It Helps to Go Back to 'Square 1' When the Problem Is a PuzzlePosted 4/14/2004
By Jeff Bach
So this guy on the phone says, "I know you don't know me, but I've sent you a lot of customers. I send you the ones that need to be rewired." He then proceeds to tell me all the things he had tried on this car he's sending over on the wrecker.
For some reason, I just don't have that, "Oh wonderful, another customer coming to help pay my bills" feeling. I guess I should know better than to allow these cars to come here, but I'm still in the throes of addiction to the challenge that enables me to justify the expense of the diagnostic equipment and technical classes I attend.
I facetiously quizzed this guy a little about how he came to the conclusions to replace the parts he had hung on the car. When I get one of these jobs in that has been worked on extensively by a guy with an obvious passion for fixing cars, but the disadvantage of not having proper equipment and training, I try to get enough information to establish the original complaint and what all transpired. I do this to get it to do what it's doing now, which is inevitably different than what it started out doing. The story in this case ended with, "and now it won't even ..." - which seems recently to be a common scenario.
I was attempting to collect all the original parts from the guy so I could try to piece this thing together in its original form when I get this suspicious question, "What do you want them for? They're no good or I wouldn't have replaced them." I quickly counter with, "I know they're bad, but sometimes I can autopsy the old parts and figure out which circuit shorted and took them out." That seemed to satisfy him and achieved the desired result. Now I could start out with the original parts and the original problem.
It's not that I don't appreciate someone getting the problem narrowed down to the wiring - which is what I was told this problem had to be - it's just that I kind of like working the whole puzzle from scratch. When you start out with a lot of new parts on a car that has never run since the ordeal began, it's kind of like buying a jigsaw puzzle from a yard sale. It's just not a comforting thought to be trying to work a puzzle and have in the back of your mind the idea that "I wonder if this could be a piece from one of those other puzzles he had laying there."
"Get back to Square 1," I say, and then start diagnosing. This particular car was a Chrysler with a history of intermittently quitting and restarting, sometimes right away and sometimes after setting a few minutes. It got progressively worse, then finally had to be towed. The other "technician" said when he first got it, it would start sometimes when he twisted the harness by the computer. After doing a fair amount of twisting and trying to start it, I concluded that it was just one of those things like when I showed my granddaughter that you can blow on the doors at the grocery store to get them to open. It has a lot to do with the timing.
I like to establish a jumping-off point on a "no start" to narrow the search by dividing the problem to fuel or spark when possible. In this case, I had neither. It was a Chrysler product with a V6. I consulted my data source for some diagnostic information on a "no start" situation and got some "out of the box" ideas with which to begin my trek. My checklist included:
This helpful information looks like it was taken from right out of the back of the owner's manual.
I looked for some description and operation info to try to devise a test for the fuel pump relay to see if I'm getting referenced by the crank sensor and I got this paragraph that said, basically, that the computer used various sensors and stuff to run the engine. If it doesn't work, you might want to test them individually.
Information on this car's crank and cam sensor signals seemed to be somewhat scarce in my information data source, but I was able to dig up that the automatic shutdown (ASD) relay de-energizes after two seconds of cranking if it doesn't see a reference signal from the distributor. I know this car had a distributorless ignition system (DIS), but I'm figuring on the same logic. We determined that the ASD relay was "De-energizing" so I did some scoping on the cam and crank sensors (which were on the list of replaced parts) to find that the signals were well below the 5 volts that were supposed to operate them.
"It heard the relays clicking as soon as I disconnected the A/C pressure switch," he said.
"Well, now, that wouldn't have been high up on my list of suspects for current leaches," I said.
Figure 2 shows how the signal looked when the shorted A/C pressure switch was disconnected.
Maybe we should send that in as a "no start" suggestion to put between cause and correction Nos. 6 and 7. Check for low voltage crank and cam signal circuit and replace A/C pressure switch.
When you finally find one of these problems that seems to defy the normal trouble tree and diagnostic logic paths, you get down to the bill, and it invariably ends up being difficult to charge someone for all the time you put into finding their troublesome problem. I'm getting better, but I know I didn't get enough for this job when the customer says at the end, "That's all?" But there is some consolation: You know whenever this customer hears that someone has one of those "hard-to-find" problems, he or she will be recommending us to them.
AutoInc. Web Site |
ASA Web Site |
State Legislatures Focus on Insurer-Owned Shop Legislation |
ASA Launches New Legislative Web Site |
Change in the Air for Climate Control |
Hybrid Safety |
Growing Your Business |
Guest Editorial |
Tech to Tech |
Tech Tips |
Around ASA |
Shop Profile |
Net Worth |
Stat Corner |
Copyright (c) 1996-2011. Automotive Service Association®. All rights reserved.