Phil Hunter diy-efi-d at t-n-e.com
Tue Nov 16 17:30:47 GMT 2004

Using an ohmmeter to follow traces on a populated PWB/PCB is using the
wrong tool for the job, akin to using pliers on a nut or a hammer to
drive a screw. Yeah, you can do it, but we're talking ECMs here,
things you probably don't want to fail miles from home because you
initially damaged it some time earlier.

Do a simple test if you have 2 Multimeters, set one as an ohmmeter,
the second as a voltmeter, and measure the voltage across the ohmmeter's
leads, note the polarity also. Switch through the ranges, and notice
how the voltage changes. Often the highest voltage is on the lowest
scale, the one you would likely use to follow a trace.

Now reconfigure the voltmeter as an ammeter and measure the current.
A "perfect" ammeter would be a dead-short, so we're measuring the
maximum current from the ohmmeter for that range.

Most every time I can figure out where a trace goes just by looking
at it w/ a strong back light. Even 4 layer boards, w/ power and ground
internally, are fairly easy to follow since the traces generally do not
change direction much except at a feed-thru. Same holds for traces under
IC's, if there are no feed-thrus in line w/ it, the trace generally
comes out straight across from where it went in or it went to one of the
near-by pins.

My thunk on what ought to be done is to come up w/ a DIY Continuity Tester.
Something that, say, puts out 0.1 Volts w/ an output resistance of 1Mohm
for the source. Not enough voltage to forward bias virtually anything,
and only 0.1 microAmps current if it does. For finding the other end of
the trace, use a comparator w/ a threshold set at 0.05V to drive an LED.
Anything below 0.05V, you're not on the same trace and the LED is out.
Above that, you're on the right wire and the LED lights up.


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