I can NOT figure out why my battery wont read over 12v when on (3800 swap) (Page 1/4)
Threedog AUG 05, 05:58 PM

My battery only reads at ~11.9-12 volts when the car is running. I can run the car forever and it will start again, I can do this a few dozen times but eventually it wont restart. Accessories dip the voltage momentarily, but it stays at 11.9-12v.


1997 Engine, 3 wire connector.

- Red to PCM
- Gray to PCM
- Light blue (Terminal S) to +12v

(If I unplug this connector, the alternator stops charging completely)

I have replaced the alternator(x2) and the battery(x2).

I have three ground straps from the engine to the chassis, one from the battery to the chassis and one from the battery to the engine.

The positive cables test out just fine.

I ran jumper cables from the alternator peg to the battery positive cable, and from the negative battery terminal to another grounding point, didn't help.


Anyone have any ideas why this just won't charge?
sanderson231 AUG 05, 07:01 PM
The same voltmeter reads 13.5V or so on another car with engine running or about 12.5V with engine off
Threedog AUG 05, 07:03 PM

quote
Originally posted by sanderson231:

The same voltmeter reads 13.5V or so on another car with engine running or about 12.5V with engine off



Yup! Tried two different multimeters. Swapped the battery with my jetta as well and it continues to read at 12.1 on, 11.9 off.

Its somewhere in the system.

olejoedad AUG 05, 07:42 PM
What year is the car?
theogre AUG 05, 07:50 PM
Alt isn't turning On. Should see ~ 14V when On @ the battery.

What alt is it?

CS 130 and related needs alt lamp to turn on. Some versions use I pin as backup to turn on. See my Cave, CS Alternator
If S wire/pin is bad then often won't turn on. S wire is how the regulator reads volts. See my Cave, Alternator Sense

Many Alt's for newer engines are control by PCM and have a harder time to find what is wrong.
PCM may use any or all pins on alt side plug including P to monitor Alt shaft RPM.

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

[This message has been edited by theogre (edited 08-05-2022).]

Spoon AUG 05, 09:11 PM
You mentioned 3800 swap. Does the 3800 use a CS Alternator or other? I did same swap but haven't bothered to notice type alternator.

You may want to verify this.

Spoon

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"Kilgore Trout once wrote a short story which was a dialogue between two pieces of yeast. They were discussing the possible purposes of life as they ate sugar and suffocated in their own excrement. Because of their limited intelligence, they never came close to guessing that they were making champagne." - Kurt Vonnegut

sanderson231 AUG 06, 12:47 PM
I checked the factory service manual for a 1994 Pontiac Bonneville with a 3800 supercharged engine. The wiring schematic shows no wires to the P, F and S terminals on the alternator. As Theogre previously mentioned, there needs to be power to the L terminal to turn on the voltage regulator. In the Bonneville the source of this power is a "hot in run, bulb test or start" circuit from the ignition switch. Next it goes through a 15A fuse. Then through either a volt indicator light or a voltmeter in the instrument panel. Next it goes to connection C1-B1 on the PCM. A wire comes out of C1-B4 connection on the PCM and goes to terminal L on the alternator. The manual does not tell why power is passed through the PCM. If the voltage regulator is powered up but there is low voltage, high voltage, or a stopped generator, a switch closes in the voltage regulator causing the volt indicator light to illuminate.

In the 1988 Fiero there are wires to S, F, and L on the alternator. No wire to P terminal. As with the Bonneville, the source of power for L is from the ignition switch through a 10A fuse to the to either a charge indicator light in the instrument panel or the voltmeter in a rally pack gauge cluster (which contains a volt indicatore light). From the volt indicator light the wire goes to terminal B3 in the C500 connector (just behind the battery). So what I would do is separate the C500 connector (1/4" nut driver) and connect a jumper wire from the ignition switch side of the B3 terminal to ground. Turn the ignition switch to "run" and see if the voltage indicator light illuminates. If it doesn't it could be a burned out bulb, the gauges fuse, a dirty connection or a broken wire.

EDIT - failed to read the original post. If the S circuit is similar to the Fiero, it is a "hot at all times" from the battery via fusible link "E" at he battery junction block. The fact that the other two connections on the alternator are to the PCM, leads me to suspect that the PCM is doing the voltage regulation or at least influencing it. Perhaps dirty connections or a PCM problem? See the below link:

https://js-alternators.com/...lternators-explained

Really would help to have the factory service manual for the 1997 engine.
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formerly known as sanderson
1984 Quad 4
1886 SE 2.8L
1988 4.9L Cadillac
1988 3800 Supercharged

[This message has been edited by sanderson231 (edited 08-06-2022).]

sanderson231 AUG 06, 01:29 PM
More on PCM controlled alternators:

http://www.gonzostoolbox.co...atedAlternators.html

You might try pulling the plug from the alternator and running a 12V+ jumper wire to terminal S. This would isolate the PCM from the alternator and see if the regulator defaults to charging at something like 13.5V.


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formerly known as sanderson
1984 Quad 4
1886 SE 2.8L
1988 4.9L Cadillac
1988 3800 Supercharged

[This message has been edited by sanderson231 (edited 08-06-2022).]

Threedog AUG 06, 08:12 PM
A couple notes and things I tried today (thanks for the suggestions!).

I have an alternator with an external fan, it came from a 1997 Riviera(car is an 87 Fiero). The alternator is the same type and the wiring is the same since I did the swap in 2018, it worked fine for two years at 14v and charged fine, I can't pin down when it stopped working though (I have since replaced the alternator twice to no avail). I THINK it stopped working when I swapped out a starter? I can't confirm though.

Things I tried today:

- Removing the plug from the alternator completely. Voltage dropped to 10.5-11, when I plugged it back in it went back up to 12.1

- I tried having only the S terminal plugged in (the thicker wire that needs a +12v). It still ran at 12.1

- I tried hooking a fat 0 gauge cable from the negative battery to the chassis ground and engine ground (both again have multiple straps inbetween), and I hooked up a jumper cable from the battery positive to the alternator output peg in case that connection was weak, didn't help.


I should note that I can turn the A/C on max, and headlights on, and everything else electrical, and while the voltage drops for a second it returns back up to 12.1. So the alternator is working, its just not going to 13-14. I have put 2000 miles on it like this and only had the battery die once or twice (different batteries) but it never read over 12.1 volts.


This just doesn't make any sense to me. The alternator is turning on and charging, it is regulating itself to 12.1 when accessories are turned on regardless of if it is controlled by the PCM or the internal regulator, it just wont go above 12.1. The only thing I can think of now is the belt maybe? The alternator is in the original location and there is a power steering pump delete pulley. It required me to get a slightly smaller belt, and I made sure that I have less than 1 inch of play on the belt (the A/C compressor works fine, but it does slip on hard shifts once in a while at 5800+ RPM).

[This message has been edited by Threedog (edited 08-06-2022).]

olejoedad AUG 08, 09:43 AM
If you could provide information on how the alternator is hooked to the PCM it would help.

Where is C500 B3 (brown wire) terminated?
Where is the red wire from the small alternator plug terminated?
Where is the grey wire from the small alternator plug terminated?