A common piece of automotive test gear is a light to plug into the fuel
injector harness [and other places], that simply flashes when the ECM tries
to fire the injector. It confirms that the injectors have power and that
the ECM is firing them. The light can also be a very rough-resolution
voltmeter, i.e. if applied across power and ground, it turns on. The light
is generally designed to load the circuit slightly, as an injector would
but probably not quite as much, but at least we can see that the circuit
in question is able to provide real current as opposed to carrying a
The problem is, some events that happen in a car do not last very long,
and it's easy to fool our eyes. Our ears, on the other hand, respond
to much higher-frequency stimuli and our brains are very good at picking
out patterns from sound. It may also happen that one needs to check
power at one point in a car while fooling with part of the electrical
system at the *other* end, and it's not often convenient to look up and
see if a light is on or not. Wouldn't it be handier if some sort of beeper
could go off instead, allowing full attention to finding the appropriate
connections rather than having to glance back and forth? This solves
that problem. It is derived from a very early quick-n-dirty prototype
from several years back, when it was used with a needle to puncture
tiny holes in harness wiring when I needed to find *which* of about a
dozen green wires within a huge bundle was carrying a signal of interest.
That rig rattled around in the trunk of two subsequent cars and finally
fell apart, so I decided to rebuild it better, faster, stronger, etc.
Assembly almost finished. It's pretty simple. Two leads come in and go
to an old "Sonalert" type beeper, and also to a bi-color LED with a 470-ohm
dropping resistor. The combination of those draws all of 30 mA at 12 volts,
and the hack here is to make the circuit actually draw some "meaningful"
current -- in the automotive context, that usually means something over an
amp. A 10-ohm resistor is in parallel at the other end of the box, able to
be switched in or out. So it can be a fairly high-impedance circuit, or a
low one. This allows testing for bad grounds or harness connections under
some appreciable load. It could also serve as a current-protected "power
probe" style supply to connect to some suspect device, to give it a little
bit of power but avoid shorting +12 through something that's defective
[such as a fried injector]. A similar hack is used to test household
AC-powered devices -- wire two outlets in series, plug a light bulb or
toaster [depending on desired current capability] as a feed load, and then
it's impossible for the device under test to blow the circuit.
The large resistor is floating in the other end of the box, loosely
supported by a phenolic ring so that if it gets warm, it won't melt the
box [at least not right away]. And if left on 14V in low-impedance mode
for a while, it *will* get hot. Some of the heat may escape through the
large hole nearby that used to admit a power plug in this box's previous
life as a laptop power supply or something. This is test gear, designed
strictly for intermittent duty. Heck, it's not even fused. I was going
to, but ran out of room to install a fuseholder.
The completed box, howling away when fed 8V or so from a spare Prius module.
The sonalert is *really* loud, necessitating the piece of tape over it for
now. Without it, you would easily drive all your shopmates out with their
hands over their ears, but it's nice to have that on tap for debugging in
noisy environments. Reverse polarity makes the LED go red and no sound
come from the sonalert, so that's another hookup option.
The only minor problem is that the sonalert doesn't respond instantly; it's
got a little bit of a fadeup and fade-down. But it's fast enough to audibly
detect flakey wiring when wiggled, and the volume varies enough with supply
voltage that one can distinguish, say, a 12V supply vs. a 5V supply. And
it *does* still have a visible light -- nice and fast since it's an LED.