Tuesday, November 20, 2012

When cars break, and break again


Land Rover service. © copyright JE Robison

My friend Bob called me to ask about his most recent car repair experience.  He said, Last week I noticed my brake fluid was low, and the pedal went to the floor.  I took it to the shop, and they replaced a leaking caliper.   Two weeks later, the same thing happened.  This time they say I need a master cylinder?  How come

The experience of sequential repairs is all too common in the automobile business.  Sometimes it’s due to technician error.  But more often, it’s the inevitable result of service policies or the step by step nature of some repair operations.  Let’s look at some examples to see how it happens.

When Bob brought his car in for service the first time, he had a visible problem.  Brake fluid was leaking at one of the wheels and it’s didn’t take much to see the source was a failed caliper.  Replacement was a straightforward job and the garage sent the car on its way assuming it was fixed for good.

Yet it failed again, just a few weeks later.  This time the failure was not so obvious.  There were no leaks at any of the wheels but the fluid was low.  When that happens there is only one place to look in most cars – the power brake booster.  Indeed, when the master cylinder was pulled back from the booster the technician could see where fluid was leaking and he ordered a new cylinder.

The question is, why did this happen two weeks after the first repair?

The answer was clear with a little bit of thought.  When everything is normal the guts of the master cylinder don’t move more than a few thousandths of an inch to actuate the brakes.  When the system gets bled following repair it moves through a much bigger range, half an inch or more.  If the seldom-used reaches of the master cylinder have become roughened by corrosion the act of bleeding the brakes may lead to the internal seals failing.

That’s what happened in Bob’s case.  I asked myself how the repeat failure might have been avoided.  The first answer that came to mind is more frequent brake fluid flushes.  When the fluid is flushed the cylinders are run through their full travel.  Doing that periodically would prevent buildup of corrosion and presumably prevent failures like this.  But at what cost?  The more flushes would have cost more than the cylinder, and required three service visits instead of the one repair.

Some would say the brakes could be bled through air pressure, without moving the pedal.  That’s possible on some vehicles but the problem with that is that you never see the system work through its whole range and that may blind you to another more serious problem.  So that may work, or it may replace a harmless problem like this with a worse one.

In the end I concluded there is no way to assure a “one-step fix” in a situation like this.  Most of the time, when the first job was done, the master cylinder would not fail.  But every now and then, it will.  When that happens you have a two-step repair.

Then there are the so-called emission problems – those “check engine” lights that just won’t go away.  Many motorists believe that scan tools point us directly to failed parts, but all too often that is not the case.  Fault codes are symptoms, many of which have numerous possible causes.  Sometimes we can test and verify a failed part and we have a simple one-visit repair.  Other times the tests all come back good – the problem is intermittent or not picked up by our testers.  Other times we reach a point in the diagnostic tree that says “Replace A and re-evaluate.”  So we replace A and see what happens next.

The problem is, low level emission faults cannot usually be detected on a short drive.  Many require that the car be run through a tank of gas, sometimes more.  As a practical matter that makes the owner the test driver – and if more work is needed, the car comes back.  Sometimes twice, sometimes three times.

I wish there was a way to streamline this, but with the ever-increasing complexity of cars, we face a choice between exchange of major systems where we swap out all the possibilities (prohibitively expensive) or multiple repair efforts (a hassle for motorists). 

1 comment:

GretaCargo said...

What an excellent explanation! Thinking about these things from a systems perspective rather than individual parts helps at least plan or not feel as surprised when these breakdowns take place.