[T3] gas mileage, voltage, etc.

[Jim Adney]

On 24 Jun 2017 at 19:15, Daniel Nohejl wrote:

> Jim........I was just thinking about what you said about an unfused wire
> shorting out to something in the brain. I have that hot start relay
> installed and you might remember that the fuse blew twice in close
> proximity to your house last summer and that you replaced the 15 or 30
> amp fuse with a bigger one. Any chance that excess voltage i??reaching
> the brain via the harness from wire 18 at the starter? I assume not,
> but beyond the harness, that relay is the only non-stock thing weTMve
> installed. 

I think you mean wire 19. That wire should not be a problem, since it 
expects to get full unfused 12 V every time you start the car. A 
weakness would have to be a brain input wire that connects to some 
circuit that only expects to see tiny voltages/currents. That would 
include most of the sensor wires.

If the drive wires to the injectors got accidentally grounded, that 
MIGHT cause a problem, but I'm not sure, since there is a series 
internal resistor for those outputs, which would limit the current in 
case of a short to ground.

Keep in mind that all this is pure speculation. It's just as possible 
that the problems are due to a broken solder connection or a marginal 
component that tends to fail, or some kind of voltage spike that your 
car's charging system is putting out. (But there is some level of 
filtering for that kind of problem built into the brains.)

All of this is made more difficult due to the lack of schematics for 
any of our brains. The only one we have is for a much later 914/4 
brain.

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