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Old 10-28-2008, 12:10 AM
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Voltage drop. Please help.

I have a 1977 Firebird with a Chevy 305. The curb idle speed for this vehicle is 500 RPM. When the car is in park the Voltage is up at 14.4 Volts. When the car is put into drive the Voltage drops to around 13+ Volts. If the headlights are turned on the Voltage drops to 12+ Volts. I have had the alternator tested and it shows good. ( I have replaced it with a spare and still the same ). The battery is alomst new and both terminals are clean. I have checked the ground at the alternator bracket and it is good too. If I hold my foot on the brake and rev up the engine to around 800 - 1000 RPM the voltage climbs back to 14.4. Is it normal for the Voltage to drop this much (does anyone have a similar car with this engine) or is there something else that could be causing it? Any ideas would be greatly appreciated.

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Old 10-28-2008, 01:25 AM
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That sounds normal to me. If you want to help things along, put a smaller pulley on the alternator.

Ideal numbers I like to see are 12.7 from the battery, 12.7+ at idle, and 14.4 at anything above idle. Its not an exact science, but if you're getting over 13v at idle, call it a win.
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Old 10-28-2008, 08:01 AM
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Is this a factory gage, if it is they are notoriously inaccurate? Get an external gage or actually check voltage with a digital volt meter.

Vince
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Old 10-28-2008, 09:09 AM
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I do have the factory gauges BUT I have also checked this with a meter.
I have boosted the idle speed to around 700 RPM and at idle the voltage stays higher but again when in gear and with the headlight and/or the rear window defroster the voltage drops to around 12 Volts. Boosting the idle is not a great fix as the car seems to be harder to stop and just lifting my foot off the brake from a stop it wants to RUN away (not crawl as it should).

I had contemplated the smaller pulley idea but was told by a friend that at high speeds it could be too fast and possibly burn it out prematurely. Could that happen? I am willing to go to a local machine shop and have a tiny pulley made to keep the voltage up at all times.

Thanks for the speedy reply guys. This site rocks.
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Old 10-28-2008, 02:26 PM
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sounds very normal, you're running more items on an alternator that might not have that much output. You can also try a higher amp alternator to compensate instead of the pulley.
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Old 10-28-2008, 10:02 PM
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It's all good

You didn't mention if you were experiencing charging problems and facing a car that won't start in the morning - if it starts for you don't sweat it.

You're putting a pretty heavy load on the alternator with headlights and the rear defrost on. When it's idling in gear there just isn't enough current out of the alternator to drive the voltage up. What you're measuring under these circumstances is still acceptable, as Curtis & Bubba had mentioned.

You've got a passenger car that isn't meant to run a lot of accessory loads at idle. If it was a fleet vehicle that would idle at the side of the road for hours and run all kinds of lights a solenoid would sometimes be fitted to bump the idle speed up.

Once the car is moving and not idling slow against the torque converter your system is charging and all is well.

I was told once that the old "charge / discharge" ammeter was discontinued as a dashboard instrument because drivers became concerned with it dipping into the 'discharge' zone. So long as it make it into the charge zone the rest of the time - no problem.
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Old 10-28-2008, 10:44 PM
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I had similar problems with my '75 GMC, but my alternator was so tired that it sometimes dropped to about 10 volts at idle with the lights on and blower motor running.

I made a direct swap from the original 63 amp 10si alternator to a 94 amp 12si, and then upgraded the charge wire to an 8 gauge wire. After that upgrade I've had no problems with low voltage.

All you need is a 12si with a V belt pulley and the same power connector orientation (mine was at 3 o'clock, I think). I'd have to verify the price, but I think it was only about $75-80US.

Bruce
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Old 10-29-2008, 04:45 PM
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Thanks a bunch guys. Really appreciate the info.
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Old 10-29-2008, 05:46 PM
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Is this a 1 wire or a 3 wire alternator. If it is a 1 wire they are prone to this type of issue. Make sure you have the correct belt. A belt that is to thin will sit down deeper in the pulley.
If you do need another pulley, you should not have to have one made. If you can find a local rebuilder they will probably sell you one.
A higher capacity alternator will probably solve most of your issues and make your battery last a lot longer at teh same time



Chet
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Old 10-29-2008, 05:56 PM
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I picked this link at random just to show you a typical alt amps output versus rpms curve,,,about half way down the page.....

http://www.rpc.com.au/products/batte...le/carfaq5.htm

at idle, it's easy for the total amps for all the devices (ign/lights/electric fan/stereo/etc) that are operating to require more amps than the alt is putting out at low rpms so the voltage potential shown on the guage drops...

the bad news is when the guage is only showing 12.4 batt only volts you are starving the coil (for volts and amps) from making a best possible spark at idle....

the good news is that by just adding copper wire grounds from most/all of the high amps devices in the car (dist body!!!!/heater fan/brake lights/etc) ground screw connects back direct to the batt neg often helps alot!!!

typically on a older car the head lights will be 15% brighter as a example (IF the lamps sockets aren't totally corroded causing resistance).....

doesn't matter how you run the grounds, in series, parallel or in a spider web,,,,just as long as there is a excellent no resistance "copper" path back to the batt neg post....

it's a pita and expensive to rewire the positive harness side which does wear out and have higher resistance but all the devices will work better and last longer with a lower resistance "supply" source on the negative side with copper grounds wires....

if it is the original headlamps switch, fair chance it is getting toward the end of it's life,,,the contacts inside get pitted due to arcing (due to the harness age) and that adds alot of resistance to the lamps circuit....
put it on your xmas list?

do check the belt tension!!!!
that can be a full volt difference.....
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Old 10-30-2008, 01:52 PM
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77',
I forgot to post:
common for a store to sell just one rebuilt alt part # with a "one size pulley fits all" for many applications....

which do infact have different damper pulley diameters which changes the alt rpms to crank pulley ratio....

do what you can to confirm your alt pulley diameter is a correct for the damper diameter oem ratio match,,,,and that the pulley "V" degrees is correct for your belt "V" degrees.... (another common problem)

by measuring the voltage drop with the motor off at each point in a circuit, you can determine if a positive side wire (or plug) should be replaced (or supplemented with a parallel wire).....
examples:
12.4 at the batt
12.2 at the headlights switch output=normal and switch is ok
12.1 at the firewall plug=plug is ok
11 at the the lamp socket=wire from the firewall to the lamps is toast (due to many years of engine compartment heat mostly)

$8? for a 6 fuse panel from your local store, to make a batt direct second parallel "primary" power feed circuit into the car for the batt direct devices (headlamps/brake lights/etc)....
"if" you do find a major voltage drop in the main power feed circuit when all is on and motor off....

what your seeing is pretty normal....
don't loose sleep over it....
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Old 10-30-2008, 04:42 PM
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Quote:
Originally Posted by T-bucket23
Is this a 1 wire or a 3 wire alternator. If it is a 1 wire they are prone to this type of issue. Make sure you have the correct belt. A belt that is to thin will sit down deeper in the pulley.
If you do need another pulley, you should not have to have one made. If you can find a local rebuilder they will probably sell you one.
A higher capacity alternator will probably solve most of your issues and make your battery last a lot longer at teh same time



Chet
It is a three wire alternator. The belt is correct. I hope the headlight switch and wiring are still okay as the car has only 23,000 original miles on it. Thanks for all the information and the link. It was very useful. I am going this weekend to see a local rebuilder for a quote for a higher amperage alternator with a smaller pulley.
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Old 10-30-2008, 05:34 PM
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77,
only 23,000 miles= all the positive side wiring and headlights switch should be fine....

just clean all the grounds connect screws you can find....
(basically, just think: where could 30 years worth of daily condensation cause exposed contact corrosion, on a glass tube fuse holder as an example or the headlamps socket and clean them for least resistance)

adding the easy to do #12 grounds wires to the head lights grounds screws and dist body back to the batt can only help....
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Old 10-30-2008, 05:59 PM
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Thanks again for the speedy reply. I have already tried to add a ground wire from the headlight directly to the battery and it made no difference. I never thought about the fuses tho. I will change them all. I would post a pic of my toy but I can't seem to be able to upload one here.

Thanks again Red. I appreciate all your advice.
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Old 10-30-2008, 07:51 PM
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Do you know if the headlights and rear window defroster are run through relays?

If not, wiring in relays will eliminate some voltage drop through the circuits.
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