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Old 03-16-2010, 10:03 PM   #1
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Default how stable is your voltage?

i'm beginning to question whether i have an issue or not. through megasquirt or tunerstudio, what does your voltage read and how stable is it?

mine idling at 900rpm with the headlights on it jumps between 13.25v-13.75v. a continuous jump of .5v seems like a big sweep to me.
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Old 03-16-2010, 10:09 PM   #2
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That sounds low, I think 14.5 volts is normal, which is where mine is. It jumps less than .5 volts too, I think somewhere around .25 volts, very rapidly.
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Old 03-16-2010, 10:13 PM   #3
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Sounds just a little on the low side. I would recommend confirming your voltage with a dmm on the battery before doing anything further, but it might be new alternator time. Your ecu doesn't have any control over this.
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Old 03-16-2010, 10:14 PM   #4
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Ok scratch that. Just cause I'm a nice guy, I went out and checked. At 900rpm I was getting 14.00volts, jumping up to 14.2 for a split second every 1-2 seconds. Didn't change at all when I turned the lights on.
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Old 03-16-2010, 10:18 PM   #5
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at the battery 14.22-14.23 stable

here's a random log, they all have always looked like this
Attached Files
File Type: msl 2010-02-01_17.12.48.msl (1.49 MB, 83 views)
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Old 03-16-2010, 10:29 PM   #6
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Well I was about to check mine and compare, but found 12.6 volts :( My poor altima red top has been through a lot. 6+ years old and when I still had a radio, I'd listen to it while I worked on the car, including months of rebuilding the engine, recharge the battery, repeat. Even so I'm getting higher voltage in TS :P
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Old 03-16-2010, 10:48 PM   #7
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Quote:
Originally Posted by Mach929 View Post
at the battery 14.22-14.23 stable

here's a random log, they all have always looked like this
Well the charging system sounds solid then. Maybe you have an ecu ground that's a little unhealthy.
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Old 03-16-2010, 10:56 PM   #8
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yay wires not my strong point
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Old 03-17-2010, 02:38 AM   #9
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Was it cold or warm. When its colder the alternator works harder and chargers the battery bet 14.2-14.5 when its warm usually 13.7-14.1
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Old 03-17-2010, 02:40 AM   #10
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My scangauge normally reads 14.5 with the engine running.
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Old 03-17-2010, 09:15 AM   #11
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it was about 50 degrees out. looking at some other's logs mine actually isn't far off though i'd like it to be cleaner, i'll clean up a couple grounds, if it doesn't help oh well the car runs fine, or with any luck it will burn to the ground
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Old 03-17-2010, 09:18 AM   #12
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Ben or Matt or Jerry, is this something the 3700uF cap might help?
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Old 03-17-2010, 09:31 AM   #13
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FWIW Megasquirt adjust certain parameters (like injection opening time, ignition dwell etc) according to battery voltage. If your battery voltage is 14.2V and your MS thinks it's 13.2V I suggest adjusting the MS voltage divider so that MS reports 14.2V as well.
Battery voltage is determined with a resistor voltage divider to bring the 12 volts down to under 5 for the CPU to convert. R3 (50K) and R6 (10K) form this voltage divider. These are both 5% tolerance resistors and in my case, the ratio was 5.22 instead of 5, causing similar differences between actual and MS values for battery voltage. I swapped the resistors out for 1% values and now it's much better (0.2V difference). If you're paranoic, you could replace one resistor with a trimmer and adjust that to the exact voltage.
$0.02 etc.
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Old 03-17-2010, 10:09 AM   #14
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Quote:
Originally Posted by Braineack View Post
Ben or Matt or Jerry, is this something the 3700uF cap might help?
Probably not--though it also wouldn't hurt anything.
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Old 03-17-2010, 10:10 AM   #15
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regarded the flux, not the voltage drop across the battery and MS...
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Old 03-17-2010, 10:23 AM   #16
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Yes, but I'm kind of worried that the flux is caused by a bad ground rubbing around.
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