99 Legacy Electrical Gremlins

[Legacy9903]

Ok so to start, I have a 1999 Legacy Postal Wagon, automatic transmission. I’ve had it about 6-7 months. And since I got it it’s had gremlins with electrical.

 

Explanation of the issue:

Bought a voltage gauge (that reads accurately) installed it, and noticed voltage reading fluctuating. And by fluctuating I mean it would read voltage like this: 14-13-13.5-13.3-13.8-14. And that’s at 2500 rpm with no accessories on at all.

Was driving the car and it dropped to 8.5v on the gauge and the car wouldn’t shift and almost died. I had previously replaced the alternator that was working but very squeaky with a Oreilly’s rebuilt one so I took the alternator back and had them test it. Tested good.

Had them charge and test the battery, tested good.

Replaced the alternator anyway with another Oreilly’s rebuilt, still fluctuating.

Rebuilt the original alternator (25yo Fuji, original to the car) to brand new condition inside and out and had it tested by a trusted professional shop, tested good.

Installed a brand new Interstate Battery, tested good.

Replaced the battery-to-engine ground, cleaned up the battery-to-frame ground, and added a battery to strut tower ground, all making clean and solid contact….

STILL FLUCTUATING. I’m gonna lose my mind. And today on the way to the store and back it wouldn’t go above 13.5 volts. I turned lights on, lights off, fan on, fan off… no difference. 13 volts at idle. 
 

I know it’s supposed to fluctuate a little bit with stuff on or off and it’s only a volt, but, the consistency isn’t constant at all and it makes no sense.

Someone please help. And thanks in advance.

[Infosecdad]

In my opinion, that's basically normal.

I chased this rabbit for over a year trying different alternators, batteries, and even a fuse that will under report voltage to the ECU so that it asks for more voltage from the alternator (pretty sure I ended up frying some coils after a year with the boosted voltage).  At the end of the day, I typically track 13.5-14.2v.  On rare occasion, it'll go outside that range for a short while, then return.  The voltage is managed by the ECU and it has a bunch of code and sensors to determine what voltage.  The difference in 13.5 and 14 may simply be the charge state of your battery, or different sensor readings.

If you have a known good battery, alternator, and grounds; and the voltage is >13, then I would consider that good.  It's a 25yo car, newer ones may have a narrower window of fluctuation, but what you are describing, I would consider good. 

[Legacy9903]

1 hour ago, Infosecdad said:

In my opinion, that's basically normal.

I chased this rabbit for over a year trying different alternators, batteries, and even a fuse that will under report voltage to the ECU so that it asks for more voltage from the alternator (pretty sure I ended up frying some coils after a year with the boosted voltage).  At the end of the day, I typically track 13.5-14.2v.  On rare occasion, it'll go outside that range for a short while, then return.  The voltage is managed by the ECU and it has a bunch of code and sensors to determine what voltage.  The difference in 13.5 and 14 may simply be the charge state of your battery, or different sensor readings.

If you have a known good battery, alternator, and grounds; and the voltage is >13, then I would consider that good.  It's a 25yo car, newer ones may have a narrower window of fluctuation, but what you are describing, I would consider good. 

I agree and that’s what’s odd when it drops below 13v but maybe I’m worrying about it too much. I’ll keep an eye out. Thanks for your help

[doublechaz]

Engine off would be 12.7 for a good and full battery.  12.6 with some age or frequent starts and very short runs (not all the way full...).  Running 13+ is normal as described above.

The 8.5 brief excursion is abnormal and caused problems.  That was probably a bad ground or bad positive lead at the battery where it corrodes.  Sounds like you got that fixed.

The reason 13.0 to 14.7 is normal is that depending on the battery fullness, total system load, and temperature, 13.0 might not be enough to refill, or in other conditions, 14.7 *will* boil away all the acid and destroy the battery in as little as an hour.  The voltage *must* follow the total system state to keep the battery happy and it would be technically failure prone and totally unnecessary to clamp the voltage to a specific level between the battery and the rest of the systems.

Edited Friday at 10:50 PM by doublechaz

[silverton]

5 hours ago, doublechaz said:

Engine off would be 12.7 for a good and full battery.  12.6 with some age or frequent starts and very short runs (not all the way full...).  Running 13+ is normal as described above.

The 8.5 brief excursion is abnormal and caused problems.  That was probably a bad ground or bad positive lead at the battery where it corrodes.  Sounds like you got that fixed.

The reason 13.0 to 14.7 is normal is that depending on the battery fullness, total system load, and temperature, 13.0 might not be enough to refill, or in other conditions, 14.7 *will* boil away all the acid and destroy the battery in as little as an hour.  The voltage *must* follow the total system state to keep the battery happy and it would be technically failure prone and totally unnecessary to clamp the voltage to a specific level between the battery and the rest of the systems.

this is why newer cars have battery sensors