Audio Systems: Head Unit only turn on sometimes when Car ignites, honda civic 1992, digital voltmeter

Question
hi, i intend to get a voltmeter but can u give me some instructions how to use it? as there are many selections
of things im not too sure of. thanks.
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The text above is a follow-up to ...

-----Question-----
Hi, my ride is a Honda Civic 1992. Lately after changing the head unit to one that plays ipod, its not turning on whenever the car ignites. Sometimes it does, sometimes no. But when it doesnt, it may turn on after a while sometimes not. I have ensured the cables behind the head unit when assembling was secure. So im not too sure can be the problem of this turning on and off. Seems like power loss to me some times. How can i check it? Thanks
-----Answer-----
Could be many things, most likely a loose ground connection. You could add a ground wire from the metal dash support to the case of the radio to make sure. You should get a cheap digital voltmeter to test what voltage is reaching your radio on the accessory power wire also.

Answer
Radio Shack usually has inexpensive voltmeters for sale, just make sure to get reasonably long leads, or some alligator clip test leads to use. They have little metal "alligator mouths" on each end of a wire to aid keeping your wires hooked up while testing. You can usually buy a whole bag, with different color test leads, for $5.00. Read the owner's manaul that comes with your digital multimeter, it will tell you what buttons or switch settings to use to measure voltage, resistance, current, etc. You really just need DC voltage to start with, so make sure you get more than 12 volts DC on the red and yellow poer wires when the key in on. If that's OK, then make sure you get a very low resistance measurement from the case of the radio to the metal dash of the car, like less than 0.5 ohms. Then get back to me with your results.