Jeff Bier’s Impulse Response—Fried Fahrvergnügen

Submitted by Jeff Bier on Wed, 11/14/2007 - 18:00

Do you ever look at a piece of hardware and wonder, “Why, oh why, did they build it like that?”  This is what I’m thinking as I look at my 2001 Volkswagen Passat, a car that is now completely dysfunctional because of an unfortunate (yet easily foreseen) intersection of water and electronic circuitry.  Let me explain.

The car has a sunroof, and the sunroof has two gutters and tubes that route rainwater through the body of the car and dump it outside. But like all gutters, they can get plugged with debris—and when that happens, water leaks through the sunroof and onto the floor of the car. Which happens to be where the engine control unit (ECU—the car’s brain) lives, right under the front passenger’s feet.  And ECUs, as you might suspect, don’t do well in water.

I first realized there was a problem when everything in my car quit working.  The car started stalling and spitting out error messages about every system it had—airbags, stability control, etc. etc.   A helpful mechanic pulled the ECU board out and showed it to me – yup, basic Motorola (now Freescale) microprocessor plus all the fixin’s, all fried. (See picture) Volkswagen wants $1200 for a replacement board. 


Now, why would you design a car with a key electronic component—an expensive, mission-critical, water-intolerant component—located at the lowest point, in a spot where it’s likely to get wet? After all, aside from gutter malfunctions, people sometimes spill their grande lattes, or they shampoo the carpets, or they hop in with wet boots or wet dogs or whatever.

Maybe, I thought, there was some thorny design constraint and they had no other choice.

But then my helpful mechanic told me, “Yeah, when this happens with Passats we just put the new ECU board on the side of the footwell—there’s plenty of room there, and it stays dry.”  It is difficult to keep from writing ungentlemanly words here. 

To me, this is a prime example of the perils of failing to think about the whole system.  I suspect that the ECU in my Passat is a really nice design—great hardware, robust software.  But when it gets wet, none of that matters.  As systems get more complex and inter-dependent, it’s crucial to maintain a whole-system perspective.  Don’t fry your Fahrvergnügen.

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