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Old 01-07-2019, 05:30 AM   #1
tino0217
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Dimming lights, altenator or battery?

I have a 71 c10 with a 350, i noticed when i bought it that the lights would get brighter when i accelerated. I replaced the altenator (70 amp) but still does it. I started using a 760 watt Schumacher power converter and i have it running thru the lighter to plug in some 110's. I installed a stereo system, 250 watt Kenwood with 500 watt speakers, nothing fancy. I took my home theater subwoofer which has a 110 plug in. After a couple of minutes of sitting idoling the coverter statts beeping saying low battery. I installed a 100 amp altenator last week and the lights are still dim til i accelerate and the converter goes about 5 to 10 minutes before it indicates low battery. I also have a battery kill switch. With all that said, it my battery? Battery is 700 cold cranking with 95 reserved capacity. Anyone have advice?
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Old 01-07-2019, 06:39 PM   #2
Andy4639
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Thumbs up Re: Dimming lights, altenator or battery?

Quote:
Originally Posted by tino0217 View Post
I have a 71 c10 with a 350, i noticed when i bought it that the lights would get brighter when i accelerated. I replaced the altenator (70 amp) but still does it. I started using a 760 watt Schumacher power converter and i have it running thru the lighter to plug in some 110's. I installed a stereo system, 250 watt Kenwood with 500 watt speakers, nothing fancy. I took my home theater subwoofer which has a 110 plug in. After a couple of minutes of sitting idoling the coverter statts beeping saying low battery. I installed a 100 amp altenator last week and the lights are still dim til i accelerate and the converter goes about 5 to 10 minutes before it indicates low battery. I also have a battery kill switch. With all that said, it my battery? Battery is 700 cold cranking with 95 reserved capacity. Anyone have advice?
100 amp alternator pulling these kind of watts=
250 watts = 20.833333333amps
760 converter=63.333333333

I'd say your maxing out the alternator & battery pretty quickly.

Charge the battery up good and crank the truck up Have some one turn everything on and take a voltmeter and check the voltage of the battery with all of it running. Do this at idle and have them rev the motor and see what voltage you get anything less than 12.5 is not going to work.
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It sucks not being able to hear!

LWB trucks rule, if you don't think so measure your SWB!
After talking to tech support at Air Lift I have found out that the kit I need is 60811. Per the measurements I gave them. Ride height of truck inside spring and inside diameter of springs.
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Old 01-08-2019, 05:15 PM   #3
Dead Parrot
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Re: Dimming lights, altenator or battery?

If it still has the old style external regulated alt, replace with a newer 12SI style internal regulated. Several threads on subject. The 12SI has about 7 amps more output at idle speeds. Should fix the dimming lights at idle issue.

Consider getting a second battery and a splitter. Run the portable earthquake rig off the second battery.

A lot of those 12 to 120 converters have a pretty high cut off to prevent winding up with a dead battery.
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Old 01-08-2019, 10:59 PM   #4
tino0217
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Re: Dimming lights, altenator or battery?

I will test it like you described and see what happens with the voltage on the battery but at this point do you think a larger battery wouldn't help
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Old 01-09-2019, 05:49 AM   #5
tino0217
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Re: Dimming lights, altenator or battery?

I took all the steps that you said and the battery still was at 12 1/2. The lights were still dim until we gave it gasand that includes the dashboard
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Old 01-09-2019, 04:45 PM   #6
El Dorado Jim
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Re: Dimming lights, altenator or battery?

install the internal regulated alternator,I hear they work alot better
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Old 01-09-2019, 07:44 PM   #7
dmjlambert
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Re: Dimming lights, altenator or battery?

I have a new CS144 alternator in my truck, which is what they put in 1993 C1500 Cheyenne trucks, Optional Heavy Duty 124 Amp. It is a more modern model than 12SI alternator. I have no radio and only significant draw are from plain stock headlights, the A/C blower motor, and the GM HEI distributor. I have not added a relay to the headlight wiring, it is a plain stock setup. The headlights are noticeably dimmer at idle, such as when I stop at a stop light or stop sign. The same thing was true when I had a 12SI in it, and the only reason I replace the 12SI is because the bearings were wearing out. I believe this lower output voltage of the alternator and dimmer headlights at the lowest RPM of the engine is normal. I would not try to fix it unless the problem was severe.
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Old 01-09-2019, 10:25 PM   #8
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Thumbs up Re: Dimming lights, altenator or battery?

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I have a new CS144 alternator in my truck, which is what they put in 1993 C1500 Cheyenne trucks, Optional Heavy Duty 124 Amp. It is a more modern model than 12SI alternator. I have no radio and only significant draw are from plain stock headlights, the A/C blower motor, and the GM HEI distributor. I have not added a relay to the headlight wiring, it is a plain stock setup. The headlights are noticeably dimmer at idle, such as when I stop at a stop light or stop sign. The same thing was true when I had a 12SI in it, and the only reason I replace the 12SI is because the bearings were wearing out. I believe this lower output voltage of the alternator and dimmer headlights at the lowest RPM of the engine is normal. I would not try to fix it unless the problem was severe.

No it's not normal! It's a condition of the system in these trucks. The head light relay addition is a great way to get better working lights. It takes all the load off the whole wiring system. In turn it makes the components last longer like head light switches ignition switches dimmer and any other component.

The problem is you are way over loading the system.
Even if you buy the biggest battery you can it want help. Yes it will play longer at Idle before it quits but it will do the same thing because at 12.5 volts your using more than you are making.
If you are still using the original regulator system then I suggest the upgrade before doing anything else.

To fix it if it where me:
Buy at least 140 alternator with internal regulator Min and install 2 batteries.
Hook them up in parallel which will be 12 volts but double your amp hrs which in turn will run longer.
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Upstate SC GM Truck Club
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Get out and drive the truck this summer and have some fun!
It sucks not being able to hear!

LWB trucks rule, if you don't think so measure your SWB!
After talking to tech support at Air Lift I have found out that the kit I need is 60811. Per the measurements I gave them. Ride height of truck inside spring and inside diameter of springs.
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Old 01-09-2019, 11:07 PM   #9
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Re: Dimming lights, altenator or battery?

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Originally Posted by Andy4639 View Post
No it's not normal!
Let me understand this correctly. On a stock 67-72 GM truck with no accessories, without a dual battery, with stock headlight wiring, do you think it is not normal/expected/typical, for the headlights to be dimmer at idle than at higher RPM?
Or are you just suggesting some enhancements that could be made to the stock configuration that would reduce that effect?
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Old 01-10-2019, 07:36 AM   #10
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Re: Dimming lights, altenator or battery?

I wonder if it would be worth it to swap the pulley on the alternator so it is spinning faster at idle and charging more. You would want a smaller pulley on the alternator.

I would say dimming at idle is not abnormal, especially if you still have a generator.
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Old 01-10-2019, 10:31 AM   #11
gmachinz
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Re: Dimming lights, altenator or battery?

You need to upgrade your wiring to support a flood of amperage that its trying to pull. Your charging system sounds choked and cant support everything-its like trying to drink a shake using a coffee stir stick. You need upgraded wiring to support that alternator-and fwiw imo SI alternators suck-go with a CS.
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Old 01-10-2019, 10:42 AM   #12
Andy4639
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Thumbs up Re: Dimming lights, altenator or battery?

If the lights go dim on a stock truck with no mods it's the external regulator and the wiring.
The stock system has a built in excitement wire that tells the alternator when it needs to produce more voltage depending on the draw but it has to be above a certain RPM for it to kick on. At idle it kicks off and that's why the truck, & dash lights dim. Plus the fact of how it's wired.

Here is a link to some very good reading on how the GM system works and why to's. They explain it much better than I. I did mine off this page as well.

http://www.madelectrical.com/


With a internal alternator this problem is helped but not always fixed due to the wiring. Some of the earlier internal alternators needed the exciter wire also but later models didn't. The dimming is also caused by slipping belts and worn out belts as well.

Wiring is also a problem depending on how well it was maintained over the years. Take a volt meter and hook it up inside the truck on a hot wire and ground it. Crank the truck up and see what kind of voltage you get at idle with everything off. Then start turning things on and see how it does. This is a quick and easy way to see how the system handles the loads. It should stay around 14 volts with a good wiring system.
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1971 LWB Custom, 6.0LS & 4L80E, Speedhut.com GPS speedometer & gauges with A/C. 20" Boss 338's Grey wheels 4 wheel disc brakes. My Driver
Seeing the USA in a 71


Upstate SC GM Truck Club
2013,14 and 2016 Hot Rod Pour Tour


http://upstategmtrucks.com/



Get out and drive the truck this summer and have some fun!
It sucks not being able to hear!

LWB trucks rule, if you don't think so measure your SWB!
After talking to tech support at Air Lift I have found out that the kit I need is 60811. Per the measurements I gave them. Ride height of truck inside spring and inside diameter of springs.

Last edited by Andy4639; 01-10-2019 at 10:52 AM.
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