GM TB injector questions, HELP!

[Tom Cloud]

>Well I finished my carb to TBI conversion on my 86 chevy 2.8l last night.  
>Double checked everything and turned the key hoping to hear the engine 
>spring to life.  But alas, it was not to be... so now I have questions!
>
>I put a DMM (I have a cheesy little Radio shack analog meter) on the 
>injector leads, cranked the starter and the meter seemed to move to about 2 Volts. 
>I know that the injectors are pulse width modulated, and at start up I expected
>the reading to be closer to 12V.  Their is a high pulse width at start 
>up right?  the DMM should integrate the signal and show around 12V, right???

you'd probably do better with the analog meter

 - you have a variable pulse width .... at full pulse width
you should see 12 volts (this assumes you don't have a system
that puts a large initial pulse to open the injector and then
a smaller hold current), but at lower pulse widths, you can
expect to see a lower joltage.

 - the meter will read from 0 to 100%, where 100% is 12 volts
(that's how an analog tachometer works) -- (once again, this
assumes your driver is putting out 12 volts or zero and not
some other value like many do)

 - if you want to make it read 0 to 100% regardless of the
type of injector driver you have, you need a "signal conditioner"
circuit.  You want something that senses when the injector is
getting current -- any current -- and gives a digital, on or off
signal as an output.  A simple BJT or FET switch will do this.
Then you can calibrate the output to drive your meter to full
scale (or to a "1" or "10") on your meter for 100% so you can
more easily read pulse width.

>The throttle body (w/ injectors) was purchased from a junkyard.  I
>hosed the thing off good with carb cleaner and put a rebuild kit in.  New
>seals and filter screens around the injectors.  Could the injector solenoid 
>be stuck?  How common is it for these injectors to go bad??
>
>I see no evidence of fuel being sprayed into the throttle body when
>I crank the starter, I don't smell it or see a trace of it.  Is it
>the injectors, or the computer...  HELP!!:)
>
>TIA as usual
>
>Greg Woods
>gwoods at symtx.com
>austin, tx

dang Greg, you're here with me .... I'm at 223-4849

Tom Cloud <cloud at peaches.ph.utexas.edu>