[Shop-talk] noid light

[David Scheidt]

On Tue, Oct 26, 2010 at 10:52 PM, Eric J Russell <ejrussell at mebtel.net> wrote:
> I have a dead cylinder on our Alfa Romeo GTV6. Spark plugs are good (swapping
> them does not change the location of the dead cylinder), plug wires are good
> (normal resistance and I can get a good fat spark by holding the end 1/4" from
> the block)
>
> I want to check for electrical signal to the fuel injector. Can I use a
> digital voltmeter to test? (Start engine, remove connector and put the
> voltmeter's probes on the two contacts. I have a wire diagram to tell me which
> wire is + & - )
>
> I don't have a noid light. Is that an LED or some special light bulb?

Depends on the kit.  It's just a light, though.  LED ones blink on and
off faster, and you can sometimes see something from that.  A bulb
just lights up.  They're cheap, though.  A vehicle specific kit is
like 10 or 15 bucks, and I'd rather use one than try with voltmeter.

You can test with a volt meter, but the circuit works in a way you
might not expect.  There's always 12V on one lead, and the other is
switched ground. You may, or may not, be able to see the ground switch
on and off.  (If you've got an analog meter, dig it out for this.
Needle jumping is easy to spot.)

You can also measure the resistance across the injector coils, which
will let you find a shorted or open coil.  You may, or may not, be
able to find a spec, but they should all be close.  If you can, test
hot and cold.  I've seen injectors that fail cold, and work hot, and
vice versa.

If you've an oscilloscope, this is the time to use it.  (If you've
always wanted one, this might be an excuse.)  You can watch the
waveform, compare different injectors, and see the current draw.
That's very handy for things that aren't quite as dead as yours seems
to be.





-- 
David Scheidt
dmscheidt at gmail.com