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  Tech Tips

'That Used to Be Impossible'

Posted 9/1/2008
By Kevin Caple

I got a call from a shop in the desert Southwest. The owner of the car (a 1998 Cadillac Deville in mint condition) and his wife were traveling across the state with friends, enjoying the retired life and doing some sightseeing. They stopped at a large truck stop for a bite to eat, and when they came out 45 minutes later, the car was a no-start. The owner and his friend (a retired mechanic who is described as a gentleman well into his '80s) opened the hood, looked for loose cables, and tugged and wiggled wires to no avail.

Deciding they needed more in-depth diagnosis, they walked across the street to the shop and explained what had happened. The shop towed the car and started preliminary testing. Having the choice of a retired mechanic hovering over him or calling the Identifix Hotline, the technician at the shop, Mark, dialed our number in hopes of a quick fix.
"Mark explained that the radio head had an internal problem that caused the data line to shut down so the computer could not see that the security system requirement was satisfied, which would allow the car to start."

Mark had just completed his preliminary checks, verifying that the car had good fuel pressure and spark, and that the fuses and grounds were all OK. Mark could not get any information from the onboard system so he hooked up his scanner to check codes and data but found there was no communication with the scan tool either. He told me he had a helper - a retired mechanic by the name of Frank who was going to listen in on our conversation if I didn't mind (you can always tell a true car guy; he'd rather be looking over your shoulder in a 100-degree shop than sitting in an air-conditioned waiting room). Frank and I exchanged pleasantries. He had an upbeat tone to his voice, a person you couldn't help but like right away.

Mark told me about the communication problem, and that he could not get data from any of the modules on the data line. He was reasonably sure that this was at the heart of the no-start problem but wanted to bounce it off someone else besides Frank, his new-found friend. I fully agreed with his diagnosis.

We discussed how the instrument cluster is part of the security system and that it communicates with the powertrain control module (PCM) via the data line, allowing the PCM to control injector pulse. After going through the list of modules on the data line with Mark on the speaker phone, Mark laughed and said our retired friend just rolled his eyes and left the shop area. I said, "If Frank has not been in the field for a few years it's a whole new ballgame when it comes to diagnosing."

Back to the car. Since there are so many modules on the data line, we needed to isolate which module or circuit was corrupting our communications. I went to Direct-Hit and pulled up a list of similar cars with similar problems to see if there was a pattern of component failures. I figured this might lead us to a logical order of disconnecting each of the components to see if our data came back. Direct-Hit listed some radio problems so I suggested that Mark disconnect the radio first.

Communication with the scanner was restored and when Mark turned on the key, the engine started and ran perfectly. Hearing the car start, Frank came back into the shop and asked what the problem had been. Mark explained that the radio head had an internal problem that caused the data line to shut down so the computer could not see that the security system requirement was satisfied, which would allow the car to start.

Frank said, "Young fella, you're a genius to be able to figure that out!" Then Frank said those immortal words, "I was a mechanic for 43 years, and that used to be impossible: a radio causing a car not to start!" We talked cars for a few minutes and shared our opinions of where the industry has been and is going. We all agreed that some things sure seem to be more difficult than they need be, but overall cars are safer, faster, longer lasting and more comfortable than in years past. Frank added, "It used to be if you got 100,000 miles out of a car you were really taking care of it and lucky to boot. Now, 100,000 is just broken in!"

This conversation made me wonder ... 30 or 40 years from now, when all of us "young fellas" are standing in Frank's shoes, what types of automotive technology will we be talking about?

Kevin Caple is an IDENTIFIX GM specialist. He is ASE master and L1 certified in GM driveability, diagnostics and emissions, with 29 years of diagnostic and repair experience.

Experience Identifix This information is provided by IDENTIFIX®. IDENTIFIX® resources cut diagnostic time and provide repair solutions that increase the shop's bottom line. From Repair-Trac pattern failure quick fixes, to Diagram-Online wiring diagrams by fax, to the Repair Hotline staffed by 32 master techs who specialize in diagnosing complex problems by phone or fax, IDENTIFIX® helps techicians fix more cars in less time.

For more information on IDENTIFIX, call (800) 288-6210, 8 a.m. - 6 p.m. Central Time.

© 2011 IDENTIFIX. All Rights Reserved.


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