Quick low voltage alternator question

Qwkspool

Member
last night I was switching out fuel pressure regulators to test if that was the cause of my car not holding fuel pressure after shut off. I let the car run then crawled under the rear of the car and unplugged the fuel pump harness that connects to the hotwire kit to bleed off the pressure. I switched out fuel pressure regulators then proceeded to the gas station. When I got back into my car I noticed the low volt light was on and my volts were 11.5-11.9 at idle. With the car off, battery and back of alternator reads 12.4. I tested the brown wire plug with key on and have 12 volts. I even jumped 12 volts from the wastegate solenoid to the alternator and still when it fires up, my volts go to mid 11s. Can I assume it's the alternator because the exciter wire has 12 volts plus I tried bypassing and it still didn't help? Just weird that this happened the minute I touched something ( fuel pressure regulator and fuel pump harness).
 

Chuck Leeper

Toxic old bastard
Staff member
I let the car run then crawled under the rear of the car and unplugged the fuel pump harness that connects to the hotwire kit to bleed off the pressure.

I guess I don't understand this????^^
Low alt output could be a diode has gone south.
Low volts could be bad grounds.
Are you reading the system volts at a Scanmaster?
How long for the psi to bleed down?
 

Qwkspool

Member
I unplugged the fuel pump harness to depressurize the fuel rail to install the fuel pressure regulator. I was just explaining the things I touched because I never had a problem before this. I am reading volts with a multimeter and the scan master reads maybe .5 volts lower than the back of the alternator. It was just weird that I touched something and now all of sudden have a problem. Like I said. I checked the brown wire on the plug and even jumped 12 volts from the wastegate solenoid to "excite" the alt. I didn't touch anything else so I'm doubting a bad ground wire. I tested the alt with a load tester and it passes but I'm still thinking it's the alternator because I'm not sure what else it could be. Just trying to look for some input before I just try and buy an alternator.
 

gunzandgearz

gearzandgunz
Reading on your battery with a volt meter and car fired up and running should be about 14.25 volts.Turn on all of your accessories headlights radio ac and see if it will maintain charge, if it starts dropping out alt. is probably bad.Make sure that before you start doing any electrical testing battery is 100% charged, then load test it too see if it is up to task.
 

Qwkspool

Member
Ok. Before this randomly happened. I always had 13.8-14 volts reading on the scanmaster and a little more at the battery and on the back of the alt. I wired a 4 gauge wire directly from the alternator to the battery a couple of years ago and that helped my voltage. It was just all of sudden, I change the fpr and plug/unplug the fuel pump harness and now all of a sudden it's reading 11 volts with the multimeter at the battery, alt, and scanmaster. And the volt light is on constantly. I have a load tester and put a load on it while it was running and it passed so I am kinda stumped. Like I said, I also tried bypassing the exciter wire and still low volts. Not sure what else I could test. Maybe it was just coincidence that the alt does while I changed the fpr. Just wanted to see if there was anything I missed before I just try and buy an alt.
 

gunzandgearz

gearzandgunz
All I can tell you is this stuff sometimes happens, you got to the store and everythings fine you come back out and car wont start.just check it for the 14.25 volts at battery engine running and if you don't have it take it off and have it tested before purchasing new one.
 

Qwkspool

Member
The battery and back of the alt reads the same. Maybe because I have a wire directly from the battery to the back of the alt. but they are both 13 volts and then when the car is running the volts are at 11.5-11.8 at both the battery and back of the alternator. Tested with a multimeter. I am going to pick up an alternator after work and just try that because I've done everything else I can think of to test it.
 

gunzandgearz

gearzandgunz
When you say 13 volts are you meaning something like 12.6, because that would be a fully charge battery. remember you should get a voltage rading of at least 1.5 to 2.5volts at battery with engine running and it should be abot 1500 rpm to excite the alternator.also remember to turn all o youraccesories the ac, headligts radio defroster and let it run for about a minute.see what reading you get if its jst batt voltage alt isn't charging batt. lt me know.
 

Qwkspool

Member
Update. After doing the brown wire "exciter" test and finding 12 volts at the plug with key on, and checking the ohms of the negative cable and the positive wire coming from the starter, I came to conclusion that it had to be the alternator. The only alternator I could find was a 140 amp from a 1996 Chevy Impala. Hooked it up and all was back to normal. 14.2-4 volts at idle on. The battery post and back of alternator and 13.5-13.9 on the scanmaster. The only thing I wanted to add is the alternator from the Impala is pretty much a direct bolt on except the back bolt doesn't sit flush with the alt so I left that one out. Also, the positive post on the back of the alternator is much larger in diameter so the cable coming from the starter and the hot wire kit had to be cut and a bigger "o" ring terminal needed to be installed. Thanks for all that helped me. It would have been much easier if I just came out one day and the voltage was low. The fact I was messing with the car and 5 minutes later I was having a problem led me to believe that maybe it was something I did. I guess it was just coincidence.
 

GNVenom

Well-Known Member
Never, NEVER apply 12 volts to that brown alternator wire. Doing this will destroy the regulator internal to the alternator.

The volts lamp has keyed positive to one side, and the brown wire is the other side. Grounding that brown wire turns the VOLTS lamp ON.

That brown wire shows 12 volts through a VOLTS lamp filament, not directly to the 12 volt source. Notice your VOLTS light comes on with key-on-engine-off. This means the regulator is providing a GROUND to that lamp. So, running 12 volts to a non-spinning alternator brown wire will short ground to positive, and the regulator burns out.
 

Sharingan3

New Member
Never, NEVER apply 12 volts to that brown alternator wire. Doing this will destroy the regulator internal to the alternator.

The volts lamp has keyed positive to one side, and the brown wire is the other side. Grounding that brown wire turns the VOLTS lamp ON.

That brown wire shows 12 volts through a VOLTS lamp filament, not directly to the 12 volt source. Notice your VOLTS light comes on with key-on-engine-off. This means the regulator is providing a GROUND to that lamp. So, running 12 volts to a non-spinning alternator brown wire will short ground to positive, and the regulator burns out.
 

Sharingan3

New Member
Spot on. Found that out the hard way when upgrading to one of those alternators. Have a home manufactured autometer dash setup due to car not coming with a cluster. Figured a switched 12v to the brown wire was all I needed. Nope. Kept blowing regulator due to it needing the factory volt lamp in line. Lamp acts as a resistor. If you have no lamp, you need to install a 470 ohm resistor in line. Then you can just power that from key on feed if you want. A side note: when I made the swap, I got the chassis harness plug for a 96 impala too. Got 2 of 'em cause we thought they sent the wrong one due to opening the box and it having 2 wires coming off of the harness, not 1 like our cars have. Turns out on the impalas, one wire excites the field like on our cars, the other is a sensing/reference wire that goes to the battery, which we don't have. I hooked it up the way it is supposed to be in an impala. 14.2 volts solid. If I turn on heat, fan, headlights, etc, the sensing wire sees the increased demand and increases alt output. With accessories on it then puts out more like 14.5-14.7 solid. How it should be.
 

Sharingan3

New Member
Might you have a part number?
Yes sir. It was from NAPA. Part number EC82. However, I will warn you when I bought it, they said it had the resistor built into the harness/connector and the picture they pulled up in their system even showed a heat-shrunk section in one of the wires, indicating the resistor in line like it should be. When it came in, no resistor. I kept it and we assumed they sent the wrong one and had 'em order me another. Came the same way. We both kind of shrugged our shoulders. I kept both since I was in the process of doing one car and knew I would eventually do it to my other, went to radio shack and bought a 5 pack of 470 ohm resistors for like $1.20, soldered her in and was in business. Will try and upload the pic of the box (it has another number on it too) and of the harness.
 

Sharingan3

New Member
image.jpg
 

Sharingan3

New Member
The only issue, as I am sure you can see, (and I hope I wasn't misleading when I said "chassis harness") is the grey connector. You would have to have the Impala end (maybe that would be more appropriate to classify as the "chassis" harness/connector?) if you wanted to pin out/solder/etc. I know a lot of guys with their cars want as many of the factory ends untouched as possible. Something would be needed to integrate if you did not want to cut and solder or cut and sta-kon to the grey end. Maybe you could re pin the brown lead from the factory harness into the impala one? The connector is the same and has all the slots.
 
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