Charging system spikes

[Tangonator]

Hey guys! I've learked around the forums for quite a while and never really had any issues with my 4runner... until today. The voltage at my ecu and at the battery will jump erratically from 12ish volts all the way up to 17 then randomly settle itself around 13.7. I've replaced the alternator thinking the regulator was on it's way out but that didnt fix squat. As you can see from the picture it'll go crazy then settle down and then spike up again. Starting doesnt seem to be a problem nor does heavy power draw. Any ideas guys?

Thank you!

 

[Tangonator]

Heres another image after going for a longer drive. I'm thinking it may be a dead cell in the battery? It's only a year or so old but it's not the best.

 

[riceandpho]

could be both, but check all terminals. including the alternator also.
then start the cheapest swap, which is the fuse, that 7.5 amp fuse i think, then the battery, then the alternator again. check all the wires first tho.

 

[Tangonator]

So I did a few checks today. Voltage drop across body to batt, alt to batt, and engine to batt all have very low voltage drop numbers (less than .04) I did notice the voltage at the battery at idle is 14.5ish and the ecu reads around a volt less.

 

[riceandpho]

now is this with hot reading or cold reading?
i know with cold they alternator can push out over 14 volts.
at hot idle it should be in the 13.4-13.8?

i really cant tell you since im using a high output alternator.

do you have a extra battery around that you can swap out? any group 24f or 27f is a drop in.

you can drop in a h7 h8 or h9 in, itll be short in height.
had an oddy pc1700mjt back then.

 

[Tangonator]

I took these readings cold. I already swapped it with a new battery and same thing. After I swapped it, charge light stays on, the system sees a steady 13.5ish hot and then eventually goes haywire spiking up to 17 on the ecu side. When I'm done at work I want to probe some of the fuses as well as the signal wires from the alternator and see if I can see the voltage drop anywhere.

 

[traildog]

I would check all your grounds. Make sure they're tight. Giving them a good clean wouldn't hurt either to ensure good metal to metal contact.

 

[Tangonator]

Figured out what it was testing wires from the alternator. The white wire from the alternator broke inside the harness. After replacing that everything worked great. Thanks for the help guys!

 

[riceandpho]

Figured out what it was testing wires from the alternator. The white wire from the alternator broke inside the harness. After replacing that everything worked great. Thanks for the help guys!

well at least its fixed now and you have a fresh new battery too.