[Diy_efi] Switched High Voltage Drop?

Tue Nov 29 20:05:48 UTC 2005

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 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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