[T3] Re/ Mystery Fix...

[Jim Adney]

On 27 Jan 2013 at 20:03, Daniel Nohejl wrote:

> Seems odd that between you and me there are 6 ECU's that'll run a car
> if the relay is jumped to ground but not if wire #19 is plugged in. 
> 
> Does this wire plug into some "difficult" area of the circuit board in
> the ECU such that units that are otherwise fine can have this single
> failure? Or does it suggest that there are multiple failures in these
> units, just not ones that we can immediately notice? 

The brain has a single transistor that acts as a switch to short #19 
to ground. I've seen exactly one brain where that transistor was 
broken: In that case, one transistor lead had corroded away because 
of the damp local weather.  

That transistor could also be blown if the 2 relay coil leads got 
shorted together when the key was turned on, or if that lead got 
connected to a 12 V source. That would allow unlimited current to try 
to pass thru that one small transistor, burning it out instantly. 
That's why those leads come with a plastic insulator around them, 
which also makes it impossible to put them on the wrong terminals.  

-- 
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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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