Test Benches

[Gregory A. Parmer]

A "test bench" is nothing more than a way of faking the ECM into believing
it's in a running car when it's really not. With a few variable resistors
(pots) you can see how twiddling inputs affect the ECM even in situations
that'd be almost impossible to see in real life. Despite having the
formula in front of me (see the FAQ or archives), my "clue" light didn't
come on that injector dwell is *much* more closely related to MAP than TPS
until after I experimented on the bench.

My super low tech test bench consisted of the harness I intended to
install, some pots, and a variable speed drill (!) for spinning the dizzy
to any desired RPM. Injector dwell was read with an old fashioned points
dwell meter.
 http://www.aces.edu/~gparmer/efi/harness.jpg

Bruce (and others too) immediately made mine look downright foolish as he
graduated to something described at
 http://www.diy-efi.org/gmecm/projects/ecm_testbench/
The description there is quite detailed, BTW.

This works great for verifying that a set of conditions reacts as you
expect based on your research/guesses, but...

The next level would be to log data from a running vehicle, then use that
log file as input to a device which would re-create the signals seen in
actual on-road conditions. Wondering if your TCC settings may be
contributing to tranny failures? Verify it safely on the bench!

It'd be even more useful for cycling through a known set of inputs to see
how changes affect the outputs. In short, it would make reasonably
thorough scientific analysis possible.

One weekend a few years back I got the idea that I could use the LPT port
and some RC circuits to generate the needed analog (and digital) signals
based on a log file. I went as far as to prove I could modify an old Boca
External Modem and that I could possibly learn enough Visual Basic to do
it before I remembered how much I hated Circuits 101.

That weekend's hardware modification efforts are reflected in pics at
 http://www.aces.edu/~gparmer/efi/testbench/

If you're interested in building TestBench Generation III and like the
modified modem concept, I'm sure I could round up some more...if you've
got a better idea, please go for it and share your results.

-greg

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