Printable Version of Topic

Posted by: championgt1 Nov 15 2010, 02:43 PM

Installed a new battery and started the car. Voltmeter gauge reads around 13 volts. Cool. Turn the headlights on, gauge reads about 12. Cool. Hit the brake pedal gauge now drops below 11. Not cool.

The owners manual says the gauge should stay between 12 and 14 with loads applied. Mine is dropping below 11.

Any ideas? Bad Ground?

Posted by: r_towle Nov 15 2010, 02:46 PM

check with a digital volt meter at the battery.
See if your test readings match.

If not, stop trusting the gauge.

Considering you removed the old battery and re-installed a new one, I would suggest that you take a close look a the positive terminal and ensure that all the small wires (one is for the gauge) are VERY clean and the funky bolt it tight...and all the connectors are perfect.

Rich

Posted by: dr914@autoatlanta.com Nov 15 2010, 03:32 PM

totally normal (as stated in the Tech Tips 700) they are on the same circuit so with the big power need for the brake bulbs it robs the power to the volt meter!!!

Posted by: championgt1 Nov 15 2010, 05:57 PM

Checked voltage at the battery with a meter. Gauge is reading different from the readings I am getting at the battery. I will check all of connections at the battery next.

Posted by: Spoke Nov 15 2010, 08:57 PM

Ohms law says it all: V = I x R

With current (I) and resistance ®, a voltage drop (V) happens. When this voltage drop happens to and from the intended load (head lights, turn signals, fuel pump, etc), your load sees less voltage than at the battery.

The wiring is resistive ®, the old, oxidized spade connectors on the fuse panel are resistive ®, every ground and power lug connection can get oxidized and act resistive ®.

You can't do much about the wiring but you can clean all the ground and power connectors. This will help minimize the voltage drops but will not eliminate them.

You can also get a layer of oxidation between the battery terminal and its connector. I had trouble starting my car, checked the battery connectors and they were tight. When the car wouldn't start, I turned the headlights on and measured about 8V drop between the battery positive terminal and its connector. Cleaned that one off and never had trouble starting after that.

Posted by: cary Nov 15 2010, 09:10 PM

Mine does the same thing. I was thinking I needed a larger alternator. George could you take a couple minutes to type or cut and paste the tech tip.

Posted by: jim_hoyland Nov 15 2010, 09:10 PM

I went through the same thing. Finally wired the battery and volt meter to a relay and used a wire from the fuse panel to turn the relay on upon starting. Now I get a direct reading from the battery.

If the relay is mounted in the engine bay, the coil wire could be used to activate the relay.

30 = Battery in
87 = Wire to voltmeter
85 = Fuse panel hot, or coil hot
86 = Ground

Posted by: jk76.914 Nov 16 2010, 04:25 AM

I did the exact same thing. Except I put the relay in the console, and used the wire that used to go to the voltmeter to activate the relay. That's a "fuse panel hot" line, as Jim described.

In either case, you have to run a new single wire from the battery to the voltmeter. It can be pretty small gauge, as there is only the tiny load of the voltmeter on it. You need the relay to be sure no current is going to the volt meter when the ignition is off, so you don't slowly drain the battery through the meter.

Posted by: cary Sep 5 2012, 08:01 PM

Finally got around rewiring the voltage gauge.
Relay in the console.
Put an inline fuse on the 12 volts from the battery.

Works like a champ.
12 volts at key on.
Running at 13.5 volts while driving.