Date: Fri, 28 Jun 2002 17:32:36 EDT
Reply-To: Trvlr2001@AOL.COM
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: John Carpenter <Trvlr2001@AOL.COM>
Subject: Re: Installing cruise on Vanagon
Content-Type: text/plain; charset="US-ASCII"
Cruise Control Basics
Cruise control systems have been with us for years. They work very
effectively to control the speed of the vehicle, but when they fail they can
be a bear to diagnose and fix.
Cruise control systems are comprised of electronic and mechanical subsystems.
This is how they work.
We all know that the things that control the speed of the car are the gas
pedal and the brakes. And the brain that normally controls the speed of the
car is the brain of the driver. The driver senses the speed by looking at the
speedometer and then adjusting the pressure on the gas pedal or the brakes to
compensate for variations in the desired speed. The cruise control system
does the same thing with one exception. It only controls the gas pedal -- it
doesn't even know there are brakes in the car!!
The vehicle's speed sensor which is mounted on the output shaft of the
transmission (the thing that drives the wheels) sends electrical pulses to
the computer, pulses which are generated by a magnet spinning past a sensor
coil. When the vehicle's speed increases the frequency of the pulses
increases. For any given speed of the vehicle there is a corresponding pulse
frequency. It is this pulse frequency which the cruise control tries to
maintain as a constant. You think of it as the vehicle's speed.
The brains of the control box of the cruise control has three functions.
First, it stores the speed of the vehicle when you press the "set" button
while traveling at the desired speed. It keeps this value in its memory until
you turn the ignition off. Second, it receives the pulses from the
transmission sensor and compares the frequency of those pulses to the
frequency value stored in its memory -- the set point. Third, it sends pulses
to a vacuum controlled diaphragm connected to the accelerator linkage. The
pulses it sends regulates the amount of vacuum the diaphragm receives. The
more pulses, the more vacuum and the more vacuum the more force on the
accelerator linkage. The system continues to add vacuum force until the set
point speed is reached. At that point the system modulates the amount of
vacuum the diaphragm receives in an effort to maintain the number of pulses
coming from the speed sensor as close to the stored value as possible.
OK, so this "brain" works just fine in controlling the speed of the vehicle
until something goes wrong. What can go wrong?
First, the VSS, the thing that sends pulses to the brain might fail. Normally
the speedometer also fails so that's pretty easy to diagnose.
Next, the power to the brain can be interrupted. A blown fuse or a corroded
connector can prevent the brain from working correctly or at all.
Next, the brains can lose its ability to function. A faulty component can
prevent the brain from doing its thing. The brain is a pretty sophisticated
box that contains a lot of electronic components including a microprocessor.
Normally when the brains fail you need to replace the box.
The vacuum diaphragm can develop a leak. If that happens then the cruise
control might set and hold the speed for some time however if the leak is
larger than the supply line and modulator can add vacuum to the system the
system will slowly lose control and the vehicle will slow down. This can also
happen if the vacuum line to the diaphragm is cracked or loose.
Finally, the linkage that connects the diaphragm to the accelerator linkage
can fail. Some aftermarket cruise control systems use a short length of what
looks like fat key chain -- bead chain. I have seen several units fail when
the chain simply breaks.
Diagnosis of a failed system can be a complex process. Most vehicle shop
manuals have a multipage diagnostic flow chart that the dealer mechanics use
to solve failures. If there isn't an obvious problem like a broken wire, a
blown fuse or a leaking vacuum line then the problem most likely lies in the
brains of the unit or in the switch that sets the speed and contains the
other functions of resume and accelerate. Most cruise control switches are on
the directional signal stem, a multifunction switch assembly with fine wires
that break due to the constant motion of the wires as you use the
directionals in your daily travels.
If you do an initial diagnosis and can't find the problem then go to the
library in your town and get the shop manual. That is the only way you stand
a chance of diagnosing and fixing the problem.
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