volt meter reading 18 volts ? - JeepForum.com
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post #1 of 4 Old 01-17-2010, 09:06 AM Thread Starter
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1978 CJ7 
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volt meter reading 18 volts ?

I just installed a new autometer volt meter in my CJ. When I turn on the switch the meter is reading 18 volts. Im also having trouble with either the brain box or the coil burning up I dont know witch one it is. I tested the battery and its reading 12 volts. Any ideas

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post #2 of 4 Old 01-17-2010, 01:16 PM
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1976 CJ5 
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Where is your voltmeter sensing voltage from? It should be at a source that is powered up by the ignition switch. If you are indeed getting 18 volts than that pretty much means your voltage regulator is full fielding the alternator which means overcharging the battery. What you do to check that is start the jeep and check voltage at the output of the alternator that charges the battery if it is 18 volts than you more than likely need a new regulator. And I would not drive the jeep if it were mine and putting out 18 volts your battery will be toast in no time which would also explain why your electronics are going out is too much volts.
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post #3 of 4 Old 01-17-2010, 01:31 PM
Mike Romain
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Bad meter is my first thought. It can't magically create 6 volts with just the switch on.

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post #4 of 4 Old 01-17-2010, 02:43 PM
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Originally Posted by RangerOne View Post
I just installed a new autometer volt meter in my CJ.

When I turn on the switch the meter is reading 18 volts.
When you mean you turn the ignition switch 'ON'...
Is that with the engine running, or NOT running?

If your volt meter jumps to 18 volts when the engine IS NOT running, but the switch is in the 'RUN' position,
Then you have a bad gauge.

If the volts are around 12 when the engine is 'Off' but the switch is in the 'RUN' position,
THEN it jumps up to around 18 volts when you start the engine,
That means the alternator is shot and the voltage regulator isn't doing it's job.

Im also having trouble with either the brain box or the coil burning up I dont know witch one it is.
Which is it?

Either the coil is what needs replaced, the two aren't interchangeable,

And when you say 'Brain Box',
Do you mean the ignition module or the timing computer?

Usually the only way to kill a coil is to leave the ignition switch in the 'Run' position too long without engine running,
Or to NOT have sufficient 'Ground' to the engine heads.

Lack of 'Ground' to the module will make it act strange and eventually burn the module up.

I tested the battery and its reading 12 volts. Any ideas
Start with taking a battery voltage reading after the vehicle sits a couple of days.
Make sure you note down EXACTLY what the voltage was,
Then start the engine, let it run 10 minutes or so,
Then take the voltage at the battery again.

If your battery voltage starts below 12.2 volts, you have a problem with the battery.

If the voltage gets above 14.7 volts, you have a problem with the alternator/voltage regulator.

With the engine turned 'Off',
Take a RESISTANCE reading between the alternator case and the battery negative terminal.
If you show any resistance more than an Ohm or two,
You need to install a dedicated ground wire between battery negative terminal and alternator case.

Start the engine, and do the same resistance test between alternator and negative battery terminal.
If your resistance goes up, then you need a larger, more direct connection between alternator case and battery terminal...
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