VikingTrad3r

Gents, good evening, hope all have had a great summer!

My 87 has been reading very low on the dash. This is my first crack at the alternator. Age of alternator and battery unknown.

Condition:

Car off, Battery 12.6 v
Car idling, Battery 14.3 - 14.1 v, Dash reading as low as 13.6

Car idling, with full load from electrical components, brake lights, high beams, everything. Dash dips as low as 12.1 i even saw 11.8. Battery reads 12.4 at from the multimeter.

So is my alternator toast?

I was excited that maybe it wasn't when i saw the reading at the battery was holding in at 13.6 but it eventually dropped down as I added more load.

Do alternators just go *poof* or do they slowly die over time?

VikingTrad3r

Cliff Harris

The dash voltmeter is not very accurate.

Probably not. The main thing is does it start the car?

It depends on what causes the alternator to fail. Most of the time it's a sudden failure.

WW7

With a fully charged battery on a hot day, my dash reading gets down as low as 13.6 while idling in gear...At startup it's around 14.4 -14.8 depending on ambient temperture...As Cliff says , the dash gauge isn't very accurate ,so always use a hand held unit to check the voltage...Check your battery a few times a week with a multimeter to make sure it's staying charged...If it's staying charged, your fine......WW

cardo0

Ya know i have 2 ten buck battery testers and used to carry one in each car - great tool. I can check batt strength, voltage and alt charge volts all in less than 1 minute. Seriously they have a little voltmeter (that seems fairly accurate when compared to my digital VOM) and even color ranges for batt discharge strength.

How old is your batt VkTr? FYI 4-5 years is a good life for a lead acid batt.
Car off, batt = 12.6V is not fully charged - should be 14.2V (or maybe 14.4V cant recall now) full charged.
Car idling batt = alt output = 14.3 is ok as alt is charging, dash meter is low.
Loaded voltage on dash = 11.8 is probably 12.4 w/good meter.

IMHO the batt is done and going to quit soon. Also a bad batt will overwork the alt as its continually charging and the alt will die soon to or its capacity is reduced as the diodes open up.

Now to confuse u some the V meter on my '94 camaro is pegged high nearly all the time at 18V. Do i care? Kinda but i have my handy dandy batt tester to check everything if i need to.

ToniJ1960

I cant agree with what youre saying about the charged voltage of the battery.

whalepirot

A very handy voltmeter I bought a few of, plugs into the cig lighter. I used it in a newer BMW,that like so many of these new 7ers, had multiple electrical problems. I gave one to a shop-owning pal as a quick and easy tool; way quicker, uncluttered and much easier than using any VOM, making it quite instructive about voltages under normal, or not, conditions.

I believe a charged battery is 2.2vdc/cell x 6 =13.2vdc. Any load will drag it down and the alternate would normally reverse that.

Are the battery terminals tight and clean, plus all grounds?

VikingTrad3r

battery age is unknown.

i will charge battery overnight and see if i can get more than 12.6.

battery cable connections are clean and i wire brushed them.

i notice the problem is worse in the rain. either because i have to run wipers and lights, or, it could be a ground.

plan is to look in fsm for list of ground locations on the 87 and check them and the main leads off the battery.

Wouldnt be surprised if the battery is old and tired. that said, i want to be thorough and clean-snug all grounds anyway before tossing a new battery into a car and feeling like its fixed while letting the potential for corroded or loose grounds fo unchecked.

will report back later cheers.

VikingTrad3r

how do i test the alternator output on the c4? i have been testing boltage at the battery but how do i do it from the alternator to see if the numbers are the same?

Nutmeg

Posted in wrong area

vader86

To measure voltage from the alternator, you just put the + lead on the rear terminal, and a ground lead on the frame or the block.

Joe C

- engine off - a good, fully charged, battery should read 12.5 - 12.8 VDC (reading, DVM, across the battery terminals)

VT3 - from your original post - sounds pretty much normal - my 85's acts the same (newer battery and rebuilt alternator). 13.8 to 15.3 volts (idle, lights and accessories off) is fairly normal. with everything powered (lights AC, etc.) lower voltages at idle (below 12.3) usually indicates you're just taxing your alternator's limits. GM spec'd the cheapest and lowest "amp" rated alternators they could get by with - it's a bean counting thing. not to worry unless the thing drops below 11.5V (dash indicated). at that point, your "temp/voltage" readout will automatically switch to "voltage" and you'll get a red warning light. at that point, you may have a concern. as long as your voltage levels recover, IMO, no need to fix anything unless you just want to clean the battery terminals. note, a light coating of petroleum jelly on the connections is a good preventative maintenance thing.

MaineJim

I have never seen a fully charged battery above13 volts unless it was grossly overcharged by a faulty charger or alternator.
Typically 12.65-12.85 volts is what the norm is for a fully charged battery a few hours after charging.
While the motor is running battery voltage should be between 13.8-14.8 depending on how long the car sat before starting it up and the electrical load while it's running.
I would verify those voltages with a good quality voltmeter Fluke,etc,i woudn't depend on the cars gauge.
A load test on the battery will show the condition of the battery ,just a voltage reading will not but if all the voltage readings i and others typed are what you are seeing it's propably ok.