3800SC Swap - Alternator options and not charging (Page 1/2)
GodSend JUN 27, 08:54 PM
Greetings Gents,

First off, Kudos to James L Brown for helping me out over facebook over the last year. Ive been bugging the sh*t out of him and I thought I would give the poor dude a break and post to the gurus here.

History : 1988 Fiero GT, Pulled the V6 a few years ago and have been swapping in a 3800SC from a 98 Buick Regal every since. Getrag trans, all mounts from FieroRog. Wiring harness was pulled from a junkyard fiero with a 3800NA swap, Was in an accident so im assuming it ran fine (or well enough to hit the road) PCM was purchased from James with a basic tune for my car. Car has never been on the road. Its up on jack stands right now. It starts and idlles. IM working on the exhaust right now, then its off for an alignment and we are ready to go.

Issue : Alt does not charge. Alt is a 11127N from a 2005-2008 Grand Prix. It's NOT the alternator that came with the engine OR the harness. Charge light on center gauge cluster does NOT light up with key in ACC (or during crank).

Things to know : The harness had a plug for a different alternator that I left behind, so I spliced in the pigtail from the 11127N. There were two wires on my harness engine. One Red and one Orange. There are two wires on my alts pigtail, one red and one green(ish). Originally upon James' advice, I hooked Red to Red, and left the other wire(s) disconnected. In this configuration I confirmed continuity from red wire (the only one hooked up to the pigtail) and it goes back to C500 B3. The large lug on alt runs to one lug on starter. Tonight I decided to hook up the second wire (Orange on harness to green on pigtail), no change. I also swapped the bulb in the cluster with a known good one. No change.

So. Advice? Should I try switching the two wires on the pigtail? Is there anything I can/should check with my multimeter from the alt to C500 to PCM to gauge? I have no reason to believe the alternator itself is bad, and as I understand it the gauge cluster battery indicator bulb is crucial for the system to work. Should it light up with everything disconnected? I would like to start by solving the dash light and work from there.

thanks in advance everyone.
olejoedad JUN 28, 10:28 AM
The alternator requires a 12 volt battery and a 12 volt ignition source to function.

If the alternator is NOT under computer control....

If the bulb on the CHARGE light in the dash is burned out, the alternator will not see the 12 volt ignition source and will not function.

If the alternator IS under computer control, the circuit is different. I can supply that info as well.

Your first task would be to determine if the CHARGE bulb in the dash is good.

[This message has been edited by olejoedad (edited 06-28-2025).]

Dennis LaGrua JUN 28, 11:15 AM
I use the GM C130 alternator on a couple of swap applications and this is how mine are hooked up. If you purchase an aftermarket connector it will have 4 wires on it a red blue yellow and white wire. You will use only three
Blue wire -this is battery sensing that goes to the battery terminal on the alternator
Green Wire- This is your 12 Volt ignition source.
Yellow Wire - The connection to the charging light which limits the input to three volts or less. If you have applied twelve votes here you've instantly destroyed the alternator
White wire -not used
This is pretty much what Joe has said. I just added the caveat that if you have mistakenly applied much more than 3 volts to the Yellow (charge light wire) your alternator is now toast,

------------------
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

Jncomutt JUN 30, 09:49 AM
Also to add what Joe said, use a conventional bulb for the volt light, not a LED. Some people may find a LED will work fine, but I've had it cause issues in the past.

[This message has been edited by Jncomutt (edited 07-02-2025).]

olejoedad JUL 02, 07:30 AM
Any updates?
GodSend JUL 09, 05:43 PM

quote
Originally posted by olejoedad:

The alternator requires a 12 volt battery and a 12 volt ignition source to function.

If the alternator is NOT under computer control....

If the bulb on the CHARGE light in the dash is burned out, the alternator will not see the 12 volt ignition source and will not function.

If the alternator IS under computer control, the circuit is different. I can supply that info as well.

Your first task would be to determine if the CHARGE bulb in the dash is good.




Bulb is 100% good. Indicator light does not light up, gauge backlight does. If I swap bulbs, indicator light is still out, and gauge backlight works fine.

How do I determine if the alternator is under computer control?
GodSend JUL 09, 05:45 PM

quote
Originally posted by Jncomutt:

Also to add what Joe said, use a conventional bulb for the volt light, not a LED. Some people may find a LED will work fine, but I've had it cause issues in the past.




Conventional bulb..
olejoedad JUL 09, 08:54 PM

quote
Originally posted by GodSend:


Bulb is 100% good. Indicator light does not light up, gauge backlight does. If I swap bulbs, indicator light is still out, and gauge backlight works fine.

How do I determine if the alternator is under computer control?



Since your bulb doesn't light up with KEY ON, one of the two wires on the alternator plug is NOT seeing the 12V ignition signal. The alternator needs this to function.

The other wire should have 12V BATTERY power - always hot.

Check the condition of the circuit traces on the instrument cluster flexible circuit board. Are any of the 'fingers' that contact the plug surfaces corroded or not making good contact?

Does the bulb socket make good contact with the copper pads on the flexible circuit board?

To determine if the alternator is under PCM control, you would have to do some investigating with a digital multimeter. I can provide that info.
GodSend JUL 14, 09:03 PM

quote
Originally posted by olejoedad:


Since your bulb doesn't light up with KEY ON, one of the two wires on the alternator plug is NOT seeing the 12V ignition signal. The alternator needs this to function.

The other wire should have 12V BATTERY power - always hot.

Check the condition of the circuit traces on the instrument cluster flexible circuit board. Are any of the 'fingers' that contact the plug surfaces corroded or not making good contact?

Does the bulb socket make good contact with the copper pads on the flexible circuit board?

To determine if the alternator is under PCM control, you would have to do some investigating with a digital multimeter. I can provide that info.




Ill be sure to check out the connections for sure. But yes, can you please send the info for the PCM control. Ive been quiet involved with the wiring of the PCM. I back probed every pin on both PCM connections to each individual sensor and etc when I was first having problems getting the vehicle to start (since resolved) so im fairly familar with everything in that area..

Is there some kind of load I can send through the C500 B3 pin to check the status of the light without having the car on? Would a simple continuity test from B3 to gauge cluster suffice?
GodSend JUL 14, 09:04 PM

quote
Originally posted by olejoedad:

Check the condition of the circuit traces on the instrument cluster flexible circuit board. Are any of the 'fingers' that contact the plug surfaces corroded or not making good contact?

Does the bulb socket make good contact with the copper pads on the flexible circuit board?

To determine if the alternator is under PCM control, you would have to do some investigating with a digital multimeter. I can provide that info.



Just to be clear, this is the indicator bulb in the aux gauges above the radio, correct?