How do I interpret the amp meter

[RebekahsZ]

My 240z has a factory amp meter on the dash.  What does it physically mean when the needle is reading to the left of center?  On center?  Right of center?

 

I've been having some intermittent trouble with starting the car when it is hot at the track.  Wondering if it could be low voltage/amperage.  But, I've never known how to interpret that gage and I'd love to learn regardless of my starting trouble.  Any advice appreciated.  Everything I know, I learned either thru hard knocks or hybridz.

[ktm]

If it is reading towards to + side, then you are charging more than you are pulling; in other words, you are charging your battery.  If it is reading to the - side then you are pulling more than you are charging and draining your battery.

[RebekahsZ]

So if it is on the - side, does that indicate a weak alternator? My alternator is a recent rebuild on an alternator of unknown age. My starter is a factory unit with 20,000 miles and 7 years.

[Domzs]

I highly rec you to switch to a voltmeter . Are you running longtube headers ? If so , a high cranking amp battery could solve your hot crank problem . My 82 GT did the same , heat soak from longtube headers . If you have a pipe that runs too close the starter , it'll do the same .

[duragg]

Amp gauge is = to flow into or out of battery (comparing Alt to System)

Google electricity and water analogy for further.

Plus side... juice flowing into and raising system.

 

What exact hot start symptoms?

 

You really want to be able to see both volts and amps if possible.

During engine cranking you are pure "draw" since the alternator is not spinning fast enough to operate or contribute.

Can help to see batt volts during cranking to see what starter is up to.

 

Electricity discussion causes engineers to wake from the dead and my butchery will be noted. But those are the basics

Edited September 3, 2013 by duragg

[duragg]

So if it is on the - side, does that indicate a weak alternator? My alternator is a recent rebuild on an alternator of unknown age. My starter is a factory unit with 20,000 miles and 7 years.

Once you start motor and depending on RPM it should go nicely to + side showing refill.

Really need to reference voltage also.

If you have like 10.8V and a discharge somethings broke.

 

What is your resting engine off volts?

[z240]

Only thing the ammeter indicates is this.  If it swings right of center, current is going through ammeter one way. if it swings left, current is going through the other way. How much it deflects measures how much current is going through. That's it.

 

Now, if the ammeter is in the circuit where its supposed to be, ie between the alternator output and the +terminal of the battery, and hooked up the right way, then swing right means current is flowing into the battery. The opposite is also true, swing left, then current is flowing OUT of the battery.

 

For the current to flow into the battery, the voltage (the thing that pushes the electrical current) must be higher at the alternator output than at the battery + terminal (measured to ground).  The whole uphill/downhill thing. Voltage is height, current is the amount of water flowing down the hill.

 

Simpliest thing ever to measure if your alternator is working. Turn car off. Measure voltage across battery terminals. Note voltage. Start car. Do voltage test again. If voltage is higher, then alternator is "working" and battery is taking current.  Ammeter should be on right (+) side of center indicating "Charging".  Opposite is also true.  Can also check voltage at alternator output terminal (other lead to ground) compared to battery + term in both run/no run. Should be same as at battery, if not there is a problem with the connections between here and there.

 

Exact voltage measurements run vs. no run determine state of battery and charging system.  Common problems include voltage drops across bad connections, all along the wires involved, from battery cables, fusible links, ground connections to the frame and alternator connections. Like anything else, without clean connections at all points, finding problems becomes very difficult and misleading.

Edited September 3, 2013 by z240

[RebekahsZ]

So it is kind of a stupid worthless gauge with no practical use whatsoever?

[NewZed]

With a voltmeter, you would have exactly the same confusion on why your engine doesn't want to start, assuming an understanding of how both meters work.

 

The advantage of the voltmeter is it tells you if the alternator is putting out too much voltage.  Otherwise, the two meters show the same thing if you know how to interpret them.

[Domzs]

So it is kind of a stupid worthless gauge with no practical use whatsoever?

You need to replace it with a boost gauge then install a turbo setup so you can see boost psi instead of amp.  . And it'll make the car goes faster .

[z240]

So it is kind of a stupid worthless gauge with no practical use whatsoever?

 

Not so much useless, think of it as only a go/no-go device.  If its on the right, be happy and keep driving. If its on the left, go straight home. 

[beermanpete]

The ammeter shows the battery charge/discharge rate. All current into and out of the battery (except for the starter motor) flows through the ammeter. When the car is not running and you turn on the lights or other load you will see the ammeter swing to the discharge side (left). When you start the engine and the alternator starts charging you will see it swing to the charge side (right) and then taper down to zero after a short time (a minute or two). When you are driving the meter will sit on zero most of the time. When the engine is at idle (stopped at a traffic light) the alternator does not have as much output and might not "keep up" with the load (worst case: headlights on, heater on, wipers on, in the rain at night). This will cause the battery to become the current source (discharge) and the ammeter will swing to the left. When you pull away for the light and the alternator output increases the meter will swing to the right momentarily while the battery is recharging. The meter will then taper back to zero again as you drive.

 

If the ammeter is ALWAYS to the left or the right of zero a problem is indicated. Always to the left suggests a weak or failed alternator or regulator. Always to the right suggests a failed regulator that is over-charging.

 

In your case, it is possible the car is fine but you are not running the engine enough between start-ups to fully recharge the battery. Do you have the same problem driving on the street (assuming you do)?

[Miles]

My stock SBC starter used to heat soak and would not turn the engine over.  I replaced the stock starter with a Hitachi gear reduction starter and never had a problem with starter heat soak again.

 

Adding a voltage gauge can be  accomplished with  a unit that plugs into the cigarett lighter. The plug-in volt meter I have  is as accurate as a reading with a volt meter at the battery terminal.

 

Having both gauges can alert you to developing problems with the battery and charging system.

[RebekahsZ]

I love this site. Every post helpful. Just picked up rebuilt starter and will wrap head pipe on that side with lots of header wrap. Starter is already wrapped but only thinly so. Played with headlights on drive home tonight. Amp meter is centered with lights off. Turn on headlights and meter goes negative (to left) by 1/4 inch and stays there at any rpm.

[beermanpete]

Unless you have high-wattage headlamps your alternator is weak or the belt is slipping (worn, loose, oily, etc.) Either way some work on the chargin system seems to be in order.

[RebekahsZ]

Nope, stock, weak ass headlamps that are about as bright as a firefly in a fog. Thanks for the advice, will get it bench tested as well as the battery.

[RebekahsZ]

Replaced starter, wrapped starter, wrapped head pipe that passes under it. Will see how it does Saturday. Thanks everybody.

[ktm]

The stock headlights are not weak, it's the wiring out to the lamps that are the culprit.  Install one of MSA's headlight relay kits (developed by Z'sondabrains or Dave) and you'll notice a significant improvement.

[Tony D]

Bo, the stock headlamps ARE weak!

 

40/50W as I recall. I had city lights in my H4 housings that were 30W on relys and they were as bright as stock Lo-Beam Koitos that came with the cars. Try to find an original wattage sealed-beam headlamp! Even now most "sealed beams" are housed halogen capsules of higher wattage...those stockers were WEAK WEAK WEAK!!!

 

Universally people upgrade to today's standard 55/65W, which REALLY benefit from relays. And if you're like me running 55/100's or 80/120's on the 'night driver' relays become mandatory.

Edited September 7, 2013 by Tony D

[cgsheen]

Back in the olden days those sealed beams weren't competing with lighted freeways, street lamps everywhere, "city light" that drowns out the stars, & traffic, traffic, traffic all day and night. Out on the road they were adequate and it was a time when you could actually run on brights for most of your night journey - only going to dim when the occasional oncoming car approached... Completely different era.  Even with little 3W gauge lamps, when you got out on the road, away from town, the dash was too damn bright!  You needed that dimmer.  That hasn't been the case for any number of years though.