Re:L-Jetronic ECU fragility? was V6 stalling - slightly off topic!

[George Graves <gmgraves@xxxxxxxxxxx>]

On Thursday, June 19, 2003, at 10:00 AM, alfa-digest wrote:

> Date: Thu, 19 Jun 2003 18:07:05 +0200
> From: "John Fielding" <johnf@domain.elided>
> Subject: L-Jetronic ECU fragility?  was V6 stalling  - slightly off 
> topic!
>
> Hi All,
>
> George Graves today on the AD made a comment about L-Jetronic ECU's 
> being susceptible to
> probing with a test lamp.  He intimated this can cause damage to the 
> internal electronics.
>
> Well, having repaired numerous L-Jet and Motronic boxes and seen the 
> sort of circuitry
> used in them I would very much doubt Georges comment.  (I also design 
> and manufacture
> automotive electronic modules so I have also been aware of some of the 
> problems you can
> experience).
I was repeating Pat Braden's advice as outlined in his "Alfa Romeo 
Bible." If I could put my hands on my copy, I'd quote chapter and 
verse, but I don't know where I put it. Maybe some other digester can 
come to my aid here.


> Bosch are not idiots when it comes
> to designing vehicle electronics and they do a comprehensive "what-if" 
> scenario as part of
> the design effort, like most vehicle electronics designers.  A 12V 
> battery is a hostile
> environment, one semiconductor manufacturer calls it "the supply from 
> hell" as it has so
> many glitches and transient spikes due to the many inductive circuits 
> being powered from
> it.  A Defence Standard specification exists for 24V vehicles.  One 
> paragraph I remember
> is the sort of transient caused by simply blowing the horn.  Voltages 
> typically to be seen
> on the 12V supply can exceed 60V spikes!  On fuel injected vehicles I 
> have measured spikes
> as much as 300V due to the injectors being switched on & off.  
> Normally the battery is
> considered to act like a large capacitor and should damp these 
> transient to neglible
> levels.  However, lead acid batteries are rather poor when they have 
> to deal with high
> frequency transients, they appear like a soggy resistor with a lot of 
> series resistance or
> impedance and do little to clamp the spikes.  The further away from 
> the battery terminals
> the worse this becomes.  As the wiring in a vehicle is a tightly 
> bundled set of parallel
> wires running for considerable distances, if a wire carries a 
> transient generated current
> the wires also running along side this one will pick up similar 
> voltages.  Because of this
> great care and additional circuitry is normally needed to prevent the 
> transients being
> induced into other lines from reaching damaging proportions.
While this is, as you say, a bad thing, an interesting aside (which 
backs-up what you say about the system being poor at suppressing 
transients) is the fact that the G-Tech Pro performance meters all get 
their RPM tach information out of the same cigar lighter jack as they 
get their power! They tell the operator to rev the engine first to 3500 
RPM and hold it, then press the "OK" button, then let the RPMs drop to 
1700 and press the "OK" button again. The computer obviously integrates 
the noise spikes on the line to read RPMs. It works too. My G-Tech Pro 
Competition model's digital tach tracks my GTV-6's tach very accurately!
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