Alternator buzzing when car stopped?

[3sacrwd]

Hey there, The connector has 3 wires coming from it, red, blue and brown, this is along with the other red coming from the battery. So, there are 3 wires coming from the connector.

Thanks

 

[mangusta1969]

Alternator/Battery Problems

I just took a look at my 91 L98 car, which is working normally (no discharge, proper charging/voltages, etc.). Here is what I found (all measurements were taken with the alternator's plug in connector removed from the alternator):

1. Alternator connector is a three wire setup, although the factory connector itself has 4 openings. The three wires on my car are connected to the bottom three openings on the connector.

2. Topmost wire is a Brown wire. This wire has zero volts on it with the key out of the ignition.

3. Middle wire is red, with a black stripe. This wire has zero volts on it with the key out of the ignition.

3. Bottom wire is red, but is much thicker than the other two wires. This wire has twelve volts on it with the key out of the ignition.

4. There is a fourth thick red wire wire connected to a threaded bolt. This wire also has 12 volts on it with the key out of the ignition.

I suspect a relay is sometimes sticking closed when your key is removed, causing the middle or top wire to have 12 volts on it when it should not. Take a look in the shop manual or wiring diagram for your car's year and see what relays are connected to these wires. Replacing the stuck or intermittently stuck relay should cure the problem with your battery discharging. If the 84 circuit diagram above applies to your 87 car then the only real culprits could be your ignition switch or "something" that is connected to the S345/Orange wire.

If you have an ammeter, you might disconnect your negative battery terminal and measure the current flow with the ammeter connected between the battery post and the negative battery terminal. On my car, the initial reading is a current draw of 5.4 amps, dropping within about 40 seconds to about .020 amp (20 milliamps). This drop in current occurs when the interior lights slowly dim and then go out.

NOTE: Make certain that your ammeter is set to a pretty high scale (like 10 amps) before taking this current flow measurement; you may need to move the DC voltmeter/ammeter's positive lead from its normal position and plug it into a high current position.

Your at rest current draw will vary a bit according to the draw of your aftermarket stereo. Anything under 60 milliamps should be considered a normal at rest draw.

 

[3sacrwd]

Thanks for the replies....I will see if I can figure out if my Haynes manual shows the schematic for that or not.......If anyone knows what the brown wire powers, I would love to hear from ya. I think the Red wire is supposed to have power all the time coming from the alt.

Thanks for any info.

 

[mangusta1969]

The two red wires on my 91 L98 motor have power on them all the time. This power (voltage) comes from the battery, not the alternator, when the car is at rest.

When the engine is running, the alternator charges the battery through the larger red wire. The smaller red wire provides battery voltage to the alternator's internal voltage regulator. After starting, this voltage is actually provided by the alternator itself, as it is then creating a higher voltage than the battery. The brown wire is connected to the rotor through a slip ring inside the alternator. When the car is running the brown wire provides voltage to the alternator's rotor.

Under running conditions, the alternator creates a voltage that is typically between 14.3-14.7 volts. Since this voltage is higher than the battery's at rest voltage (12.5-12.8 volts), the alternator recharges the battery and provides electrical current to operate the engine and its accessories. Because of the diode orientation inside the alternator, current only flows FROM the alternator TO the battery through the large red wire. As described above at startup, the battery actually supplies current to the alternator through both the brown wire and the smaller red wire.

 

[3sacrwd]

So, the current flows FROM the 3 connector side INTO the Alternator???? I had thought it was the other way around.....So, if I have the slght whine from the alternator at Key off position, then what else could it be? I re-crimped my Red wire coming off the pole of the alternator just to be sure it had a good connection, and I still have the whine......I a stumped at this point....I am about ready to just cut all 3 wires and see if the wire stops.........I dunno.

 

[mangusta1969]

If your car/alternator is anything like my 91, you should be able to just unplug the 3 wire connector from the alternator's connector. No cutting should be required to remove all three of those wires from your alternator.

On my car, the large red wire goes to terminal connector that goes over a threaded bolt on the alternator. You can take it off by removing the nut on the threaded bolt. I would remove that wire first and see if you still have the humming sound with a well-charged battery (12.5-12.7 vollts). Maybe one of your alternator's power diodes has gone bad and is allowing current flow from the large red wire into the alternator. Normally, the diodes will prevent any current flow via this path.

To avoid problems, do all of these connections/disconnections with the negative battery cable removed from your cars battery. Once you have any measurement devices in the circuit and the wire connections made or broken, then re-connect the negative battery terminal.

Small electrical currents should flow into the alternator via the small three wires and a large charge current should flow out of a spinning alternator via the single large red wire.

 

[3sacrwd]

Thanks for the replies....The "humming, or slight whine" does stop when I disconnect the Red Wire connected to the post, or when I disconnect the harness from the back of the alt.

I can still drive the car and it charges at about 13.9-14.5 V, so it does seem to work, on that end anyway......If I let it sit for more than a day at a time, then the dash gets pretty dim when I try to start it.


Is it time to purchase a new AC Delco alternator and see if that fixes my problem?

 

[Galen]

I think you have a diode problem with the alternator. It charges because the diodes are not working but allowing power to flow both directions which discharges the battery thru the alternator windings. The alternator will fail due to the heat generated in the windings so dont wait till it is time to walk because of a failed alternator. I have to ask have you had the alternator tested. I actually have a Auto Zone alternator on my car as I was stranded one Sunday and that was the choice. I have not had trouble with it but I usually buy a new AC Delco from my supply whse. Just hard to beat new and it isnt alot more money. Of course Auto Zone only had there hi dollar lifetime alternator in stock when I needed it. There is only one switched wire going to the alternator on that and it goes thru a resistor and would be very low draw. Take it back and get a different one.

Galen

 

[3sacrwd]

Thanks Galen,

I will try to get it replaced this weekend......When O'Reillys tested it, it tested out fine. I am not sure if their equipment would find bad diodes or not. I took it out of the car and had it tested, as well as brought the car to the store and had it tested on their handheld machine. Both times it tested as "OK".

I will let you know what I find out.

 

[mangusta1969]

I wouldn't be so sure it is the alternator from the above information.

From the above reports and testing the alternator’s plastic connector on my normally operating 91 car, I think you FIRST need to correct the +12V that is present on one of the two upper most wires going from your car to the alternator via the three-wire connector. Since 86-91 cars can use the same alternator, the wire positions in this connector must be the same between the car years. You reported that two of the three wires on your connector have +12V on them when the car is at rest and the connector is disconnected from the alternator. Only the bottom most wire on this connector (thicker red wire on my 91) has +12V on it.

Perhaps others can disconnect their alternator connectors and verify the at rest voltages on the three wires. Report the voltages by wire position in the connector, not by the wire color, which may vary from year to year. It is very easy to measure on an L98 motor.

The fact that your alternator is producing over 14.5 volts at reasonable engine rpms probably indicates that its power diodes are okay, too. A diode that is leaking current from the battery to ground will typically cause a reduced output voltage from the alternator.

You can further verify that the alternator’s power diodes are not leaking battery voltage to ground by measuring current flow from the battery through the alternator with your ammeter connected between the negative battery cable and the negative battery terminal (use 10 amp scale and connections on your ammeter). This measurement should be made with the three wire plastic connector DISCONNECTED from the alternator. Only the large 10 or 12 gauge wire should be connected from the car to the alternator’output power insulated post. A minute or two after first connecting your ammeter in the circuit, the current flow should settle down to around 20 milliamps (.020 amps). Anything higher than about 50 milliamps will drain your battery pretty quickly. As you have seen, a flow of 500 milliamps will drain your battery overnight.

While I think your problem is originating within the car (+12V on a second wire in the three wire alternator connector is suspect to me), if you decide to get another alternator, I would strongly recommend something other than stock and I would not use a parts store rebuilt unit. The OEM Corvette alternator is undersized and prone to overheating and premature burnout/death. Years ago, I went through one new AC Delco alternator and about 4 of the parts store alternators in 70K miles on my 86 Corvette. Since I had a "lifetime" guarantee from a national chain, I only had to pay for one of the parts store alternators, but the pain factor of replacing a couple of these failures out on the road was considerable.

Re-look at this recent thread and try these larger./reworked alternators that have been recommended by other forum members:

Recent alternator thread:
http://corvetteactioncenter.com/for...ht=alternator and heavy and duty&pagenumber=1

Forum members/suggested non-stock C4 alternator sources:
rrubel
email - TK4Sports@aol.com

Tom3
Email - Info@corvetteclinic.com
www.corvetteclinicinc.com (click on C4 solutions)

 

[vettenick]

Correct me if I'm wrong, but, I thought I read that your brown wire has 12 volts with the key off. If that's true, then that's your problem. That wire which is powered through the "gages" 10 amp fuse is hot only when the key is on. Also the charge light on the dash is also between the fuse and the alternator. If the humming stops when you pull the gage fuse, maybe you have a bad ignition switch, which is supplying power to this circuit when it shouldn't. Now, this fuse also powers up a lot of other things such as the visor vanity lights, courtesy light delay timer, etc. which would explain your battery problem. Make a quick check to see if the visor lights work with the key off. If they do, then I would check the igntion switch first.

Good luck, and let me know.
Nick

 

[3sacrwd]

Thanks Nick. The brown wire does have voltage to it with the key off. Is the gages 10 amp fuse in the set of 3 that is behind the driver information display unit? If so, I did disconnect that fuse and still had the humming.....I left my front panels off, in case I needed to check something else. I will run out and see what the 3 fuses are, as I have forgotton.
I do not have any "visor" lights. This is an 87 Convertible, I guess I don't have that option. The glovebox light does work, (and turn off) however....same circuit?

 

[vettenick]

The fuse block is all the way on the right (passenger)side of the dash panel. It's right next to the Engine computer(silver box). The gauge fuse is the second fuse from the top, in the first column on the right. Next to the circuit breakers. There's about 19 fuses in the box, with 4 circuit breakers.
Forgive my error on the visor lights. That's what the diagram indicated. I guess it was an option. However, there's still a lot of stuff that this fuse powers up. Independent of that, I just have a feeling that the igniton switch is the problem.
Keep me posted.

 

[mangusta1969]

Vettenick,

Thanks for your input and fuse box details on this. Does your 94 car also have the three wire connector to the alternator with just the bottom wire having 12 volts on it when the car is not running?

Despite the fact that stock C4 alternators are poorly designed, you and I are in violent agreement that this does not look like an alternator problem based on the measurements reported to date.

 

[Galen]

OK I will say it one more time if the alternator whines at rest and you take the red wire on the stud off and it quits you have a alternator problem. All of you with working systems go turn your key on and listen you will not hear a whine. On some gm carlines there are only 2 wires on the same type alternator and one is a switch wire the other goes to the batt. My plug has 3 wires and a stud is on the alternator.The regulator in the alternator knows when it is turning there is a wire unused that also does a tachometer type function as well. One more check take an ammeter and check for draw on the red wire on the stud connected in series. If it reads anything it is defective. My car has 4 wires 1 red large gauge hooked to stud on alternator comes from terminal behind batt to batt hot always.(unswitched) Next medium gauge red thru plastic plug S terminal on regulator this is also always hot goes to same terminal behind batt this is used by the regulator to sense voltage.Next there are 2 brown wires one is larger then the other the small one goes to the indicator light from the L terminal on the regulator which isthe turn on for the alternator. It is only used at start up by the regulator.The larger brown is on terminal F which is power for the field coil and is connected with the A/C fuse 25 amp vin J ,gauges fuse 10 amp vin 8 .which is hot only in run position. (On some cars it is hooked unswitched)(Not Corvettes) The F terminal is not a switching terminal for the alternator and shouldnt make any difference on a properly functioning alternator.The P terminal is unused and is a tach type hookup. The I terminal is unused and is a alternate turn on for the alternator. So that being said 2 are hot 2 are not. Both reds hot. Both browns switched. I have the 1990 Chevrolet Corvette manual and that is where this information is from. Different years my be different wiring but the alternator terminals are still correct.

Galen

 

[Galen]

This is from the '84 shop manual, but I would imagine '87 is very similar:

This is correct for the old style internal regulator alternator. Only one wire is switched it is the small one in the plastic plug. The large wire in the plug and the studded wire go directly to the batt. I think I have a 87 manual at the shop I will check tonight and let you know.
Galen

 

[3sacrwd]

Thanks guys....I know where the fuses are, and have already taken each out one at a time, used a broom handle pressed against the alternator, and checked for the Whine......I even pulled out all of the silver type fuses. There are 3 other fuses hidden behind the back of the drivers info. panel. They are labeled something like...Vats, Hatch, and LCD...I am not sure of the exact names, I took those out also, and no change..still whining.

I just went and pulled my guages fuse, and it is still whining. The only thing I have not done, it to pull all the fuses at one time. I pulled them one at a time....????

 

[3sacrwd]

Ok Galen has his money on the alternator.....anyone else care to ante up...ha, ha.

I will check the current from the alt. stud tonight to check for voltage, and respond back.

Thanks for the help.

 

[mangusta1969]

Votes and Colors

Sac3rwd,

It's your money, so it's really only your vote that counts.

As I stated above, to confirm whether the power diodes (also known as a rectifier bridge) in the alternator are flowing current in the wrong direction and draining the battery, you need to measure the current flow (not voltage levels) from the battery when the 3 prong connector is removed from the alternator.

Galen is actually saying the same thing that Vettnick and I are saying:

When the car is at rest, there should be NO voltage on two of the three wires on the alternator's three prong connector.

Galen wrote: "So that being said 2 are hot 2 are not. Both reds hot".

You don't have this situation, so something else is likely to be wrong. While your diodes may also be bad, you cannot really verify that except by disconnecting the three prong connector and measuring the current flow. If you don't disconnect the 3 pronger, you may get false measurements. You can measure this flow at the alternator, but it is probably easier to disconnect one of the battery cables and insert your ammeter there.

As I previously stated, you should go by the wire positions in the connector, not colors. Galen's 90 car and my 91 car have different wire colors to the connector. I don't have two brown wires going to my alternator. See my attached picture to clarify things a bit. Only the bottom most red wire should have +12V on it when the car is not running and the connector has been removed from the alternator.

 

[3sacrwd]

You are not going to believe this.....I was disconnectng my Red wire off my Alt post to check for the whining sound, when I decided to pull the plastic wire loom apart a little from the bottom connector....There was some tape around the wires, so I started to disconnect and found a MESS. Someone had the wires TOTALLY messed up. The blue and brown wires were totally cut, and the red 12V thicker guage wire was stripped of it's vinyl casing for about 1/2 inch. On the one side of the cut wires, the brown wire going to the alt was connected to the 12 V red, the blue and brown wire coming from the regulater side was connected to the blue wire going to the alt.....I have never seen anything like it.......I have no idea how that happened or when.....noone was working on my car except the transmission shop (the guy was a real moron, so who knows). I have had the car for over 3 years, and it always ran pretty well....no alt problems.
I re-wired everything back together and lo and behold the humming stopped. I started the car and it was normal, and still charging normally. I will drive it the next few days and check the battery voltage to see if it starts to drop.

This 4 page thread may be completed!!! I will let you know.

Thanks to everyone for their responses, you kept me going and hopefully avoided taking it to the dealer.