[Diy_efi] Switched High Voltage Drop?

[Becker, Damon Damon]

Thank you so much!  
 
The battery voltage (which is the same at the alternator) is 14.6. The
ECU is seeing 13.0.  
 
He has a large number of aftermarket electronic stuff that is probably
all tied to the switched high bus.  The drop happens between the relay
and the ECU.  This is an MR2 Turbo, so there's a lot of really long
wires stretching from back to front, so it's not a small task to find
the culprit.  
 
What do you think about that voltage?  
 
By the way - Is your shop in the Denver area?  I would be more than
happy to hire you.  

  _____  

From: diy_efi-bounces at diy-efi.org [mailto:diy_efi-bounces at diy-efi.org]
On Behalf Of Tom Visel
Sent: Tuesday, November 29, 2005 6:42 AM
To: diy_efi at diy-efi.org
Subject: Re: [Diy_efi] Switched High Voltage Drop?


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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