I am new to corvettes, specially to the use of ammeters. My corvette will show -40A whenever I turn on the ignition, or just turning on the lights etc. When I start the car the ammeter goes full +40A. I have charged the battery and still get the full deflection of the ammeter, it stays there (not moving) and I am afraid of leaving the battery connected and having the ammeter burn out. Can anyone tell me how it is supposed to read (operate). I am at the point where I want to put everything back where it belongs but thinking that the ammeter may burn out I have stopped.

Please any advice, new ammeter needed?

Thanks,

Carlos

 

This should help some. The red wire between points A and B carry the electrical load, the green lines are the meter circuit. You may have a marginal connection on your horn relay.

 

If Your talking about the factory ammeter guage in center cluster, leave it disconnected it is a fire hazzard. I had a buddy have a fire in his 66 Chevy II. It was traced back to the ammeter. I am almost done a frame off and left mine disconnected and it always reads "0" much safer, but just my opinion.

 

Change it to a voltmeter from a mid 80's chevvy pickup in the junkyard....bolts right in....

it give far more information than any ampmeter does....

I dunno about '68 as I think it has that external regulator with 4 posts on the firewall, similar to A bodies of that era....IF SO, that alt needs updating...the regulator plug blades are arranged [[ if they are arranged -- the alternator is a newer one of decent design...

 

Its not a real ammeter. It ain't any more dangerous than a volt meter. In reality, it is a volt meter.

 

Just curious. What other information does an voltmeter give?

 

I would just be guessing but he might have meant more useful information , more dependable information. IMO you can trust the voltmeter because its accuracy is viewed every time you turn the key to ON , the ammeter is always a mystery until you put a meter on it.

 

I personally like the ammeter better. We have hashed this one out many times before. A volt meter tells you just that, what the voltage is. An ammeter, even the hybrid type device used on the vehicle in question, gives you both an indication of the voltage and the length and depth of the charge. This allows you to monitor and determine the condition of the charging system and battery, not just the voltage. A volt meter alone cannot do this. If you start a car with a stone dead battery that will not charge and the alternator is working, it will indicate the voltage. That voltage is dictated by the setting of the voltage regulator, and will be the same whether the battery is old, new, dead or alive. But, when you start a car with an ammeter, you see the indication of the rate of charge, and the length of time it takes to bring the battery up to charge. This indicates if the battery is in good condition or if it is failing. You get a heads up, before the thing fails completely. For example, when you install a new battery in your vehicle and start it, the charge will be higher amps and will last a long time. As that battery ages and starts to fail and you start the car, the charge will get shallow and quick. Over time you can see the change in the battery's condition.

 

Well, there is a little bit of correct info on this thread, so far. The bit about the GM ammeter really being a voltmeter...actually, a milli-voltmeter. That meter reads the amount of voltage drop over a prescribed distance of the main power line in the wiring harness. The vehicle's current actually flows through the harness, not the meter. Only the voltage drop over the length of harness wire...terminated by the connecting points from the meter...goes to the meter itself. In essence, that length of wire acts as a calibrated 'shunt' (a known resistance value...very low, of course, because it is copper wire). Since the resistance of that wire is known and the voltage level is being read by the meter, E=IR so the level of current in that wire can be displayed on the meter scale--which just happens to be calibrated in "amps".

The system is simple and reasonably accurate. There is NO RISK OF FIRE BECAUSE OF THE METER ; false info on that one.

As far as how to use an ammeter: The meter will show charge or discharge whenever the ignition is turned on (engine running or not). If engine not running and lights, radio, etc are ON, the meter will show a discharge in the amount of the total amps being absorbed by the equipment that is ON. If you start the engine, the alternator will start to recharge the battery and you should see a positive charging condition. As the battery recharges, that amount of charging will continue to decrease until the battery is fully charged, as indicated by the ammeter having a reading of "0". If all is well with your system, the meter should generally stay at "0" with engine running.

Your description indicates that the meter reads full scale (exceeds 40 amps) when the engine is started. That is rather unusual...unless you have lots of electrical 'stuff' that is running at start-up [lights, radio, radiator fans, etc.] OR you have a very weak battery. With a weak battery, it will show a high charge rate at start-up but quickly get to a full-charge condition (0 amp reading). In fact, the battery likely has very low storage capacity and it is depleted and recharged very quickly. Of course, it will not crank very long before it 'peters-out' if your engine has any difficulty at all starting. Get your battery checked (using a hi-load test).

P.S. If you have ignition ON, engine OFF and low-beam headlights ON, the meter should be showing a discharge of about 10 amps. If your ammeter shows considerably higher drain than that [and you still have stock headlights], someone has done some re-wiring as related to your ammeter. Perhaps the meter feed wiring has been re-located along that main power line...I can only guess. But it shouldn't be reading that high with only the low-beam lights on.

 

My best guess, based only on your above description, is someone has miswired your car. Your data suggest the car electrical loads are essentially connected to the battery; hence you get a big discharge current when switching on the ignition (battery current drain) and a large charge current with the engine on. The posting by SIXFOOTER shows how the wiring done properly.

On your 68, all electrical loads (except battery and starter motor) are connected to the horn relay as SIXFOOTER diagram indicates. There is one wire which goes from the horn relay to a terminal on the starter. This is the wire that supplies current to charge the battery. It has a small resistance so that current through it produces a voltage drop. The ammeter, a microvoltmeter really, reads in paralles across the resistance and the resulting small voltage tells you if the batery is charging or discharging.

When you are driving at cruise RPM and all electrical loads are on; headlights, radio, air conditioning, wipers, etc..the ammeter will read 0 amps because the ammeter is producing all the power your loads need. (0 amps assumes your battery is charged after having driven it some after you last started the car). The ammeter should read only when the battery is charging or discharging. The ammeter will normally read a charge current for a little while after starting the car with the battery and read a discharge when you are idling at a stop light with a lot of electrical loads on. At idle RPM the altenator can't produce enough power, hence a discharge.

 

{There's an echo in here....}

 

Thanks to all for the information posted. I have been on travel so I have not done much with the car. One thing is certain, when I bought the car it had been rewired using a Painless harness so the horn relay is no longer where the old one used to be it is part of the fuse box. I will have the battery check. By the way I have already burnt an ammeter that showed -40A, I was checking the lights and I guess I left the draw in too long but it definetely was not good, a lot of smoke and it was gone.

 

Another condition where you will see a high charge rate (needle high on the + side) is if the voltage regulator has failed and the alternator has an abnormally high output. I have had this happen a couple of times over the years, with a relatively good battery. If the condition isn't corrected, it will ruin the battery. This is with my '69 with an internal, solid state regulator, but the symptoms would be the same with a relay type regulator.

Pete

 

Thanks for your input, I do have a question that you may be able to answer. When I bought the car it had been rewired using a Painless system and the horn relay is no longer where it used to be it is now inside the car by the new fuse box, any thoughts on how to check proper wiring to the ammeter? Thanks

 

You might need to open the panel up and follow the wires from the meter. If your meter is pegging on either way it is in indication that the normao current path is weak or open. That would be the Red #10 wire from point A to B. ALL the current for the car goes thru that wire with the exception of the starter current. The Amp meter measures the voltage drop across that red wire and displays it as amperage Charge or Discharge. If you disconnect the wires from the back of the meter, everythiong should work normal, if it doesn't then there is definately an open along that red wire somewhere.

 

If I remove the ammeter from the circuit the car will not start, as a matter of fact nothing that is electrical will work. Is that my problem??

 

Wow, the timliness of the posts on here continues to amaze me!

Ever since I have owned my car the "battery" gauge (because I don't want to get into the discussion of whether it is a voltmeter or ammeter) has worked properly. When I would start the car, it would show a small charge for a while, and slowly move to zero as I drove. When I turned off the engine in the garage with the brake lights on, it would show a small discharge until I let off the brake, when it would go to zero.

Literally yesterday, I was cruising around and noticed the battery gauged pinned to the right. When I got back to the garage and turned off the motor, it was pinned to the left when my foot was on the brake, and went to zero when I took my foot off the brake, hence looking on here today for info on my battery meter. So, if I understand the above correctly, the most likely cause of this is a bad connection at one of the two points where the green wire connects to the red wire, correct? If so, where on the car are those two connections?

Thanks,

PK

Edited to add: My car is a 1970.

 

Most people aren't going to believe you can start the car through the ammeter.

In the pic above that Sixfooter posted is the horn relay,there is an orange wire fusible link and a black wire fusible link on the left in the pic. Pull on the black wire fusible link and see if it stretches,if it does its burned in two and you will have to replace it.

 

Uh-Oh .... Hey Sixfooter I have to apologize because I think you ask me to look at the diagram a long time ago when you made it and I know I should have caught it then but I just found an error. Sorry. :o
Look at the 16 black fusible link coming from the horn relay , it doesn't tie into the 10 red going to the starter it goes to the bulkhead connector and supply's the cabin with power. Again I'm sorry I didn't catch this sooner.

 

OK, but to ask a stupid question, where is the horn relay on my car? I assume the other connection is at the alternator?