[Diy_efi] Switched High Voltage Drop?

Tue Nov 29 16:21:30 UTC 2005

Yeah, you are preaching to the choir about the wiring.  I have given him
a lot of crap over it.  Before I could get as far as I am, I had to fix
a lot of hacked up wiring that used those cutting splicers.  *shudders*

I hear you on the emissions thing.  It's needed here because it's REALLY
polluted (stupid SUVs).  I wouldn't mind living somewhere that doesn't
have quite a few of our retarded California-esque laws.  Of course, this
place is the ultimate outdoor playground, so that counters it.  

Anyway, thank you for your help!  

Damon

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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 9:14 AM
To: diy_efi at diy-efi.org
Subject: Re: [Diy_efi] Switched High Voltage Drop?

I'm in Prescott Valley, AZ - 5300 feet, so I don't mind altitude, but
I'm allergic to restrictive emissions inspection programs.  Pretty happy
with my own biz, too.  Had to open it so I could get paid for
electrical/driveability diagnosis - the shops I was working at before
didn't recognize the value.  Sorry!
TomV
P.S.  You could install all that added key-on stuff on a relay, make the
whole thing neater, and save trouble.  Just a thought.

Becker, Damon (Damon) wrote: 

	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.  

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	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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