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'69 camaro...original 350ci and pretty much unmolested engine compartment. (all stock) charging system works great, battery normally tests at 12.3v and alternator is putting out 13.4v...but noticeable dimming in instrument lights, dome light, head lights etc unless rpm's are above idle. any ideas on what would cause this????? thanks for looking, and any input you may have. Rick
 

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Too many amps being drawn at idle with alternator not putting out enough.
Although a 12v system, the alternator should be putting out 13.5-14.5v. Some alternators don't charge at idle.
 

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Yep, as Doug suggests, the original design left a lot to be desired by today's technology.
Check fan belt tightness and adjust accordingly - 1/2-5/8 inch deflection on the longest run.

Your battery measurement of 12.3 is low, and very well may be due to low alt output as indicated, 13.4 volts.
Several things could affect this event.
Loose belt or worn belt slipping
Poor connections of red wire from alt BATT terminal to battery post - disconnect battery, clean clamps and posts, disconnect wire from alt, horn relay buss bar, battery junction block, and negative battery cable , and wire brush all terminals, reassemble.
Volt regulator needs adjustment - let an electrical shop do it, or
replace the VR with a electronic VR, Wells P/N VR715 and don't forget to use the ground strap from the original VR.

If you change out VR, save the original one.
Any questions, keep asking.
Years ago, the VR was adjusted for the driving routine of the car.
Once a week, battery level was checked visually.
If acid was low, battery was being overcharged, reduce voltage setting on VR.
If battery was dead, or low on power, adjust VR up in voltage.
This design was not for stop-go traffic and idle time.
Start the car drive it 10-15 miles at highway speeds.
Adjust accordingly.
 

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install the smallest pulley that you can get on the alternator
 

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thanks a bunch guys! i bought a new solid state voltage regulator today, (just in case the old one wasn't tweeked just right), so i will install that tonight, and check connections to make sure they are clean and making good contact. will also check alt belt and adjust if needed. hopefully this will take care of the problem. i will let you know! Rick
 

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My original system was pretty good and never let me down, it did the same thing though, lights dimmed at night etc.
But when I did the engine swap I put a CS130 alternator and a new front harness to eliminate the rats nest for the external regulator on the core support and relocate the alt to the passenger side (not necessary but considerably cleaner). The lights are bright and rock steady now, it made a big improvement.
 

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My orginal altenator also did the same thing..
and was getting 'tired'.. so it was put away into storage
A internal alternator dropped in increasing the original amp out put from spec of 64amps (think it was) to 100 amps....
Remove the old one, bolt up new one, connect red wire, connect one of the others(blue from memory... a jumper wire between a couple terminal on the back (to excite the field coils) blank off the last wire.....clocked up well over 100K miles
Down side on a cold wet night.. wipers heater lights etc drew a little more than the alternator... so this alternator is 110 amps... and the internal regulator now runs an electric fence from a 6/12/24/ 36v ac source off an old electric blanket and a indicator flasher with points contacts.
 

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This has always been the way I was taught from sometime in the early 1970's. When the electrical system is in good working order and the lights are dim (headlight and dash) and get brighter as the engine rpm comes up it's a sign of a weak and soon to die battery.
 

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thanks a bunch guys! i bought a new solid state voltage regulator today, (just in case the old one wasn't tweeked just right), so i will install that tonight, and check connections to make sure they are clean and making good contact. will also check alt belt and adjust if needed. hopefully this will take care of the problem. i will let you know! Rick
Rick, The solid state voltage regulator was a good decision, in addition a headlamp relay setup as Mark suggested will help even more if you want brighter headlamps. He provided a link for Summit but there are others available as well. They make a big difference and take a great load off the old factory wiring harness.
 

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Dirty and corroded connections, including grounds will cause less current to flow making things work poorly. Going through and cleaning +/- connections will usually make things work better.
A charged battery is 12.6V. Anything less is not fully charged.
 

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My 67 used to do that. I replaced the original style alternator with an SI alternator. Very easy to swap over with a short adapter harness. and a jumper at the external regulator plug. No problem any more even with new halogen headlights. The harness is a good option too. I have one on my jeep, really brightens up the headlights.
 

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UPDATE.....just wanted to follow up on this thread (because everyone was kind enough to help, and i hate it when others don't come back and give conclusions, lol)

i pulled battery cables, grounds, and horn relay connections and cleaned them, put on a new solid state regulator, made sure that battery was charging up to 12.6 and holding charge, and now lights DO NOT DIM at idle. thanks to all who had input! Rick
 
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