For too many technicians, diagnosis. A Logical Diagnostic Procedure Saves a Fishing Trip! DRIVE ABILITY

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1 DRIVE ABILITY by Dave Skora A Logical Diagnostic Procedure Saves a Fishing Trip! According to the repair order, the rebuilder replaced the torque converter and the converter clutch solenoid during the rebuild. Now it s back with an intermittent surge. For too many technicians, diagnosis is more a matter of hit-ormiss testing and parts replacement. If they get lucky, they ll hit the right part early in the diagnosis; if not, it may take days or even weeks to come across the root cause of the problem if ever. Handled properly, diagnosis is a logical series of steps, designed to isolate the root cause of a problem through a process of elimination. This series of steps should always include: Verifying the complaint Checking for obvious causes Retrieving codes Searching for published bulletins or articles And, if that doesn t allow you to isolate the problem, you should identify all of the possible causes for the condition. Then you perform a series of tests Figure 1: A faulty MAF sensor can cause a surge, but disconnecting it and driving the vehicle is an easy way to eliminate it as the problem. NOTE: Some models won t run with the MAF sensor disconnected. designed to eliminate those possible causes, one at a time, until you find the root cause of the problem. Here are a couple examples, taken from real life: It was a beautiful clear and cool Friday morning. I went to work hoping to leave early. I d planned to go fishing and wanted to camp at the lake before sundown. I got the boss to agree to let me off early, as long as there was nothing else the other guys couldn t handle. There were only two things that stood in my way Vehicle #1 A 1997 Chevrolet truck with a 5.7L engine and 4L60E trans. The truck had about 120,000 miles on it and we rebuilt the transmission about two weeks ago. According to the repair order, the rebuilder replaced the torque converter and the converter clutch solenoid during the rebuild. Now it s back with an intermittent surge. I drove it around and finally got it to surge. It seemed to come and go while cruising around MPH. It felt like the torque converter was being pulsed on and off, and then it would go away. I connected a scan tool; there were no codes and the data stream didn t show any obvious problems. Next I searched my ATRA and On- Demand databases, but didn t find any bulletins that matched the problem. I asked my coworkers for suggestions. One said it might be a lockup shudder. Another one heard that a MAF sensor might cause it (figure 1) or the PCM might need to be flashed. All good suggestions. 12 GEARS September 2005

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3 A Logical Diagnostic Procedure Saves a Fishing Trip! I reconnected the solenoid pack; once again, the engine died whenever I put it in drive. There was no doubt that the PCM was calling for lockup. I knew our shop uses good quality parts and rebuilt converters; while that doesn t necessarily rule out a converter clutch problem, the odds of the new parts having the same problem as the original ones are pretty slim. For now, we ll assume they re okay. A MAF sensor problem is possible, but there s an easy way to eliminate it as a suspect: I disconnected it and went for another test drive. Same problem, so that rules out the MAF sensor. Flashing the computer might be a possibility, but only if: there were no other problems there was a re-flash available the update had the potential to correct this particular problem Figure 2: At a steady RPM, the timing fluctuated a good 10º as seen by the timing mark. The problem turned out to be a worn distributor gear. There was no update available, which eliminates flashing the computer as a possible fix. What now? I called the customer and learned he had been to the dealer for this problem. They told him at the very least the truck needed a torque converter and probably a complete overhaul. That s why he brought it to us. At this point, I knew I was going to have to work for the fix. I made a list of all the possibilities that could cause a surge. First I had to determine whether the surging was related to the converter clutch or was a driveability problem. My Mastertech scan tool lets me operate lockup with its bidirectional controls. The surge continued, even with lockup applied 100 percent. So it wasn t a converter clutch problem; now we re talking driveability. I started thinking about what could cause this. The list included the ignition system, mixture controls, engine emission controls, the alternator, traction control, ABS, the radiator fan, the brake system, 4WD system, and the A/C compressor. Wow! That s a lot of possibilities. I almost wanted to try something and hoped I made a lucky guess. But it was still early and I knew if I stuck to my diagnostic method, I d find the problem. To narrow the suspects down, I disabled as many devices as possible that would still allow the engine to run. This included the EGR, AIR, EVAP, traction control, ABS, 4WD controls, and A/C compressor. Still surged. Next, I disabled the alternator, unplugged the electric radiator cooling fan, and drove the truck just long enough to see if it still surged. It did. This left either a fuel mixture or an ignition problem. After reconnecting everything, I got the truck to surge and made a movie of the data stream. The only thing that seemed erratic was the ignition timing. Hmm was this the cause or a result? I had to find out if the ignition was actually fluctuating. To test it I simply pointed an old fashion timing light at the front pulley. Then I held the engine at the same RPM as where the surge occurred; the spark timing flicked back and forth about 10º (figure 2). The source was a worn distributor drive gear; a new distributor fixed the surge. It was just after lunch. One down, one to go. Vehicle #2 A F150 with an E4OD. The complaint was that it would kill the engine when you put it into drive. Sure enough, every time I put the selector in drive, the engine shuddered violently and died. Obviously the lockup was stuck on. There were no codes in memory. I knew a bad transmission filter or a worn pump would cause this in reverse. But this truck was going right into reverse with no problem. To make sure the problem was inside the trans, I unplugged the solenoid pack harness. Now the truck didn t die; it went in drive just fine. Well, at least it wasn t our builder s fault. But if there are no codes, why would the PCM be commanding lockup at a standstill? I reconnected the solenoid pack; once again, the engine died whenever I put it in drive. There was no doubt that the PCM was calling for lockup. I figured my scan tool would pick up something. But it happened too fast to see what was going on. I tried catching the problem two more times, but each time the engine died out before I could gather much useful information. Boy, it was looking bad for catching fish tomorrow! Before the problem turned into a cat-and-mouse game, I decided to do a bulletin search. Sure enough, ATRA 14 GEARS September 2005

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5 A Logical Diagnostic Procedure Saves a Fishing Trip! Figure 3: While this might have been a tough problem to find from scratch, ATRA already provided the details of this diagnosis in TSB-706. had published TSB-706, which covers a faulty PSOM or instrument cluster that sends a false vehicle speed signal to the PCM (figure 3). The PCM thinks the vehicle s going fast enough to turn lockup on, even at a stop. I replaced the PSOM: problem solved. And still plenty of time to get to the lake a drop a line in the water If you re still having trouble getting your head around the concept of a logical diagnostic procedure, check out Road Testing for Results, by ATRA CEO Dennis Madden. It s available through the ATRA BookStore, and it can help you learn how to work out a logical diagnostic process for virtually any vehicle or problem that makes its way into your service bay. Because a careful, well-reasoned diagnostic procedure will beat a wild guess any day of the week. 16 GEARS September 2005

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