I drove my 1986 L98 convertible for a couple months this Summer, then left it for a month or so...I had bought a brand new battery for it in July.

When I tried to start it last week, there was *no* charge in the battery. I charged it overnight, and today it started right up. But after idling for a minute or two, a red warning light came on in the dash...



The switch was set on Temp, not Volts...When I manually switched it to Temp, the red warning disappeared, and the temperature was displayed...but it switched itself back to Volts, and the warning light came back on...

I hadn't been having any problems with the electrics since I got the anti-theft module replaced, and the battery is brand new...

So...What else could be causing the warning light to come on??? Bad alternator??? Diode bridge burned out???

I don't want to drive it on the battery...been there, done that...

What I'm asking is...Does this warning light indicate it's not charging??? Or just that I have a low battery??? I know it will bleed off volts just by sitting...Will it charge back up by driving and the light will go off???

 

The red bar indicates low or high battery voltage. In your case it is low at 11.2 volts. The voltmeter reads low if the engine is not running . Normal battery voltage, engine running, is 14.7 volts cold and this drops to 13.7 volts when the alternator gets to its operating temperature. The dash voltmeter will read 0.3 volts lower than this because it reads the voltage after the ignition switch contacts which have about 0.3 volts drop. You clearly have something wrong with the charging circuit. Engine running, measure the voltage on the output terminal of the alternator, it should be 14.7 volts and this should also be the voltage on the battery terminals. If the fusible link in the alternator output wire is blown, you will read 14.7 volts on the alternator but much less on the battery, like 12.0 to 13.0 volts. If it appears that your alternator is defective, you can remove it and take it to a parts store, most can test the alternator for you.
You can tell the state of charge in your battery by measuring its no load voltage on its terminals. 12.0 volts and below, discharged, charge your battery up overnight. 12.9 volts and higher, fully charged and linear in between.

 

The digi-dash has a red warning bar when the indicated value exceeds normal operating perimeters. In this case the voltage is too low with the engine running.

If your going to park your car for extended periods of time get a "battery tender" or charge the battery before taking your Vette out for a ride.

 

So it's not a charging [alternator] problem???

Exactly what I do...On a monthly basis, during Winter...

But this is the first time I've seen that red warning bar/light come on...and have the Temp indicator snap to the Volts display all by itself...

Weird...

 

Your alternator is not charging the battery! Also, get a battery tender and keep it on your battery. Driving your vette once a month will not keep the battery fully charged and you will experience short battery life.

 

Try cleaning all of the alt. conections. The plug in conector can loose contact and cause this prob. wiggle the wires before spending alot of time and money. The gage will switch it's self if there is a prob. good luck.

 

You have a bad alternator. I kept having to change my alternator in the summer, so I had 3 of them to swap out if the replacement got bad.
I went to a 92-96 alternator in the 'ol '86, and no more problems.
To swap it over, you need an '85 alternator bracket and fabricate the new alternator to exhaust brace that it will need along with new bolts that are long enough.
The later alternator is 140 amps as well. Oh, they are cheap to buy. I got mine for $35 used. There are plenty in the wrecking yards for about $50 a pop if you ever need one. They are almost bulletproof anyway.

 

OK, I charged the battery up for a day. Battery now reads 12.37v, engine OFF, switch OFF. Switch ON, engine OFF, light does NOT go on. Switch ON, engine RUNNING, light is ON. So now I try to diagnose...

Switch ON, engine RUNNING, output term of alt reads 11.97V; battery reads 11.95v.

HUH??? Battery reads 12.37v with engine OFF, but drops to 11.95v while RUNNING??? With alternator connected???

I'm running off the battery??? No charging at all???

I tried to run some of the tests in the Helms Manual, but they were inconsistent...I couldn't tell if the alligator clip was on the "L" terminal or not (small terminal; larger alligator clip)...or whether the manual was referring to the alternator terminal or the harness terminal/lead...

I'll pull the alternator off this week and take it to AutoZone and also to CarQuest and see what they say (and compare their prices)...

That sounds interesting...What makes the later alternator so much better???

 

You don't need anymore opinions on your dead alternator! What you do need is a good alternator.

 

...ARRRGGGHHH!!!... I hate electrical systems!!!...

 

I have the exact year and model vette as you do and when my alt went out the warning light came on and the display changed to volts as well. New alt cured it.

 

The new alternator is larger in diameter, approx. the same size as the 85 alternator, and due to it's larger size, The manufacturer installs more segments to the stator and it charges higher amps at idle due to the increased load carrying ability. There is also more aluminum to draw away the heat that is built up because corvettes aren't metal skinned and hold in a lot more engine temp.
The increased stators allow for more segments on the rotor and the internal wiring is increased with the wiring totally encapsulate within the alternator. This allows better connections eliminating internal heat as well.

FYI,

There are two alternators that look like yours. one is the heavy duty one, and one is a standard or light duty.
THe light duty has a cheapened diode bridge that has the wires internall spotwelded to the diodes on copper material, cut from a copper sheet of about .012" thick, and the heavy duty one has the larger copper wire that is soldered to the stator windings.
Overhauled alternators sometimes just get the wires cut and replacement wire connectors get crimped onto the wires allowing for increased conductor heat which frys the diodes, and makes them short or open which ever the case at the moment of failure.
When shopping for a rebuilt alternator, pop off the plastic piece on the bottom of the alternator, and look at the wires. Soldered is great, but crimped tin sleeves is a warning sign that the rebuilder cut some corners. Sometimes the lifetime warrantied parts are made up of parts that never came with that particular alternator, but checked good at the time of remanufacturing.
The gamble is that the alternator parts will be good for a long time, lessening the need for ALL new parts.
The things they change always are brushes, brush holder, bearings and diodes. The rest is up for interpretation.

 

Thank You for all the information! Will any 92-96 GM alternator work, or is it Corvette specific??? Will I need a new pigtail as well as bolts & bracket arm???

I've a good junkyard near work that has ample supplies of those years, but not Corvette...maybe an odd Camaro/Trans Am or two, but GM station wagons seem a more likely source...

 

Would that be this one??? The upper bracket???

 

Your current alternator is very simple to rebuild and inexpensive. You just need a new voltage regulator, rectifier, brushes and a bearing. Or skip the bearing and brushes if they are good. I fixed my alternator years ago for 12 bucks, it was the rectifier (diode pack) that was bad for 12 bucks. The alternator is very easy to take apart.

 

You are a better man than me...I couldn't get the back end separated from the center section...

I took the alt off the car, removed the three long through-bolts, and separated the front from the rear...The center section doesn't want to separate from the rear, although it is loose...It looks like the three diode leads are holding it in...

BTW; my three leads are crimped, and there seems to be a coat of silver paint on the alt...(except the steel center part)...I'm guessing this is a rebuilt???...

The Helms manual doesn't show any rebuilding of the alternator (or else I'm missing page 6D3-4)...the Haynes manual does, but the pics/description are not my alternator...

Do the diode leads need to be snipped to get at the diodes???

 

I went this route...

I got what was advertised as a "1992-96 Corvette alternator LT1 CFI" that supposedly was "used" for $30 on eBay...but when I got it, it had nice bright bolts, clean aluminum, and shiny black paint...and a sticker that said "Rebuilt by Autolite"...

The pulley was clean and the paint on it was not worn at all...if this was used, it wasn't used much...

I also got an 85 upper alternator bracket off eBay, same price...Took my old alt off, bolted the new bracket & "LT1 alt" on...The bottom 86 bolt was OK as on the 86 alt; the bolt through the 85 bracket to the smog pump needed to be longer...and it needed two shims to fit the 85 bracket...one .310" thick and the other .715" thick...I also thinned the head of the bolt to get extra clearance for the smog pump pulley...

I made a new brace for the back of the alt out of an old mid-70's Honda 500 Four rear brake strut...

Belt was OK; wires were a bit out of place but stretched enough to test it...



It now charges at ~15.7 volts at the alternator terminal (battery may have been down a bit); idiot light is off when idling; voltage at battery is ~15.3 volts...

I think I'll move the wiring up a bit to take the tension off the wires, but this should work...

I've found that my particular 86 (with aluminum heads) seems to have some 84 parts on it (it has an 84 10 1/2" clutch assembly); the 85 bracket may well fit an 86 smog pump without shims...I may have some older parts here & there...

Now, the next problem...the negative battery bolt seems to be too small a diameter...it keeps spinning when I try to tighten it, but a 3/8" bolt (that I use for bench charging it) tightens up nice and tight...