ABS Diagnostics and testing

[Phil Payne]

> Question is twofold: (1) What other diagnostics can I do to narrow down and resolve the
problem with my car?  With winter and rainy season coming, I don't want this problem
outstanding for too long especially since today in the rain I experienced harrowing braking
experience when someone cut in front of me, so fix is a priority.  The Bently indicates use of
an anyalyzer that plugs in where the ABS Control Unit(ACU) goes.  I suppose this checks inputs
and performs some output to the various ABS sensors and pump controls.

Hmmm.

I have had success with an a/c millivoltmeter.  You need long leads.  I made up a lead using
an ABS sensor connector from a junkyard, but these days I wokr from the main connector.

Disconnect the ABS controller and connect the millivoltmeter to any pair of wires in the
connector that have the same solid colour - i.e., no stripes.  Do a controlled drive at
relatively low speed - ca. 10 mph?

Note the voltage.  Repeat for the other three pairs of wires with solid colour.  One will
stand out.

The quick fix (once you know which sensor it is) is to loosen the clamp very slightly and give
it a gentle tap with a soft-faced hammer.  Three out of four success rate.

Quite fun - the controller has a timeout function and allows a few seconds for its internal
amplitude and frequency tests.  If you start off and get to a reasonable speed (>30 mph) quite
quickly, it won't spot a bad sensor until you slow down.  There's a square law involved - the
slower you go, the more obvious any fault will be.

-- 
  Phil Payne
  http://www.isham-research.com
  +44 7785 302 803