1980 L48 Auto

Can someone give clear instructions on how to measure the output at the battery, alternator and to the starter. Should the vehicle be on, do I disconnect the battery etc... Electrical stuff is very difficult for me.

Thanks............

My voltmeter gauge in the vehicle with the engine running sometimes dances from low on the left all the way to the right and then back and forth back and forth. It seems to happen more when the weather is hot or the vehicle has been running for a long while. The other day I took the Vette to pick up my son at school. When I came back (after 15mins)to the car it would not start- the starter would not even turn over. I walked home brought my other car to jump it and it started right up after a minute of charging.

I took both the alternator and battery to an auto store and had them checked. Both were deemed good on the stand.

Any suggestions as to what could be causing this? I thought I read somewhere in a previous question I had, that a "diode" is going bad inside the alternator which explains why the volt needle is jumping and why it tested good at the time.

Any thoughts on this?

Thanks much
Darrel1

 

Anyone with instructions - on how to check the battery, alternator, starter..

Darrel1

 

The battery voltage should be checked with a handheld meter with both the engine runnng, and with it off. In the off condition you may read around 12.75 vdc or so, and in the running condition 13.8 vdc, which would tell you the alternator is charging.

The jumping voltage you mention could be a loose connection somewhere. Check your alternator case to some ground point on the frame of the car. Also the ground wire between the frame and engine block.

 

When checking the battery do I just put the meter between the + and - posts and read the meter or ??? These are the details I need help with- sorry for the dumb questions.

When you say "check the alternator case to ground" please explain?

Also, I have read that I could also have a problem with the starter solenoid- how do I check that- details please.

Darrel1

 

check the belt tension.

 

When checking the battery using your voltmeter [VDC setting] (both for engine running and for engine off), put the + test line on the positive battery terminal and put the - test line on the negative battery terminal. The test of "alternator case to ground" that was mentioned is a continuity check using the ohmmeter [ohms/resistance meter setting]; you should do this with the positive battery terminal disconnected from the system and put + test lead on the alt. case and - test lead to a 'known good' ground point. You should read "O" ohms. If you read any resistance, your alternator is not properly grounded...suspect the ground cable running from the starter frame to the right-side engine mount [not the solenoid lead]. If that checks OK, then re-connect the + battery terminal, fire the engine and use your voltmeter [VDC] to check the output voltage of the running alternator. {Note: be careful not to get anything in the rotating drive belt and do not allow any metallic object to touch both the (red) voltage output terminal AND the alternator case. } With + test lead connected to the (red) output terminal and - lead connected to the alternator case/ground strap, you should be reading a little less than 14 volts with the engine running...and the voltage should be stable.

Now, if the running voltage at the alternator is not a steady 13+ vdc, the alternator diode set is defective (regardless of what the auto store boys told you). You can repair your alternator or do an alternator exchange to fix the problem. If your alternator running voltage is steady 13+ vdc...but it is not the same at your battery, you either have a wiring problem between the alternator and the battery or your battery is bad. You will have to figure that one out, if need be. Good luck.

 

Thanks - that gives me the detail I need. I am going to check the system over the weekend

Darrel1

 

I went for a drive Sunday and noticed my voltmeter was going from around 8 to 13 while driving. After checking everything with a hand held meter, I started checking all the connections at the alternator, and sure enough, when I touched the hot wire, it came loose in my hand, separated from the connector. New connector fixed it. Something to check.
KC

 

Ok so I did all the checks- the only question is on the alternator case to ground. Do I remove the + lead at the battery or the alternator? And this is with the vehicle off?


Yesterday here is what I did:

1. re-installed the alteranotor and battery- made sure all connections were good and belt tight.
2. Checked battery engine off- about 12.2 volts
3. Check battery engine on- 14.3 volts
4 Checked alternator output- 14.3 volts
5. Checked alt case to ground (+ lead on battery disconnected) 0 on Ohmmeter- so ok if properly done

At this point the needle on the gauge is not jumping so I figured the issue was the belt or one of the connections. I go for a 15 minute drive to warm the engine bay up. Pull into the driveway and shut off engine. After about 15 minutes of the vehicle sitting I go to start the car - when I turn the key- and nothing from the starter- but power to everything else (radio- lights- gauges). No click..click...click- nothing on the starter. I hook up a battery charger to the battery- and 1 turn of the jey and it starts right up?

I go for another drive- needle is steady at about 14 volts. Now I let the car sit in the driveway for about 30 minutes-let everything heat soak. I start the car and now the needle on gauge is bouncing all over everywhere- only when the car is in drive with the foot on the brake. I get my wife to keep the car in drive I check the alternator output and it is bouncing all over everywhere- 14.7 volts down to 1 volt?

So Is my issue the alternator?
Any thoughts on why the starter did not move- is it going bad also?




Any suggestions would be appreciated


Darrel1

 

Try taking a volt reading on the battery with the engine not running, but with the headlights on high beam. The 12.2 vdc you mentioned sounds a bit weak. If the battery is good, the voltage level should remain fairly steady for quite a while. If you see the voltage falling, replace the battery.

 

I would check the battery connection at the ground or - side of the battery or where the cable is bolted to the frame for your stater cranking problem. When you connected the battery charger you probably completed the ground through your charger. As for the needle jumping, I would think the voltage regulator in the alt. is going bad.. Very easy to replace along with the brushes. Parts are available at most parts houses and starer rebuild shops.

 

And your lead to the starter is clean and tight. Also make sure your battery ground connection is clean and tight (as mentioned earlier). I would also check to see that the ground lead from the frame to the engine is clean and tight. 12.2 vdc is what my battery reads when the car is off, it seems low, but my system is working fine right now and that's what it reads....you might want to get battery load tested at your local parts place anyway...they'll do it for free.

After reading this a couple of times, I can't decipher what it is that you have done? Other than take readings? Did I miss it somewhere.

 

Yes that is all I have done is make sure the connections at the alternator and battery are good and take readings- tonight I will check the grounds mentioned above. I did have the battery and alternator checked-auto store said they are good.

The weird part is that these things happen when the outside air and vehicle are warm/hot. When things are cold they work fine.


Darrel1

I will check the ground

 

Or take the alternator to autozone and they will run free check on it. My needle flickered only a little and the reading from autozone was the alt was bad. Left with a new (refurb) one (warranty for life) and haven't seen a flicker since.

 

A problem with the alternator should not prevent the car from cranking. You problem is one of a [warm] wire or connection not allowing enough current to flow to operate the starter... OR ...you could have a bad starter solenoid... in addition to a wire/connector problem (which is causing the gauge needle to jump).