I was driving to church one Sunday evening in heavy traffic. When I came a few driveways before the church's parking lot, I activated my turn signal, applied the brakes, and turned into the parking lot. I was startled to hear a horn blow from the car behind me. It was odd that I hear any horn in my city, other than a general chorus of car alarms that go off on windy days.
When the service ended, I went out and put my foot on the brake. It was dark, so I would have been able to see the brake lights glowing. But there was a problem -- no lights. I got down on the floor and pushed the brake switch with my fingers. The tail lights lit up like a Christmas tree. But when I pushed the pedal, no lights.
I drove home (carefully) and disassembled the assembly. With my ohm meter I measured and found it working with minimal resistance. I grounded the brake wires and found the lights were working. I put it back together and again found that they did not light up.
It turns out that the switch was bad, but only when pushed at the right angle and with the right pressure all while mounted in the brake pedal frame. A $20 switch at the dealer solved the problem.
This entry was submitted by John Bate and edited by Jennifer Campbell.
John Bate has been a system engineer at Volvo Trucks since 1999, working with electrical harness design and process improvement. His background includes embedded computers and software, HF radio design, telematics, and writing copious amounts of simulation models and processes improvement software.
Tell us your experience in solving a knotty engineering problem. Send to Jennifer Campbell for Sherlock Ohms.
John, I will bet you that the time you spent diagnosing the problem was longer than the time you spent fixing it. This is the problem with many automotive systems. With the advent of very inexpensive microcontrollers, this should be the next wave of automotive improvements.
Isn't that always the case-? Diagnostics taking longer than the corrective action-? Makes me think of a recent issue I had on the product I was designing. I had hand-assembled the very first (10) working prototypes, but every time I tightened down the outer housing screws, the display blanked-out. I spent literally 2 solid weeks of assembly evaluations and diagnostic trouble shooting before I narrowed the cause to the lack of insulated solder-resist on the PCB top layer. Tightening the housing screws simply squeezed the metal modem casing onto exposed circuitry which should have been insulated during the PCB fabrication process. Corrective action was a 1c piece of Kapton tape under the module. Two week investigation; 10 second fix. Of course that is a natural part of development and this type of issue must be completely resolved months before production. I hate to see silly issues like this affect the consumer end-user.
On a tangent; I once was asked to help when a car would not shift out of Park when the brake pedal was pressed. And also, the brake lights were not lighting when the brake pedal was pressed. It turned out to be a loose wire at the switch. The brake lights turned out to be a troubleshooting test.
Speaking for the people that two years from now that have to figure out why your product suddenly stopped working, you should never rely on soldermask for insulation (been there done that). Or are you giving someone else an opportunity for a future 'Sherlock Ohms' submission.
The specific occurrence was a small array of plated thru via's (to the main LCD flex ZIF) anchored through the board with annular rings. The annular rings were <.015" O.D. but completely exposed. Meanwhile, the transceiver module had a die cast zinc outer casing and laid directly over those via's, flush to the PCB surface.
There are nearby areas of other printed circuit traces also under the zinc casing, but they are under the layer of solder resist. (I don't get why those traces were covered but the ZIF vias were not; oversight by the PCB designer, I guess) But that's a moot point if your concern is true (which I do believe you).
Solder resist/mask is not a mechanically robust component and is not designed to provide any mechanical isolation. As its name implies, it's designed to prevent solder from going places you don't want (e.g. wicking from pads).
If you cannot confine your enclosure's contact with the PWB to purpose designed areas of the board (e.g. a mounting pad), you need a mechanical component like an insulated washer or a coverlayer of kapton or equivalent.
On a tangent, for higher reliability, you should avoid via's (or plated holes) under mounting pressure anyways since the thru hole barrels tend to crack under the compression.
I had a minivan that I put many miles on doing field service work. Got to where the cruise control would not stay engaged one 11 hour trip. When I got back home (another 11 hours without the cruise control) I started poking around. Wiggled the brake switch and things behaved for about 2 weeks. Then I had a problem with intermitent brake lights. Finally replaced the switch. Turns out it was a few years of accumulation of crud and minor arcing that finally led to switch failure.
Putting the bad switch back IN was a waste of time except as confirmation that it was mechanically bad. The failure was in a very small bounding box containing only the switch and its connector and the vehicle's harness side. Going to the dealer for a $20 replacement was also suboptimal since almost every vehicle has $8 aftermarket replacements available at any parts store.
I'm a volunteer tech specialist on a Chevy Trailblazer / GMC Envoy forum, and we get these sorts of questions monthly. Brake lights and ignition switches are well-known high failure rate items.
But I have to comment on the microcontrollers changing automotive manufacturer's design goals any time soon. The capability is there for plug and play peripherals on everything except vehicles. Install a new intelligent driver's door switch module (that dies over time because the switches are horizontal where rain and snow falls right on them with every time you open the door), or a 4WD transfer case control module, and you THEN must go the dealer, who has the "magic" Tech II tool, to download the personality firmware into the module. Minimum charge for this 5 minute process? $75-200!
GM and the like will NEVER do something to decrease the revenue stream for their dealer network for outyear support by assisting the DIY repair owners. I get more and more bitter when I see design decision made that thwart DIYers in order to drive the owners back into the clutches of often-abusive dealers. I know it's not the designer's fault - they are under management orders. But it's an economically-driven cycle, not technical, and therefore not open to the usual rational discussion we would have about other products and systems.
Just LOOK at the insane number of Delco/Delphi connectors and ESPECIALLY the little safety catch/latch mechanisms that thwart mechanics and diagnosticians at every turn. Irrational.
Inexpensive micro controllers will not ever, at any time, under any condition, improve automobile anything! BUT they will certainly increse the profits gained from replacing the failed parts, since the code will be unavailable to anybody except the auto companies repair parts makers. While emissions and economy have been improved through electronic controls, all of the other car functions have been rendered less reliable and nonrepairable by the use of them.
Recall that a few years back it was predicted that automotive electrical systems would go to a seriel buss and just a pair of power wires, in order to reduce the weight and cost of all those wires. BUT what we have now is a few central modules and huge bundles of wires. We also have a few accessory modules for those extra-priced expensive options, which only become more expensive each year.
An unfortunate number of things that sort of sound like good ideas are actually some vendors idea in search of acceptance, instead of something that actual customers would have any desire to implement.
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