I had a Lincoln Town Car come to me for repair that required some real detective work to figure out what was wrong. The issue wasn't actually electrical in nature -- my expertise. It turned out to be a clogged filter and faulty logic in the ride height computer control. Let me explain.
It was a mid-80s Town Car with the air bag suspension. When the car was started and driven for a while, the suspension would go to its full travel height, similar to when all the kids were putting "Hijacker" air shocks on their cars. This made the car very unstable at highway speeds (not to mention it looked ridiculous).
If the car sat overnight, it would be level in the morning. By the time the car was brought to me for repair, the air bags, ride height control module, ride height sensor, etc. had all been swapped out, and now the customer was running out of patience (and money). I tested the wiring, the control solenoids, the height sensor, and the pump. All checked out electrically.
Eventually, I put a test light across the vent solenoid to verify when it was being activated. Apparently, the control computer would attempt to vent the air bags until the height sensor said the car was level. If a certain amount of time elapsed while venting without the car lowering to level, the computer would assume the height sensor was bad, and that the car was sitting on its suspension stops.
The computer would then inflate the air bags for a pre-programmed amount of time. Once this cycle was repeated a couple of times, the car would be at full travel. It turned out to be a clogged filter in the pump housing assembly that allowed the air out of the system. I popped the filter out, and pfffftttttt, the car went to level and stayed there, even when driven.
This entry was submitted by George Tempesta and edited by Rob Spiegel.
George Tempesta is a former automotive electrician who is currently a network engineer/architect with an IT consulting services company based in Philadelphia. He spends most of his time trying to figure out how things work, fixing things, or building something.
Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.
Seems like going to a dealership or auto shop for repairs is like calling for computer tech support. The person assigned to your ticket may never have done the hands on trouble shooting and is simply following a troubleshooting "tree". You can also pay an automotive tech less than you can pay a "real" mechanic. None of this bodes well for the customer, unless you really have a good component level knowledge of the misbehaving system.
From this discussions, it appears that more than a few of the folks who post here are DIY mechanics as well as technocrats. So they are well aware of how enjoyable - and increasingly rare - it is to find a machine which has been deliberately designed for easy diagnosis and repair.
I'm aware that in my own career I've designed devices that were appreciated for their excellent serviceability.....as well as some which were frankly terrible. We can probably all recall presenting finished designs which worked, but which needed improvement right from the start..... if only we had a bit more time.
Thinking back on my own education, I don't believe that the idea of designing for serviceability was ever considered. I wonder why? Aren't some of the basic features of a serviceable part quite simple? And possibly even universal?
Diagnostic messages tell what the designer thinks is happening. Machines don't lie to be malicious, but because they are dumb. When I hear that machines will be able to fix themselves, I don't believe it. A machine can't tell if a sensor is detecting a problem, or if the sensor has failed. Part of troubleshooting is thinking like the machine - recognizing what sensors the machine has and what it can, or can't, detect.
@GlennA: I probably do not remember every auto repair I ever had or every mechanic who has worked on one of my cars/trucks. That said, it does seem as though today if there is not a computer code stored somewhere, the mechanic has no idea of what the trouble is. It is really cool when a machine can be plugged in and tell the mechanic that X, Y or Z has failed and therefore needs to be replaced, but it is absolutely frustrating when my truck keeps dying, at any speed, and none of the five garages I took it to (including two dealers) have any idea why because there was no computer code to tell them.
As far as machines fixing themselves, I can visualize rare occasions where a machine could bypass a faulty circuit, but unless it includes a welder and spare parts I do not see how it is even remotely possible.
Tool_maker; My brother is an auto mechanic. He often tells me stories about other mechanics changing parts indicated by the error code, but not fixing the problem. The first thing he asks is if they pulled all of the codes, not just the first one that popped up. Then did they look at the fault table to see what the combination of codes indicated. Then, decide which part is most likely, easiest to change , and the cheapest.
I had a rental car that had the engine warning light come on the first day. I found out previously that a loose gas cap can cause an emissions code, which turns on the engine fault light. Since the car ran fine, I checked the gas cap, and continued to use it, waiting for the fault to reset itself - it finally did on the 3rd day.
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