Date:         Sat, 7 Dec 2013 17:19:49 -0500
Reply-To:     Steve Cotsford <cotsford@AOL.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Steve Cotsford <cotsford@AOL.COM>
Subject:      Re: making a little headway but need advice
Comments: To: David Beierl <dbeierl@attglobal.net>
Comments: cc: Neil N <musomuso@gmail.com>
In-Reply-To:  <52a39aa5.0476310a.7d9b.520d@mx.google.com>
Content-Type: text/plain; charset=us-ascii

Thanks David.      I think when I went looking for the B+, I found them on both connections of the l/h plugs.   At that time I still had the r/h injectors plugged in so I unplugged them to look for B+ on the r/h injector plugs.  I could only find one on each plug.   It did not occur to me that unplugging the injectors on the r/h side would effect the l/h side connections.   Your AAAANNNDD comment I think makes that easier to understand.
cheers,  Steve

On Dec 7, 2013, at 5:00 PM, David Beierl wrote:

> See oops below...
> 
> At 04:43 PM 12/7/2013, Neil N wrote:
>> Thanks for the correction David! I assumed the white wire from ECU 12
>> to fuel injectors only supplied a switced ground. I think I understand
>> what you're saying; this wire switches from + to -  (engine running)
>> to open/close the fuel injectors.
> 
> Not at all.  It does indeed switch the ground, because this is electronically easy and efficient to do.  When the transistor is conducting (injector firing) the great bulk of the voltage drop will be across the injector, since the transistor has a low resistance.  But when the transistor is *not* conducting, then all of the voltage drop will appear across the transistor.  Thus if B+ is not present on the negative side of the injector connector, it means there's an open circuit somewhere upstream of it.  If B+ *is* present on the positive side, that locates the open circuit somewhere between the two sides of that connector.
> 
> Same trick you use to locate a break in a window defogger -- on a good one you can run your probe along the paint stripe and see the voltage gradually decline (or rise, if you're going in the other direction).  But on a broken stripe you'll get either nothing or B+ right up to the break, where it will switch to the other one.
> 
> AAAAANNNDD...I forgot to mention that you have to unplug the other three injectors, because all four of them are feeding that negative side through their common connection to the ECU.  So only plug in one injector at a time for this test.  Big Oops.  Sorry, Steve.
> 
> Yrs,
> d