying homage to Chart A-3, I ask them to look at the cranking rpm on the scan tool and crank the engine before disconnecting the speed sensor. Sure enough, no cranking rpm is indicated - now we're finding clues.

Figure 3 shows the location of our Jeep's rpm sensor, which is mounted to the top of the transmission bellhousing. The spec for the speed sensor is 125-275 ohms, and there are 0 measured between terminals “A” and “B.”

Figure 4 shows the ohms reading with the old sensor on the bench, and Figure 5 is the new sensor with acceptable readings of 249 millivolts AC, with the engine running at idle. The old sensor is showing 261 ohms, but only with my hand pushing the wires together just right. My students have found the root cause for our no-start: an open speed sensor.

The update comes with new speed sensors for these Jeeps, which consists of a new sensor-to-computer harness, making it necessary to remove the old wires from the power control module (PCM), install new ones, and bore a new hole in the firewall to keep these wires away from electrical interference. Figure 6 shows the PCM with the updated wiring harness installed.

Logical Troubleshooting Sequence

I loved this no-start because it illustrated the fact that in the absence of no A-3 chart, a logical approach to repairing a failed system on the car actually works. You and I can apply this to any vehicle that comes into our bays for service. Yes, even if an old MG rolls in the door with a “Lucas-Prince of Darkness” wiring system on it, we can pull a schematic - although I'm not sure where from - and systematically trace down the circuit fault.

A Little AC Sensor Information ... Usually, when you think of AC waveforms, you think about a typical sine wave signal, like the one you'd get from your home outlet. This is an AC signal, but not the most common one on a car.

Typically, automotive AC signals are timing signals. They tell an ignition module when to fire a spark plug, or tell the computer when to fire an injector. Because of the precision necessary for these operations, the AC signals have to be sharper and more precise than a typical sine wave signal.

Typical automotive AC signals include ignition pickups, crankshaft sensors and camshaft sensors.

Automotive signals, which appear more sinusoidal in shape, are from ABS wheel sensors and vehicle speed sensors.

AC signals all have traits in common: they are all repetitive; that is, they repeat themselves in cycles. Variations in these cycles can indicate problems in the circuit being tested.

In addition, AC waveforms must rise and fall below a specific base level. The household AC sine wave rises and falls equally above and below zero; this is what defines the signal as an AC waveform.

What does the computer look for in an AC signal? The computer looks for:

• Peak-to-peak voltage.
• Spiked wave, which it can interpret better than the sine wave.
• Frequency and amplitude of the waveform, and if it's a signature PIP (Ford), sequence of events.
• Cranking AC voltage, which should be 300 millivolts or more.
• Voltage peaks, which should have even height on the scope.

Three things can influence waveform amplitude on an AC pickup:

• Magnetic field - Broken magnets (GM pickup) will have a weak magnet.
• The speed the reluctor passes the pickup.
• Separation between the pickup and the reluctor (CKP on Jeeps).

Brian Manley is a vocational automotive instructor for the Cherry Creek school district in Aurora, Colo. He is an ASE master certified automobile technician and a former member of the National Automotive Technicians Education Foundation (NATEF) board of trustees. He can be reached at manley_brian@hotmail.com.

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