still not charging…

[prizm01]

recently replaced my alternator on my 99 chevy prizm with a custom built cnc machined apex alternator due to the stock one failing to charge anything above 12.3/12.5 after replacement it ran did freaking amazing anywhere from 14.9 to 15.2 with headlights and ac on. about 3 days later i was playing music and my system completely cut out, and i look at my voltmeter and its down to 12.3 again! the car has a assload of grounds, one straight from the alt to the bat, to alt to chassis, and a 00 charging wire straight to the battery positive, my amps in the truck have a dedicated 00 ground to battery and a chassis ground aswell. all of my connections seem to be super solid and have no play. i cant seem to figure out what the **** is going on with this thing and its driving me nuts cause i daily it in the summer and have a show to attend on the 5th, if anyone has had any similar issues please let me know if you ever found a solution. or if you have any ideas as to what could be going on. thanks for reading

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[1aespinoza 10+ year member]

found a solution. or if you have any ideas as to what could be going on. thanks for reading

Did you measure the voltage at the alternator post?

 

[prizm01]

yes aswell as the rear batteries, everything is the same

 

[prizm01]

yes aswell as the rear batteries, everything is the same

 

[1aespinoza 10+ year member]

yes aswell as the rear batteries, everything is the same

I assume the alternator has a 4 wire harness. AFAIK, chevy alternators are ECU controlled. The fact that you got above 13.8v is beyond me. Others here have 15+ resting voltage but they use lithium cells and whatnot.
I recall a member here had the same issue and he traced it down to the exterior ambient temperature sensor. They told him the high reading caused the computer to lower the output to protect the alternator or battery.
The fix had something to do with reprogramming the ECU in some way beyond my knowledge. I'll try to find the thread.

 

[2023 LTZ]

My impala had a similar issue. It has a lead coming off a voltage sensor that was crimped into the main ground attached to the battery. I replicated this in 0ga but found the connection although tight to be the problem. I remade that wire in 0000 then separated that voltage sensor lead to the chassis and the problem was solved.

 

[prizm01]

just replaced the power cable lugs and fuse and seems to be helping, however after ebout 10 minutes of play time it completely cuts out and doesnt deliver power at all to the rear, but is now charging my front battery, definitely some sort of wiring gremlin

 

[2023 LTZ]

Tge several different ground paths I tried, connections and continuity were in check. Changing a single 12ga sensor ground lead location made the difference.
The more dug in the gremlin is the harder it is to find.

 

[metalheadjoe 10+ year member]

yes aswell as the rear batteries, everything is the same

Did you bump the throttle while testing? Might be low idle output.

 

[2023 LTZ]

Is that a gm drive train or Toyota?

 

[prizm01]

Is that a gm drive train or Toyota?

its toyota

 

[prizm01]

Did you bump the throttle while testing? Might be low idle output.

no difference, the alt is rated at 400 amps idle

 

[2023 LTZ]

Ah
My suggestions likely won’t work. I’d hoped if it were gm they’d have a similar setup but if it’s Toyota probably not.

 

[HardofWhoring]

no difference, the alt is rated at 400 amps idle

NO... WAY..

 

[some dude]

Yeah, 400A at idle seems very very unlikely.