[Diy_efi] Switched High Voltage Drop?

[Tom Visel]

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. 
>
> ------------------------------------------------------------------------
> *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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>>  
>>
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