Battery Voltage at Idle
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Battery Voltage at Idle
What should my battery voltage be on an L98 1985?
It sits around 13 to 13.5 when I'm moving, but sometimes when it idles in the Atlanta heat (with the A/C on of course), the voltage dips to as low as 11.3. When I get moving again it goes back up to the 13 range.
It sits around 13 to 13.5 when I'm moving, but sometimes when it idles in the Atlanta heat (with the A/C on of course), the voltage dips to as low as 11.3. When I get moving again it goes back up to the 13 range.
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Am I in danger of stalling with that voltage at idle? Am I in danger of starting issues?
Race Director
Joined: Apr 2002
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From: Anaheim CA
Turn on the ignition switch without starting the car. The voltage on the display is your battery voltage. It will be roughly 11.6 to 11.8 volts.
After you start the car it will most likely be higher because the voltage is coming from the alternator. I said "most likely" because the alternator is just barely working at idle. The voltage will go up pretty rapidly as you increase RPM, even just a little. The normal output of the alternator at higher RPMs is around 13.5 to 14 volts. The exact voltage varies, and is determined by the voltage regulator in the alternator.
Lower than normal alternator output can be a bad voltage regulator, bad winding(s) or bad diode(s).
After you start the car it will most likely be higher because the voltage is coming from the alternator. I said "most likely" because the alternator is just barely working at idle. The voltage will go up pretty rapidly as you increase RPM, even just a little. The normal output of the alternator at higher RPMs is around 13.5 to 14 volts. The exact voltage varies, and is determined by the voltage regulator in the alternator.
Lower than normal alternator output can be a bad voltage regulator, bad winding(s) or bad diode(s).
Thread Starter
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Turn on the ignition switch without starting the car. The voltage on the display is your battery voltage. It will be roughly 11.6 to 11.8 volts.
After you start the car it will most likely be higher because the voltage is coming from the alternator. I said "most likely" because the alternator is just barely working at idle. The voltage will go up pretty rapidly as you increase RPM, even just a little. The normal output of the alternator at higher RPMs is around 13.5 to 14 volts. The exact voltage varies, and is determined by the voltage regulator in the alternator.
Lower than normal alternator output can be a bad voltage regulator, bad winding(s) or bad diode(s).
After you start the car it will most likely be higher because the voltage is coming from the alternator. I said "most likely" because the alternator is just barely working at idle. The voltage will go up pretty rapidly as you increase RPM, even just a little. The normal output of the alternator at higher RPMs is around 13.5 to 14 volts. The exact voltage varies, and is determined by the voltage regulator in the alternator.
Lower than normal alternator output can be a bad voltage regulator, bad winding(s) or bad diode(s).
Le Mans Master
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From: One day you're a Comet...the next day you're dust... Arkansas
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Team Owner
Joined: Mar 2001
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From: Cincinnati, Oh USA
Dash voltmeters read low when the alternator is not putting out normal voltage(engine off). Batteries are considered discharged at 12.0 or lower volts. Engine off dash voltmeter readings have no value because the voltmeter is inaccurate. Normal battery voltage is 14.7 volts engine idling and this drops to 13.7 volts when the alternator gets to its operating temperature. The dash voltmeter reads 0.3 volts low due to its measuring the voltage after the ign sw whose contacts have about 0.3 volts drop. A/C on and idling, you should see 14.3 volts dash meter, engine cold and for the drop you observe, you probably have too low idle rpm or your alternator has low output. At engine idle, the alternator cannot generate its normal output current and with high current loads, A/C, high fan speed, engine fan on (comes on with A/C) you will see voltage droop, but it should not be below 12.0 volts or even slightly higher.
Le Mans Master
Joined: Sep 2006
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From: Phoenix AZ
2021 C4 of the Year - Modified Finalist
before you go off and spend big money, go to harbor freight and buy an inexpensive voltmeter.
start your car and put the red lead on positive battery post and black on negative post.
your readings will probably be a half volt higher than the one in the car.
I vote your car is normal.
the extra wire in the car will drop the voltage reading a little bit. If your car reads 11.3, keep it in gear, and put the parking brake on full. then what is your reading with your foot off the brake? probably 12.6, then turn off the air conditioning, and it jumps to 13.2.
If this happens, put it in park, and save your money. your car is normal.
start your car and put the red lead on positive battery post and black on negative post.
your readings will probably be a half volt higher than the one in the car.
I vote your car is normal.
the extra wire in the car will drop the voltage reading a little bit. If your car reads 11.3, keep it in gear, and put the parking brake on full. then what is your reading with your foot off the brake? probably 12.6, then turn off the air conditioning, and it jumps to 13.2.
If this happens, put it in park, and save your money. your car is normal.
Advanced
Joined: Jun 2011
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From: Charleston SC
Adam,
I think the guys are right BUT, my 84' does the same thing. If I'm sitting at a traffic light, with the AC fan on high, my volts will drop to around 12... and if the headlights are on, I'll see 11.7 to 11.8... If I turn the AC fan sw back one notch from high, it goes up to 12.2-12.3. I've tested my charging system, completed a battery test, and everything checks out. I hooked up a tester to the battery as described above, and the reading's I was getting were just over 1/2 volt higher than the dash was reading... So, considering the power hungry electric fan mounted on the front of the motor...coupled with the AC fan, that alternator has to work hard to keep up with the AMP load. I think you're OK but it never hurts to have a battery & charging system tester from Harbor Freight tools, it's a great tool for anyone maintaining his/her own car.
I think the guys are right BUT, my 84' does the same thing. If I'm sitting at a traffic light, with the AC fan on high, my volts will drop to around 12... and if the headlights are on, I'll see 11.7 to 11.8... If I turn the AC fan sw back one notch from high, it goes up to 12.2-12.3. I've tested my charging system, completed a battery test, and everything checks out. I hooked up a tester to the battery as described above, and the reading's I was getting were just over 1/2 volt higher than the dash was reading... So, considering the power hungry electric fan mounted on the front of the motor...coupled with the AC fan, that alternator has to work hard to keep up with the AMP load. I think you're OK but it never hurts to have a battery & charging system tester from Harbor Freight tools, it's a great tool for anyone maintaining his/her own car.
Burning Brakes
Joined: Jun 2008
Posts: 892
Likes: 1
From: New Smyrna Beach Florida
Adam,
I think the guys are right BUT, my 84' does the same thing. If I'm sitting at a traffic light, with the AC fan on high, my volts will drop to around 12... and if the headlights are on, I'll see 11.7 to 11.8... If I turn the AC fan sw back one notch from high, it goes up to 12.2-12.3. I've tested my charging system, completed a battery test, and everything checks out. I hooked up a tester to the battery as described above, and the reading's I was getting were just over 1/2 volt higher than the dash was reading... So, considering the power hungry electric fan mounted on the front of the motor...coupled with the AC fan, that alternator has to work hard to keep up with the AMP load. I think you're OK but it never hurts to have a battery & charging system tester from Harbor Freight tools, it's a great tool for anyone maintaining his/her own car.
I think the guys are right BUT, my 84' does the same thing. If I'm sitting at a traffic light, with the AC fan on high, my volts will drop to around 12... and if the headlights are on, I'll see 11.7 to 11.8... If I turn the AC fan sw back one notch from high, it goes up to 12.2-12.3. I've tested my charging system, completed a battery test, and everything checks out. I hooked up a tester to the battery as described above, and the reading's I was getting were just over 1/2 volt higher than the dash was reading... So, considering the power hungry electric fan mounted on the front of the motor...coupled with the AC fan, that alternator has to work hard to keep up with the AMP load. I think you're OK but it never hurts to have a battery & charging system tester from Harbor Freight tools, it's a great tool for anyone maintaining his/her own car.
I have same problem in my 89 with a new battery. It only drops below 12v at idle with A/C on. (11.9 -11.8v) Not as low as yours, but still below 12v. Goes up to 13.5v when driving down the road.Take Alt. out of car, and take it to Auto Parts store where most do a free load test. I plan on doing it 1 of these days...
Team Owner
Joined: Sep 2003
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From: Freedom is never more than one generation away from extinction.
St. Jude '03 thru '24
What is your idle RPM?
With the electrical system at full load (standard equipment) there is no appreciable drop in volts on my 86.
My idle is 750-800 with A/T
With the electrical system at full load (standard equipment) there is no appreciable drop in volts on my 86.
My idle is 750-800 with A/T
Burning Brakes
Joined: Jun 2008
Posts: 892
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From: New Smyrna Beach Florida
That idle seems high to me. You must have to use a lot of brake pressure when stopped at a light with trans. in gear. My idle is in the 600 rpm range (A/T) when stopped in gear.
Team Owner
Joined: Sep 2003
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From: Freedom is never more than one generation away from extinction.
St. Jude '03 thru '24
600? Too LOW!
150RPM can be enough to drop the voltage regulator off line.
Try holding the throttle to get revs up so the voltage goes above battery.
Note the voltage and RPM.
150RPM can be enough to drop the voltage regulator off line.
Try holding the throttle to get revs up so the voltage goes above battery.
Note the voltage and RPM.
2nd Gear
Joined: Jul 2011
Posts: 2
Likes: 0
Battery Replacement L98 C4 1989
I've read that when the battery is disconnected for replacement that the "computer" needs be reset.
If this is so, then what is the reset procedure?
Can this procedure be bypassed by removing the battery while the
engine is running and powered from the alternator?
If this is so, then what is the reset procedure?
Can this procedure be bypassed by removing the battery while the
engine is running and powered from the alternator?
Instructor
Joined: May 2011
Posts: 102
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From: SE SD
Most come on at 2400rpms. Remember you have to take pully ratio into account as well. Some are on a 3 to 1 ratio. So 600 rpms at the crank will get you the 2400 at the alternator.
Burning Brakes
Joined: Jun 2008
Posts: 892
Likes: 1
From: New Smyrna Beach Florida
I'm sorry, but I believe 750-800rpm w/auto trans. in gear at idle is too high. I know I wouldn't mine that high.
