Well, 3 days into my ownership of my 86 Corvette a problem has developed. The voltage is low. I haven't gotten a chance to check the voltage at the battery but the gauge shows 12 volts while driving and 11 volts at idle. I was running the car a few minuets ago for about 10 minuets and the voltage fell to around 10. The lights get brighter when I press on the gas so I'm guessing this isn't an alternator issue but a battery issue? The battery is a 1.5 year old Duralast unit. I called the previous owner and he is currently looking for the receipt for the battery. What do you think it is? I really need it to be a dead battery right now.

 

Check the voltage of the battery when not charging and when charging , with a voltmeter. Should be above 12v at rest and 13.xx when charging via the alt.
Could be the gauge.. I doubt the car would start with a 10 or 11v battery....

 

Check the voltage of the battery when not charging and when charging , with a voltmeter. Should be above 12v at rest and 13.xx when charging via the alt.
Could be the gauge.. I doubt the car would start with a 10 or 11v battery....

 

Check the battery voltage and note how much the volts increase, if the battery is not holding a good charge 12.5 volts overnight then look for any excessive current draw.
If the battery is good and has 12 volts the alternator should charge it and bump the voltage up over 13 volts, if the battery is half dead and only around 11 volts even with a good alternator you may only see 12 volts.

I have the 86-91 type alternator on my 85, they have some issues.
I had a brand new one that started to struggle to charge my battery.
I found the shaft would move forward slightly due to poor bearing fit
resulting in the brushes loosing contact and charge stop completely.

I usually see the voltage around 12.5 volts, when starting the car it will go up to 13.4 or so and hover around 12.5 driving. With lights on and a/c the voltage can drop to around 11.8 and if the idle drops below 600 rpm it can bring the warning light on, however that is only after very long idle periods with a lot of load.

My dash shows around a volt less than the battery,
(is a common issue) i have found.

Good luck

 

Well, when I was running the car last night I drained the battery and I had to jumpstart it. After jumping it, the voltage read at 13v (on my voltmeter) while it was idling at 1100 RPMs. When the idle dropped to ~700 the voltage fell to 12.5v and continued to fall to 12.2v and hovered between 12.2v and 11.8v. With the AC on it would hover at 11.8v. The weird part is that the voltage increases when the RPMs go up. When I would rev it to 1500 RPMs the voltage would go up to 13v. Sometimes it seems like the alternator is the one that's dieing but at other times it's like it's the battery.

 

Why guess? Take the battery and alternator to a shop, and have them tested..... it's usually a free test.

 

I guess I'll do that later today when I have another car to use! lol that's probably what's going to end up happening.

 

no chance in hell my 89 will start with 10v. Battery should be at 12v when not running and jump to around 14 when running. I have had battery issues for years now. I thiink i found the issue this week. I need a tad over 11v for my car to start.

 

A reputable auto repair shop will fully charge the battery and do a load test on it and also check the alternator output.

The low voltage (assuming the dash display is correct) you are seeing typically indicates a fully discharged or bad battery. Another thing to check is the tightness of the serp belt. A very loose belt, caused by a bad tensioner, may allow the alternator pulley to slip and that would not provide enough power output to keep the battery charged.

And if you do need a battery, get a good one. The chain parts store batteries are not noted for long life. Our shop sees a lot of these cheap batteries that die after just a couple years. Get an Interstate, Sears DieHard Gold, or a professional-grade AC-Delco (these are sold at aout repair shops and not chain stores) battery in the correct group size for the car.

 

I was using a voltmeter to do the testing. The gauge reads about .5v lower than the voltmeter. I already have the alternator out so I guess I'll be removing the battery next.

 

While you have the battery out check the fusible links on the junction block behind the battery.. Make sure all are good and the connections are clean..

 

I think you will find that most batteries are manufactured by Johnson-Controls..... DieHard, Optima, Interstate, AutoZone, Walmart, etc. are all the same battery with different labels.... most have the same warranty. Interstate is just a marketing company, they don't make batteries.

If you intend on keeping your car for a few years, buy the best battery (cranking amps) that will fit in the space allowed. Most quality batteries have a 3yr full replacement-10yr prorated warranty, and should last that long with proper maintenance.

If you are a Costco member, their batteries are the same as you would buy at Sears, etc. ........ but at a much lower price. Same warranty.

 

If you are testing voltage a the batt, it will always read higher than your gage due to current passing through the ignition.

 

Clean ALL elect copnnections at the alt, battery AND especially the jumper post hot wires and the Grounds on the block by the oil filter...HARNESS GROUND colection...NOT the battery cable.
Clean the alt plug-in and the cable.

 

Well I took my alternator and battery to get tested and sure enough the alternator was -almost- dead. It would pass the easier tests but fail the harder ones. $100 later and I'm happy.

 

Good fix....... now, make sure all connections are clean and tight.

 

That shows the alternator is working, the voltage will rise with rpm.
You can get a powermaster or such alternator that will give more charge at lower rpms, but sounds like your battery is bad.
You need to get the acid's specific gravity checked, does the battery voltage drop overnight?.

 

Read post #15..... alternator failed test.

 

I picked up the alternator today and...... Wrong part! Ugh. Now I have to wait for 11:30 AM to roll by.

 

So after receiving the wrong alternator twice, can anyone tell me what the ac delco part number is for the alternator? It's an 86 with iron heads.