No alternator output (Page 1/1)
Rsvl-Rider OCT 15, 01:06 PM
I've been struggling with a charging system problem on Jenny's 86 GT with 3.4L swap with a 5 speed manual and looking for some help. The battery is not receiving charge voltage from the alternator. The battery and alternator are brand new. I identified a blown fusible link below the C500 connector and replaced it with a good one. All other fuses and links are OK and I am not seeing a parasitic drain. The battery will maintain voltage while sitting. It just doesn't receive charging voltage from the alternator.

I'm not getting battery voltage (zero volts) on either of the two wires in the small connector that connect to the alternator. As I understand it, one of these wires senses the battery charge level and controls the alternator output voltage to recharge the battery. I was told that this sensing voltage is supplied by the ECM so I swapped the ECM from a stock 86 V6 with a 4 speed manual. Although the car still starts and runs there is still no voltage on the connector and no charging voltage from the alternator.

I see that part of the charging circuit relies on continuity through the battery warning light in the guage. The PO that did the swap replaced the aux gauge set with aftermarket gauges and the warning light must have been bypassed. I don't think that's the problem because the car ran great for several months after we aquired it.

Any help appreciated. Thanks.
theogre OCT 15, 06:02 PM
New alt's can be dead out of box or soon after. More so if buy from Eflay and others. Get it tested at local store.

Most Fiero, Alt needs dash light working to turn on.
But Engine Swaps may have problem(s) w/ PCM and not turning On even when Dash light works.

------------------
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

olejoedad OCT 15, 09:21 PM
What ECU are you using?

The stock Fiero ECU has no interaction with the alternator.

The red (or brown) wire gets 12v from the gage fuse via the CHARGE light with KEY ON.
The orange wire gets 12v constantly from the Battery Junction Terminal.

If those conditions on the small plug are met and the alternator isn't putting out anything, the alternator is defective.
fierofool OCT 15, 09:49 PM
One time the solution I found when I was about to remove the alternator on my 87GT to purchase a replacement was that the harness had come unplugged from the alternator.

Way before that on an 85GT I had, when I took the 'dead' alternator in to Advance Auto to get a replacement, the young girl asked if the BAT light in my instrument panel had burned out. She told me that if it was burned out, the alternator wouldn't charge. I drove back home, checked the bulb and it was burned out. Replaced it and the alternator charged. That girl was so young I was surprised she even knew what a Fiero was. She also saved me about $80.

So, check the bulb in your gauge and check the harness where it plugs into the alternator.
olejoedad OCT 16, 09:37 AM
The CHARGE light bulb must be good for the IGN voltage to be sensed by the alternator.

The bulb doesn't turn on unless there is only voltage on one side of the bulb. When alternator voltage output decreases, the light glows because the voltage across the bulb filament is weaker and allows current to flow and illuminate the bulb to varying degrees, depending on the voltage imbalance.
Raydar OCT 16, 12:16 PM

quote
Originally posted by olejoedad:

The CHARGE light bulb must be good for the IGN voltage to be sensed by the alternator.
...



I've been doing alternator stuff, most of the weekend. The 88 manual shows a resistor bridged across the Charge lamp bulb.
I didn't bother to chase that, since I was off chasing a different problem. Just noticed it in passing.
theogre OCT 16, 02:14 PM

quote
Originally posted by Raydar:
I've been doing alternator stuff, most of the weekend. The 88 manual shows a resistor bridged across the Charge lamp bulb.
I didn't bother to chase that, since I was off chasing a different problem. Just noticed it in passing.

FSM? Where? Because I can't find that in E-diag section. Shows a Diode is series w/ change light but may not really be there. If there, can be just about anywhere in the cabin.



Just ground B or L pin in alt's side plug to test the light works.

Most think FSM is a Bible but are often wrong. Because "authors" copy/past data for a page or a whole section and never check is correct for the car.

Even If has this is a mute point because of engine swap that can be wired in several ways and including wire(s) to PCM that turns On the alt.
Many Swaps use CS130 w/ I pin wired to PCM or other "12v" sources sim to 87-88 Fiero 4cyl to turn on if the charge lamp dies.

[This message has been edited by theogre (edited 10-16-2022).]

Raydar OCT 16, 05:09 PM
It was in the "Charging System" section. It gave a quick diagnostic procedure.
This might just be a generic diagram, though. Obviously there's not one in the electrical diagram that is further back in the book.




Edit - your pic seems to show the SI alternator. Not that it matters for this discussion.
Here's the one for the CS. (This also seems to have minor differences from my hard copy manual. <shrug> )

[This message has been edited by Raydar (edited 10-16-2022).]

theogre OCT 16, 07:13 PM

quote
Originally posted by Raydar:
Edit - your pic seems to show the SI alternator. Not that it matters for this discussion.

Yes because that came in 86 GT...

Even if has CS130 now... Fiero CS130 wiring often doesn't = to whatever wiring for a swap.

Fig 10 is generic and little to do how wired in real world. Very likely GM Bean Counters won't approve that resistor in Fiero and most others.
olejoedad OCT 17, 03:55 PM
The diode is located in the C500 harness near where the ECU is located.