ZR-1 Net Registry Forums - View Single Post - AC Ripple Voltage and why you should be concerned

Ccmano · 09-05-2022

I recently changed out my differential for a 4.10 and as expected my speedometer was off by about 5mph at 60mph. I had done this on a previous car and solved the issue with the installation of a Dakota Digital SG-5C electronic speedo adjustment box. The installation is straight forward splicing it into the yellow wire from the VSS and providing power and ground. After completing this on the current car I found that my speedo would intermittently register varying speeds while standing still. I contacted Dakota Digital and was told I likely had a ground loop issue. I won't bore you with the week long diagnostics. Suffice to say I determined that the issue was caused by something called AC ripple voltage generated by a defective alternator. I also discovered that the condition had shorted out my battery.

I'm sure I was taught about AC ripple current back in the day but I had completely forgotten about it till now. A car alternator produces AC voltage which is changed into DC voltage by the rectifier in the alternator via a set of diodes. As we know a car functions on 12volt DC current. The rectifier makes sure that 99.99% of the AC voltage is transformed into DC voltage. When the rectifier starts to fail it allows increasing amounts of AC current to enter the vehicles electrical system. This is a very bad thing. AC current can quickly damage the electronic components of the system. As we know some of these components on our cars get quite pricy should they fail. Add to that the age of our first gen electronics and it's a recipe for disaster.

Had it not been for the speedo issue I would have had no idea it was happening until something more major would have failed. Battery DC voltages are normal even under alternator load. My battery would have been the next to go as it finally did during testing.

Unfortunately testing for AC ripple voltage is tricky. It requires one of 3 methods. First, using an AC volt meter while the engine is running, probe the positive and negative terminals of the battery. You should see an AC voltage no higher than 0.05 ac volts. (50 mVolts, some say up to 100 mVolts). The problem here is that most modern digital volt meters don't have DC voltage blocking (filtering) on the AC voltage setting. Remember your hooking an AC voltage meter to a DC system. Without this blocking you will see a voltage of 27 to 30 volts on your AC volt meter. Completely useless for this AC voltage reading. Finding a DVM with DC blocking is not easy.

The second way to diagnose this is to hook the charging system to a scanner that provides oscilloscope functionality. The voltage wave pattern can then be analyzed. Ripple voltage gets its name from this oscilloscope wave pattern with "ripples" at the peak of each wave. Again, unfortunately, most of us do not have access to an oscilloscope.

Lastly, in my case I had an idea that ripple current might be the case through a process of elimination. I removed the alternator, not easily done on our cars, and took it to my local Autozone. They put it on their test machine where it promptly failed due to.... AC ripple current.

I should add, alternator diodes can be tested with an ohm meter. However the alternator needs to be removed to do this. You may find that (like with mine) the diodes ohm out good but the alternator still failed the ripple voltage test.

Sorry for the long dissertation. So... my point here is, if you have your plenum off your car for any reason and the alternator is original to the car (~30yrs old) take the time to have your alternator tested. It may save you a burned out ECM, DIS module or some other electrical gremlin. I should add that my alternator was only a 5 year old rebuild. Makes you wonder what is actually rebuilt in a "rebuilt" alternator.
H

09-05-2022	#1
Ccmano Join Date: Jun 2006 Location: Sparks, NV Posts: 2,852	AC Ripple Voltage and why you should be concerned I recently changed out my differential for a 4.10 and as expected my speedometer was off by about 5mph at 60mph. I had done this on a previous car and solved the issue with the installation of a Dakota Digital SG-5C electronic speedo adjustment box. The installation is straight forward splicing it into the yellow wire from the VSS and providing power and ground. After completing this on the current car I found that my speedo would intermittently register varying speeds while standing still. I contacted Dakota Digital and was told I likely had a ground loop issue. I won't bore you with the week long diagnostics. Suffice to say I determined that the issue was caused by something called AC ripple voltage generated by a defective alternator. I also discovered that the condition had shorted out my battery. I'm sure I was taught about AC ripple current back in the day but I had completely forgotten about it till now. A car alternator produces AC voltage which is changed into DC voltage by the rectifier in the alternator via a set of diodes. As we know a car functions on 12volt DC current. The rectifier makes sure that 99.99% of the AC voltage is transformed into DC voltage. When the rectifier starts to fail it allows increasing amounts of AC current to enter the vehicles electrical system. This is a very bad thing. AC current can quickly damage the electronic components of the system. As we know some of these components on our cars get quite pricy should they fail. Add to that the age of our first gen electronics and it's a recipe for disaster. Had it not been for the speedo issue I would have had no idea it was happening until something more major would have failed. Battery DC voltages are normal even under alternator load. My battery would have been the next to go as it finally did during testing. Unfortunately testing for AC ripple voltage is tricky. It requires one of 3 methods. First, using an AC volt meter while the engine is running, probe the positive and negative terminals of the battery. You should see an AC voltage no higher than 0.05 ac volts. (50 mVolts, some say up to 100 mVolts). The problem here is that most modern digital volt meters don't have DC voltage blocking (filtering) on the AC voltage setting. Remember your hooking an AC voltage meter to a DC system. Without this blocking you will see a voltage of 27 to 30 volts on your AC volt meter. Completely useless for this AC voltage reading. Finding a DVM with DC blocking is not easy. The second way to diagnose this is to hook the charging system to a scanner that provides oscilloscope functionality. The voltage wave pattern can then be analyzed. Ripple voltage gets its name from this oscilloscope wave pattern with "ripples" at the peak of each wave. Again, unfortunately, most of us do not have access to an oscilloscope. Lastly, in my case I had an idea that ripple current might be the case through a process of elimination. I removed the alternator, not easily done on our cars, and took it to my local Autozone. They put it on their test machine where it promptly failed due to.... AC ripple current. I should add, alternator diodes can be tested with an ohm meter. However the alternator needs to be removed to do this. You may find that (like with mine) the diodes ohm out good but the alternator still failed the ripple voltage test. Sorry for the long dissertation. So... my point here is, if you have your plenum off your car for any reason and the alternator is original to the car (~30yrs old) take the time to have your alternator tested. It may save you a burned out ECM, DIS module or some other electrical gremlin. I should add that my alternator was only a 5 year old rebuild. Makes you wonder what is actually rebuilt in a "rebuilt" alternator. H __________________ 90'ZR-1 #1461 Red/Black Ported Intake, Degreed Cams, OBX Headers & Exhaust, Fidanza FW, Secondary Delete, Custom tune, C6 GS Wheels. Sold 08 C6 Z51 Crystal Red Metallic Sold Vararam Intake, Tune, Sold 05 C6 Z51 Red/Black Sold 90'ZR-1 #1723 Black/Gray Sold but not forgotten 91' Z51 L98 White Vert. My First, you always remember your first. Sold.