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Jim Grant's Tech Tips
89
Buick Century, Eight Months of Mystery
Q: My '89 Buick Century
has been fine until the last 8 months. The problem: while driving, the
car does NOT lose power but the speedometer drops down to zero for a few
seconds. I can even accelerate. Just as the speedometer goes back up to
the current speed, two lights on the dash (Check Engine Soon and Temperature)
flash on briefly. Just at that moment the car seems as though it will
lose power, but it doesn't. There's not much left to change on my car
now. Have you ever heard of such a problem? Rich
A: You're
describing the possible warning signs of a computer failure. The symptoms
in your vehicle can vary, these are a few of the erratic (I stress "erratic"),
conditions you may experience; loss of speedometer, high idle and/or low
idle speed, stalling, bucking at highway speed, Check Engine Soon and/or
Temperature light. Often a diagnostic check of the computer will fail
to provide an explanation, no codes in the computer's memory. Or, the
computer gets real creative and invents phantom codes that diagnostically
lead nowhere.
So,
how do we diagnose a problem computer? It has to be unbolted from its
hiding place behind the dash. This is done for a couple of reasons. First,
to impress the customer, next to check the electrical connections, but
most importantly to have it out where you can hit the darn thing! Just
like jump starting a newborn, the computer will get a slap. Of course
the engine is running during this high-tech test. If the computer has
circuit problems, the slap test can cause the engine idle to become erratic
or even stall, which means the computer needs to be replaced.
GM
computers in the late '80s had a few problems. That was corrected, more
than once, with an improved computer design. Currently, the improved design
is only available from GM. If you purchase an aftermarket computer you
could be bolting on an old problem.
A
word of caution: The computer isn't always at fault. A perfect example
is the computer in my Buick. It has a dent in it from a flying coffee
cup, that bounced off the computer and broke the face on my test meter.
The real problem was due to corrosion on one of the sensor connections
for the computer at the engine.
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