Alternator???
Thread Starter
Instructor
Joined: Oct 2005
Posts: 244
Likes: 1
From: Brighton Michigan
Alternator???
I have a 74 BB and last night I had the headlights on for a little while and the interior lights for the gauges kept dimming while stopped and at idle. then as I took off from the light they got to the proper brightness and looked fine. The battery gauge would go just slightly negative at idle and then at proper speeds be slightly positive. Any help on this would be great.
Thanks,
Alex
Thanks,
Alex
Melting Slicks
Joined: Apr 2009
Posts: 2,632
Likes: 112
From: Great Plains Iowa
I have a 74 BB and last night I had the headlights on for a little while and the interior lights for the gauges kept dimming while stopped and at idle. then as I took off from the light they got to the proper brightness and looked fine. The battery gauge would go just slightly negative at idle and then at proper speeds be slightly positive. Any help on this would be great.
Thanks,
Alex
Thanks,
Alex
Alternators typically have a sheave that is 2-3/4" to 3" diameter so they turn at about 1/3rd the crankshaft speed. If you have an automatic transmission and idle at say 600 rpm in DRIVE the alternator speed will be around 1800 rpm and alternators need at least 2000 rpm to produce anything. That's why your lights are dimming at idle and the only cure is to put a smaller sheave on it OR a premium alternator like an 80 amp Delco like the Cadillacs and Oldsmobiles used in the 1970's. The 80 amp Delcos produced a lot more current at low rpm but they have a weight penalty as their rotors are very heavy.
Team Owner
Joined: Jul 1999
Posts: 65,492
Likes: 230
From: Orange Park Florida
I have a 108 amp larger dia. alt. from a '79 Seville with RW defogger.....I run Dual Spals and a/c and lights, and all sorts of crap with it, even at idle, it never fails to put out......but you may HAVE TO CHANGE THE MOUNTS much less I have a Serp drive from an 88-91 vette.... so the alt pulley is smaller diameter and with that large alt on there I have no problems.....
Le Mans Master
Joined: Dec 2006
Posts: 8,837
Likes: 1,028
Alternators typically have a sheave that is 2-3/4" to 3" diameter so they turn at about 1/3rd the crankshaft speed. If you have an automatic transmission and idle at say 600 rpm in DRIVE the alternator speed will be around 1800 rpm and alternators need at least 2000 rpm to produce anything. That's why your lights are dimming at idle and the only cure is to put a smaller sheave on it OR a premium alternator like an 80 amp Delco like the Cadillacs and Oldsmobiles used in the 1970's. The 80 amp Delcos produced a lot more current at low rpm but they have a weight penalty as their rotors are very heavy.
Dimming lights at idle is NOT normal. They did not do that new, nor should they now.
Melting Slicks
Joined: Feb 2006
Posts: 3,105
Likes: 96
From: Sunrise FL
