Correct Way to Measure AC Voltage

[BarryG]

As the title says what is the correct way to measure AC voltage on a 2001 CTD. The reason I am asking is I have been having some weird electrical problems and have checked just about everything. I have read on here somewhere that some of the issues can be due to excessive AC voltage from bad/leaking diodes in the alternator. I realize that there will probably be some ac voltage present but I would think it would very minimal. However I cant seem to find the post that told how and where to measure the ac voltage at. Not sure the battery is the place to test it maybe the alternator, if so how. Have a multimeter just not sure where to hook up to for this test.



Any help would be appreciated

 

[JLandry]

Barry,



The best way to detect AC voltage on a DC line is to use an oscilloscope. That way you can visually see the AC component intensity and characteristics. Now if you don't have an oscilloscope, another less-desirable method is to use an ordinary multimeter set to measure AC volts. You may have to connect a capacitor in series with the positive meter lead to block out the DC voltage component so that only the AC voltage component gets to the multimeter. Some multimeters do this automatically when set to AC volts, but the el-cheapo models probably don't. The capacitor blocks the DC voltage component but will pass through the AC voltage component so the multimeter reading will be the amount of AC voltage on the line.



Note that some small amount of AC voltage is normal on most alternators, so ideally you'd want to compare your readings against another truck or two. When the AC component gets too strong is when the problems occur. Excessive AC voltage is caused by failing diodes in the alternator.



Good luck,



John L.

 

[Bubba7492]

Barry,



The best way to detect AC voltage on a DC line is to use an oscilloscope. That way you can visually see the AC component intensity and characteristics. Now if you don't have an oscilloscope, another less-desirable method is to use an ordinary multimeter set to measure AC volts. You may have to connect a capacitor in series with the positive meter lead to block out the DC voltage component so that only the AC voltage component gets to the multimeter. Some multimeters do this automatically when set to AC volts, but the el-cheapo models probably don't. The capacitor blocks the DC voltage component but will pass through the AC voltage component so the multimeter reading will be the amount of AC voltage on the line.



Note that some small amount of AC voltage is normal on most alternators, so ideally you'd want to compare your readings against another truck or two. When the AC component gets too strong is when the problems occur. Excessive AC voltage is caused by failing diodes in the alternator.



Good luck,



John L.

A Motorola Radio repairman once told me the AC meter will read Zero volts. Any bad diodes etc. will show AC voltage. He proved this to me while troubleshooting alternator whine in Pa speaker on police car.

 

[CKelley1]

each brand of meter will read differently, that is why he said oscilliscope but the way to poor boy it is to get a meter and check the amount of ac volts you see on another dodge and compare with yours

 

[Bubba7492]

each brand of meter will read differently, that is why he said oscilliscope but the way to poor boy it is to get a meter and check the amount of ac volts you see on another dodge and compare with yours

A lot of better meters will have frequency , witch will do similar.

 

[CKelley1]

bubba the frequency does not help and most of the time read erratic. the meters that read true rms will read the same no matter the brand. but for the average person with a cheap meter to trouble shoot the problem just use the meter you have and compare, the leaky diode will cause a large increase in ac over a normal truck.

 

[Wingate]

Fluke 87-V will do everything a do it yourselfer will ever need.

 

[Bubba7492]

bubba the frequency does not help and most of the time read erratic. the meters that read true rms will read the same no matter the brand. but for the average person with a cheap meter to trouble shoot the problem just use the meter you have and compare, the leaky diode will cause a large increase in ac over a normal truck.

Correct me, would not the frequency be zero. ? I'm a resident field mechanic for the Caterpillar dealer in Pa. so I hope my skills are above average. We have kicked around putting scopes in our trucks , however cost has stopped it . The meters we use are excellent quality Cat labeled and Flukes. Our watches are mostly PWM's, duty cycles, and frequency.