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

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

Matt B · 09-06-2022

Very interesting read, thanks for sharing this.
Maybe the quality of the rebuild suffered? Electronic components today have a wide range of obsolescence ratings. Maybe some cheap ingrediences found their way into that rebuild?

XfireZ51 · 09-06-2022

Great read Hans. Should be part of the registry stickies.

Jagdpanzer · 09-06-2022

Provided the alternator is functioning properly the battery is key for keeping the alternator output ripple within acceptable limits.

Sent from my iPhone using ZR-1 Net Registry

Ccmano · 09-06-2022

Quote:

Originally Posted by Jagdpanzer

Provided the alternator is functioning properly the battery is key for keeping the alternator output ripple within acceptable limits.

Sent from my iPhone using ZR-1 Net Registry

Good point, the battery acts as a buffer absorbing the normal small amount of Ripple voltage that is produced. A good rectifier still allows a small amount through. However as Ripple voltage increases the battery will be the first element adversely affected.
H

Ccmano · 09-06-2022

I should add, I took the opportunity to install a 200 amp alternator, upgraded the wiring to the battery and to the blower motor per Mark Haibecks excellent write up about improving Air Conditioning efficiency. Thanks as always, Mark!
http://www.zr1specialist.com/HAT%20W...the%20ZR-1.pdf
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.

09-06-2022	#2
Matt B Join Date: Jan 2021 Location: Beautiful Bavaria Posts: 518	Re: AC Ripple Voltage and why you should be concerned Very interesting read, thanks for sharing this. Maybe the quality of the rebuild suffered? Electronic components today have a wide range of obsolescence ratings. Maybe some cheap ingrediences found their way into that rebuild? __________________ ZR-1 #638/1991

09-06-2022	#3
XfireZ51 Join Date: May 2007 Location: Chicagoland, IL Posts: 9,667	Re: AC Ripple Voltage and why you should be concerned Great read Hans. Should be part of the registry stickies. __________________ [SIGPIC][/SIGPIC]Former Membership Chairman Former ZR-1 Registry - BOD 1972 Corvette 4speed base Coupe SOLD long time ago 1984 Corvette Z-51/4+3 SOLD 1992 Corvette ZR-1 Aqua/Gray #474 SOLD 1992 Corvette ZR-1 Black Rose/Cognac #458 2014 Honda VFR Interceptor DX

09-06-2022	#4
Jagdpanzer Join Date: Apr 2007 Location: Northern VA Posts: 2,644	Re: AC Ripple Voltage and why you should be concerned Provided the alternator is functioning properly the battery is key for keeping the alternator output ripple within acceptable limits. Sent from my iPhone using ZR-1 Net Registry __________________ Phil Wasinger 1994 Torch Red ZR-1 WAZOO Member George Braml Intake

09-06-2022	#6
Ccmano Join Date: Jun 2006 Location: Sparks, NV Posts: 2,852	Re: AC Ripple Voltage and why you should be concerned I should add, I took the opportunity to install a 200 amp alternator, upgraded the wiring to the battery and to the blower motor per Mark Haibecks excellent write up about improving Air Conditioning efficiency. Thanks as always, Mark! http://www.zr1specialist.com/HAT%20W...the%20ZR-1.pdf 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.