12.5-12.9v when the engine is running, bad alternator?

[Dylan JZ]

I'm pretty sure it is, but to be sure I wanted to ask.

Car had 12.3v when I started it earlier, after being in stop and go traffic with the headlights dimming and such when I put on the brakes, the reading was bouncing from 12.5v to 12.9v. When I got home, I turned the car off and waited a little and then turned the key, and the AFC read 11.5v (not good).

bad alternator, case closed?



the battery is brand new, and I have been noticing strange fluctuations in the voltage after the car warms up lately. before this, the car would intermittently not start, and after knowing what I know now, I think it was the alternator not charging the last battery properly either.

 

[toyotanos]

How much electronic equipment are you running? If you're running a big pump, lights, gauges, sound system, etc you might be drawing more power than the Alt can put out. Either way I'd suggest a bigger alt (I've heard good things about MR2 and Driftmotion's 140 units) But yes, anything below 12.6v running is too low. In all actuality I expect to see 13.0-14.2 volts on most vehicles at 2500.

 

[Dylan JZ]

Thank you for the quick and informed response. As to what I'm running, just stock with the addition of an AFC and gauges, though it ran for years with the current setup, no issues.

Just started one day after idling for like an hour, went and tried to start it and nothing happened. This continued intermittently until I replaced the battery, but the charging volts were only 13v whilst driving, and now this that it's getting warm again I assume.

Anyhow, I'm gonna pick up a 92-94 sc300 alty, supposedly 90-100amps from what I've heard/read.

 

[Poodles]

More likely to be bad wiring (i.e. grounds and battery connections).

 

[mkiiichip]

Ground terminal F, and report the voltage.
http://www.cygnusx1.net/Supra/Library/TSRM/MK3/manual.aspx?S=CH&P=5

 

[Dylan JZ]

yeah, I checked the leads for corrosion and such Poodles.. they seemed to be fine from the portions available to the naked eye.

as far as checking the regulator, I was pretty sure that was the cause after I noticed the strange voltage fluctuations after driving for like 15mins or longer.



I ended up buying the 92-94 100A SC300 alternator.. there was however a 145A version, but I wasn't sure if that was too much for the stock system to handle (probably not?), so the 100A seemed to be the one of choice because others have used it for quite some time as a direct replacement.

 

[Poodles]

What's it's rated for it's it's peak, that's why they have regulators

 

[Dylan JZ]

ah, I have realized my mistake.. thanks for all the input guys.

 

[Poodles]

Regulator failure can can fry your electronics, that's why the test looks for high and low voltage.

 

[Dylan JZ]

You fellas may have been on to something else here..

I replaced the alternator with one from a '92 SC300 100A, and the car started up with 14.2v (already an improvement over the previous 13.7), but after driving for a while, it dropped to 13.5v, and near the end of the drive I was getting 13v flat rate. why is it dropping as the car warms up? more resistance? -I tried turning on the lights and any other accessories, and there seemed to be no voltage drop due to that nor during an rpm increase/WOT.


This car has a battery relocation in the hatch, with the positive lead (2 or 4 guage IIRC) running directly to the alternator, battery is only grounded in the hatch. The plug going into the alternator had some corrosion where the pins go in, so I used some CRC to remove it, but this may be something that extends farther threw those particular wires (an easy fix for the most part, if that turns out to be the case).

Should I be doing anything else? The battery read 12-12.4v after I turned the car off, which again is another improvement over the previous 11.5-11.7v.



Things I'm considering:
1. I'm going to replace my 100A alternator fusible link because the top plastic was missing, and as a result it was slightly corroded.

2. I've heard sometimes replacing the positive lead can work wonders, the line is 4 years old at this point. :shrug:

3. Grounding the alternator up front.




RECAP: Battery is a brand new 720cca optima red top, and alternator is a refurbished '92 SC300 100A. So, any issue is probably with leads/wiring/grounds etc.

 

[Poodles]

Battery needs to be grounded directly to the block (as it is stock).

 

[Dylan JZ]

Battery needs to be grounded directly to the block (as it is stock).

Alright. I'll do that along with changing the alternator fuse. Could that lack of ground be causing an issue such as the one I'm having? I'm still trying to figure out why the volts progressively drop the longer the car is driven.

 

[3p141592654]

You need to do a voltage drop test along the 12V and ground paths between battery and alternator. Find out where your missing 1.2 volts are going. Throwing parts at it is an expensive troubleshooting approach.

 

[Dylan JZ]

if the line between the battery and alternator is the main area containing the problem, then replacing the cable altogether doesn't bother me at all.

Now, if there are other paths to check, then those may end up becoming rather expensive in comparison.

 

[mkiiichip]

Why do you have the positive cable going to the alt, and not the starter? How are you powering the starter? Where did you run the small wire that used to go to the positive battery terminal?

 

[Dylan JZ]

The battery is also connected to the starter AFAIK.. As for the little wire, I think it now runs from the post where the (+) battery cable connects to the alternator.

 

[3p141592654]

Measure some data, use it to identify the problem, and fix it. Don't guess.

 

[Dylan JZ]

Figured out another problem, and from speaking with BigzAvs, I think we might have the issue..

So aside from wondering how the car functioned properly until now, I asked Ben about the fact that my battery is connected directly to the alternator. He asked why? I said, I have no idea, is that irregular? -he said absolutely.

Apparently, the battery is supposed to be wired to two places OE style. The main cable goes to the starter and another wire (he thought was blue?) is supposed to be hooked up to the fusebox from what he said.




Other than that, the alternator is plugged in and has the normal white wire running to the B+ post.

Why do you have the positive cable going to the alt, and not the starter? How are you powering the starter? Where did you run the small wire that used to go to the positive battery terminal?

you were on to something here, thanks for noticing that and saying something. It may be the source of these issues..

---------- Post added at 10:50 PM ---------- Previous post was at 10:42 PM ----------




Can anyone confirm what wire (and color) in the fusebox is connected directly to the battery in stock form? This is an '87 chassis.



I need to fix the routing of the battery cable in order to see if that changes anything.

 

[Dylan JZ]

Anyone?

 

[mikes_88_supra]

there should be a white wire with a ring terminal on it and a a black wire that is actually two (8ga wires i think) with a weird ring terminal on it. the black gets extended to b+ on the alternator the white goes to the pos side of the battery ( i think that if you want to run this to the starter along with the pos wire from the battery that would be fine as well) as long as the white one is connected to the battery pos then it should be good also your battery neg has to go the the block like previously mentioned and just a little side note the more grounds the better. i have the battery to block, bell housing to chassis, transmission to chassis and another going from the block on the passenger side to the chassis, and the battery to the chassis and all of them are 4ga wire except for the battery to chassis. this is defiantly overkill but ive had grounds go bad in the past and over heat the other grounds and then things tend to catch on fire so the more the better. if you have any more questions feel free to ask