Fooling the alternator...


Senior Member
May 24, 2001
As per the suggestion to my question in the scan tool forum, I am going to try to fix my alternators varying voltage problem, by bypassing the volts light in the dash.

As I understand it, the alt gets a feedback 12v line to determine the output voltage.
Would someone explain this to me? If the line is 12v, then wouldnt the dash volts light be on all the time?

12v source------(resistor)------alt 12v input. <- this I understand.

But is the 12v in switched or constant?
And is there a way to fool the alternator into thinking its putting out less voltage, so I get a steady higher voltage overall? Right now its about 12.0-13.3 at 2000rpms, but Id really like to see 14.0-14.4.
NO, the alternators in our cars are internal sense UNLESS they get an external sense T? I forget but can look it up at work. The volt light is used to excite the alternator. It provides the electricity to start the magnetic field which is used to generate voltage. Volt boosters basically use a pressure sending unit and a voltage divider. When boost hits the pressure to close the contacts in the particular sender, it puts a voltage on the external sense terminal which fools the alternator into thinking the system voltage is low, which causes the regulator to bump up voltage/current.
I thought they were internal sense too, but the whole thread I started about the varying voltage on the s/m leads me to think it isnt.

Maybe my alternator is just dying. I think its getting plenty of tension, and it is charging. I drove it about an hour each way to my dads house without a problem. But, the voltage was all over the place from a best of only 13.3 to quick drops to 11.9. This was at average of 55-60mph. I would think it should be about 14v or better at highway speeds.
The volt light is switched...turn the key on, voltage passes thru the light and on to the alternator...this excites the field which causes the alternator to start charging assuming the engine is running and the alternator spinning...
So then the alternator does sense an external voltage to regulate the ouput?
Then if the wiring between the bulb and the alternator is suspect, to take it out of the equation it just needs an ignition switched feed source to tell it what the car is seeing for voltage? Does it need a resistor in the chain?
Since the alternator is making voltage, enough to keep the battery from dieing out, then the alternator is not the problem. Its just bad wiring that is feeding it mixed signals, or an unstable signal.
I see 14 volts on my work van, but never saw much above 13.3 volts on the G/N. I've seen 11.9 on the scanmaster a few times as well...
John Spina told me it does need a little resistance in the chain to keep from damaging the regulator over a period of time. I guess the resistance of a small bulb is required...I have wired a couple of swaps..and have used a small bulb in a holder from the parts house and mounted it in the dash in a hole...last one was a 383 chebby in a '50 Bel worked fine. I also did it to a digital dash Limited when a screwy connection in the dash. It worked.
Originally posted by SilverSleeper
So then the alternator does sense an external voltage to regulate the ouput?
Then if the wiring between the bulb and the alternator is suspect, to take it out of the equation it just needs an ignition switched feed source to tell it what the car is seeing for voltage? Does it need a resistor in the chain?
Since the alternator is making voltage, enough to keep the battery from dieing out, then the alternator is not the problem. Its just bad wiring that is feeding it mixed signals, or an unstable signal.

No it doesnt, but it CAN. The voltage from the bulb just provides the initial electricity to build a magnetic field, which the armature spins thru. As it makes/breaks magnetic fluxes (is that the right word? :confused: ) a voltage is generated. This voltage in raw form is an AC sine wave. Using a bridge rectifuer (or diode trio as GM calls it) this AC is converted to DC. The regulator varies this voltage in the field to increase or decrease output. With no external sample hooked up, the regulator uses internal voltage. If there IS a sample voltage on the external plug, it will use that. Again, thats how volt boosters work. They trick the alternator into thinking system voltage is low and it needs to increase output.
Ok, this confuses me now...

So basically if the voltage varies or "jumps" around alot, then it is to blame on the internals of the alternator and not the wiring from the dash?
Then what is the point of trying to bypass the dash idiot light to feed voltage to the terminal on the alternator? How come there are people doing this if it only is an effect when the car is just starting up?
I really dont want to have to replace my alternator to make me feel more at ease about the voltage. I actually think it used to be better. Driving around at highway speeds with the s/m reading an 11.9 or 12.3 or so doesnt give me much confidence.
our alternators are not self exciting...pull the brown wire off and they will not charge.

Some alternators will...the so called one wire models..they don'
t seem to put out as much at idle, tho.

Heck, it is simple, go pull the plug and see what you get.
Most auto parts stores have free alternator check.
This is one sure way to know if yours is good or bad.

Gary :)
The problem with AP store testers is they cant heat the unit. As with all electronic devices, performance degrades as they get hot. Same it true with alternators and heat may exaggerate a pending problem

If theres a question, I'd rather see you throw an alt at it instead of ****ing with all of this. There are some cases of plain ole cut and dry.
I'm with you on this. I have had 3 alternators on my car in 4 years. All of them doing the same thing. I think the suspect wiring may also be damaging the regulator in the alternator. My volt light comes on and I push the guage pod in and the light goes out. I have had the dash off 3 times trying to get better contacts. I am now running a 140 amp alternator off of a caddy just to get it to run right.

I am going to "hotwire" my alternator to bypass the volt light. When I drive the GN the scanmaster is always on the volt setting, because that is where my biggest problem is. If its nothing more than cutting the brown wire, adding a 1 ohm resistor, and attaching it to a power source then I'm doing it.
Now that we have gotten past the part of not needing external excitation to make the alternator charge. If the low output is being measured at the back of the alternator and not by a Scanmaster, then you should take it and have it tested.

Before you do that, tho', be sure that you are measuring voltage off the connector on the back of the alternator.

These cars are notorious for having a high resistance spice in the harness so that the voltage that the Scanmaster sees is up to a volt less than what the alternator is putting out.

If by chance the alternator is charging at a higher rate than the SM is seeing, then move to the battery and see if it is getting basically the same voltage as the back of the alternator. If it is a half a volt lower or so, then you need to examine the connection in the cable between the alternator and battery as it often goes bad, too.

This idea of throwing money at a problem and hoping it goes away disturbs me. Find out what the problem really is before you start spending.

NOT 1 OHM, 1K OHM (1000 ohms) they are almost free at Radio Shack (.39)

It is a CHEAP test. if you get a 'repair end' and make a pigtail then no cutting was
required and the vehicle can be returned to stock.

BLACK6PACK and SilverSleeper :
this sounds like your problem. It's a quick fix, plug the resistor
pigtail into the fuel pump prime connector, it's right there. I fought intermittant
alternator/voltage dropping for a while before I did this, it's ROCK SOLID now.
(rock solid means 13.8 - 14.2 darn near all the time, NONE OF THE 11.9 CRAP ANYMORE)

The volt-booster phenomenon is a different animal. And uses a different connector pin.
The voltage at the L pin is required for the regulator inside the alternator to operate.

The Scanmaster does not sense voltage, the ECM does and transmitts this as part of
the data that is displayed by the Scanmaster. I don't believe there are any splices
in this circuit (there are plenty in other circuits) but my memory could be faulty here....

I hope this helps some.


(on my other car I did have an alternator with an output stud that had a loose nut on
the inside which causes a similar problem, replacing the alternator fixed that one tho)
Bob, the feed to the ecm comes off one of the fusible links...I think there are up to six things coming off a splice in this line. The splice is in the harness somewhere behind the passenger head. Not only will the SM read lower than the battery but so will the various fuses that also come off this splice...

I originally learned this from Mr. Licht and then confirmed it with the wiring diagram. Now, there may be more than one feed to the ecm (the orange wire for instance) and I don't know where the voltage reported by the SM originates from within the ecm.

However, I know that a volt meter on the circuits involved with the splice will report the same voltages as the SM while the battery will report much higher....Of my three cars, one does this and the other two report on the SM much the same as the battery is seeing.

Thanks for the info on the resistance value. I had been wondering what to use other than a bulb. :)
Ok on the splice, I understand. I have seen one car (it was at Mike's old shop)
that had a bad fuse-terminal in the fuse block that caused this also.

The alt/battery voltage was fine but the voltage to the coilpack and
injectors was fading at high RPM/load. The drop in voltage was seen on the Scanmaster
and a tweak of the terminal fingers on that fuse fixed it right up.

So, we can add the voltage/splice issue to the troubleshooting list.

Thanks for the clarification.

To you other guys, try the bypass resistor thingy and let us know how it turns out.


I havent tried the bypass yet, but I think I have found the cause of my problem.
In poking around underhood in the few spare minutes I had, I noticed the wire leading to the alternator shorting against a support bracket. Since I have had good voltage readings before, I am just going to try and repair the wire and see where it gets me. Then I will see how much difference there is using the bypass.
I also thought it was odd, that I was getting the same readings, no matter what the rpms were. I am pretty sure I was gettng a much steadier reading around 13.7 before, and the voltage problem was a recent development. Hopefully this is an easy fix, I guess I'll find out tomorrow.
How did things go with you guys? Im having the same problem with my voltages varying alot betwwen 11.9-13.7.

I changed the battery, put gold post connectors, impalla alternator, added redundant 8 guage power wire to alternator and ground to fender. I gained .2 volts more with the extra power wire at the alternator.

Did all this because my voltage was varying and low in some instances with a good idiot light. Before i change the battery cable out i am gonna try getting a repair end and using the fuel pump prime connector.
Chime Chime....
There is an easier way:

There are two terminals related to field feed within the alternator, the F terminal and the L terminal. Internally there is a resistor on the F terminal. The above harness addresses this by "hotwiring" the alternator to always charge regardless of the condition of the VOLTS light circuit. Look at the "INSTALLATION" link in the description, it will pretty much explain it. Bob is right, don't use a 1 ohm resistor, or you'll be replacing your regulator.

So, for those of you who want to do it yourself, just follow the above note.

-John Spina