[Diy_efi] Switched High Voltage Drop?

[Tom Visel]

Try disconnecting the alternator - both the plug and the output lead - 
to see if the symptoms go away.  If they do, you most likely have bad 
diodes.  Alternatively, you could take the car to a shop or parts store 
and have them check the alternator for bad diodes (excessive AC 
ripple.)  To your ECM, a bad diode is the equivalent of a booming stereo 
blasting while a class full of students try to take a math test - it 
can't concentrate and its numbers get skewed by the ever-changing values 
it sees from its A/D converters, which are seeing ever-changing 
constants provided by the ECM's internal voltage regulators.

The overall DC voltage of the ECM's power supply isn't awfully critical; 
as long as it isn't too far away from nominal, you're OK in that 
regard.  However, the same voltage drop that allowed electrons to get 
lost on the way to your ECM might have also told your alternator's 
voltage regulator (sense terminal) that the battery's state of charge 
was lower than it actually was, leading to a long-term overcharge 
condition.  Long-term overcharging, besides being bad for the battery, 
will overheat and kill the diodes in the alternator, because they don't 
get a rest.  They have a duty cycle, and if it is exceeded for too long, 
they will fail, either open or shorted, which leaves you with an 
alternator that produces what I call "polluted DC."  Imagine the graph 
on a heart monitor - that's more or less what a failed diode looks like 
on an oscilloscope.  AC is a sine wave, DC is a flat line, and normal 
alternator output is an almost flat line, like a pond on a windy day.

I would get your diagram out, follow the power from the ECM back to its 
source, and take measurements all the way.  Place your positive lead on 
one side of a circuit section, and the negative on the other, and 
measure that drop.  When you find the bad section, narrow it down to a 
connector or fuse or whatever and correct the problem so it doesn't 
return.  Explain to your friend the benefits of not just fixing the 
problem, but the root cause, and he can buy us both a beer.

TomV


Becker, Damon (Damon) wrote:

> It's now my turn for an electrical question. 
>  
> I have an MR2 Turbo that has TERRIBLE drivability.  It's my buddy's, 
> and I can't figure it out.  One thing I noticed is it has a 1.6 volt 
> difference between battery voltage and ECU voltage.  It's not a ground 
> issue, there's less than 0.1 volts drop from ECU ground to battery 
> negative terminal.  It's pretty much all in the switched high bus, 
> which has been tapped to death, but is still fused with stock-sized 
> fuses. 
>  
> My big question is this:  The drivability issues happened on the road 
> one day, and no electrical modifications were made for months prior to 
> this happening. 
>  
> Could this voltage drop be the culprit? 
>  
> Thanks in advance for your help.  I'm completely flabergasted. 
>  
> Damon
>
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