[Diy_efi] Switched High Voltage Drop?

Steve Ravet Steve.Ravet
Tue Nov 29 20:27:34 UTC 2005


Keep in mind though that the computer implements battery voltage
compensation, so that if voltage drops it will increase the on time of
the injector to compensate.  If the sensed voltage at the computer isn't
the same as the voltage at the injector then your pulsewidth may be off.
Not having looked at the table, though, I don't know if that matters at
the voltages being discussed here.
 
--steve
 



________________________________

	From: diy_efi-bounces at diy-efi.org
[mailto:diy_efi-bounces at diy-efi.org] On Behalf Of Jim Butterfield
	Sent: Tuesday, November 29, 2005 2:06 PM
	To: diy_efi at diy-efi.org
	Subject: RE: [Diy_efi] Switched High Voltage Drop?
	
	
	Just to add my thoughts... computers only need good 12v to
operate... I would think you are just fine
	 
	jim
	
	"Becker, Damon (Damon)" <damonb at avaya.com> 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 h! ire 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 diod! e
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.&n!
bsp; 
			 
			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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