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Alternator volts

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Old 07-11-2018, 01:13 PM
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Default Alternator volts

I have a 2008 mercruiser 5.7 L carb I/O engine that is showing on the volt gauge between 13 and 14 volts at 1500 RPM but then down to between 12 and 13 volts when running at 3500 RPM. I just had the alternator replaced because it wasn't charging my battery. Does this make any sense for the volts to drop at a higher RPM ?
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Old 07-11-2018, 02:48 PM
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Originally Posted by RJJ1 View Post
I have a 2008 mercruiser 5.7 L carb I/O engine that is showing on the volt gauge between 13 and 14 volts at 1500 RPM but then down to between 12 and 13 volts when running at 3500 RPM. I just had the alternator replaced because it wasn't charging my battery. Does this make any sense for the volts to drop at a higher RPM ?
Maybe. Voltage is controlled by the voltage regulator, not by RPM. Have you checked voltage at the battery with a quality digital multi meter vs just looking at the gauge? Gauges fail all the time.
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Old 07-11-2018, 03:18 PM
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Doiuble check the voltage at the battery with a meter. No it doesn't make sense so something isn't working, reading right, or you have some funky stuff happening. Another possibility is that the drive belt to the alternator is slipping so check that as well.
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Old 07-11-2018, 03:26 PM
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I have the same motors...my helm gauges read 13 volts at idle and drop to 12 at cruise speeds.

when I measure at the alternator and the battery voltage directly I get correct voltage so I know my gauges are off (or perhaps corrosion on my terminal/harness somewhere.)

engine speed should primarily effect amperage output rather than voltage output...a typical 70 amp alternator only puts out 8 or 10 amp at idle and full output at cruise speeds but should always put out 13.5 to 14 volts
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Old 07-11-2018, 04:16 PM
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Originally Posted by Aliboy View Post
Doiuble check the voltage at the battery with a meter. No it doesn't make sense so something isn't working, reading right, or you have some funky stuff happening. Another possibility is that the drive belt to the alternator is slipping so check that as well.
likely a GM based alternator. Possibly nothing wrong. Most, not all, alternators sit at a higher voltage at higher ROM. GM stuff is all over the place, even when working correctly.
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Old 07-11-2018, 04:18 PM
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Originally Posted by outobie View Post
I have the same motors...my helm gauges read 13 volts at idle and drop to 12 at cruise speeds.

when I measure at the alternator and the battery voltage directly I get correct voltage so I know my gauges are off (or perhaps corrosion on my terminal/harness somewhere.)

engine speed should primarily effect amperage output rather than voltage output...a typical 70 amp alternator only puts out 8 or 10 amp at idle and full output at cruise speeds but should always put out 13.5 to 14 volts
sort of. It is capable of putting out higher amperage at higher RPM. it only puts out what is necessary. Most GM alternators have a theoritical set point of 13.8, but based on the regulation style can run well below that (12.5 - 13.0 volt range) and it be perfectly normal.
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Old 07-11-2018, 04:39 PM
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At 12v it is dragging the 12.8v battery down not charging? Hard to imagine that the alternator is actually designed to do that so I would suspect more that the low reading is due to where the voltage is being measured in the harness than that the alternator is designed like that. Meter across the battery will confirm what is really happening.
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Old 07-11-2018, 05:21 PM
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Originally Posted by outobie View Post
I have the same motors...my helm gauges read 13 volts at idle and drop to 12 at cruise speeds.

when I measure at the alternator and the battery voltage directly I get correct voltage so I know my gauges are off (or perhaps corrosion on my terminal/harness somewhere.)

engine speed should primarily effect amperage output rather than voltage output...a typical 70 amp alternator only puts out 8 or 10 amp at idle and full output at cruise speeds but should always put out 13.5 to 14 volts
Gauges are known to be less than accurate.
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Old 07-11-2018, 07:01 PM
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Originally Posted by Aliboy View Post
At 12v it is dragging the 12.8v battery down not charging? Hard to imagine that the alternator is actually designed to do that so I would suspect more that the low reading is due to where the voltage is being measured in the harness than that the alternator is designed like that. Meter across the battery will confirm what is really happening.
it wont be pulling it down if it is working, even if reading a lower than expected voltage. Testing voltage for the battery is generally about 12.7. Not uncommon with GM stuff for them to be putting out great amps at realatively low voltage compared to other alternator makes. Just the nature of the beast. Also not uncommon for them to appear completely normal. As mentioned several times, the ONLY way to know what’s going on is to put a meter directly to the battery. Not a cigarette lighter gauge. Not a meter elsewhere in the system. Use a good meter, at the battery.

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Old 07-11-2018, 07:16 PM
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Originally Posted by autobaun70 View Post
it wont be pulling it down if it is working, even if reading a lower than expected voltage. Testing voltage for the battery is generally about 12.7. Not uncommon with GM stuff for them to be putting out great amps at realatively low voltage compared to other alternator makes. Just the nature of the beast. Also not uncommon for them to appear completely normal. As mentioned several times, the ONLY way to know what’s going on is to put a meter directly to the battery. Not a cigarette lighter gauge. Not a meter elsewhere in the system. Use a good meter, at the battery.
Don't want to pick a fight about it but you would have to explain that one to me. You put amps into a battery by raising the voltage difference between the battery voltage and the alternator voltage. That is why the standard alternator specs usually show around 14.4v for bulk charge and around 13.2v for float charge. If the alternator voltage is lower or equal to the battery voltage you are not putting amps into the battery. That would also mean that your battery was supporting the engine needs without getting charge until the battery dropped to the 12.0v alternator level which is actually an almost fully flat battery voltage level. .Take the battery out of the system (alternator wouldn't work unless self exciting somehow) and yes you could power pumps, engine etc at 12.0v and produce amps for that, but once you have a battery hard connected to the alternator you are 12.7/12.8v to maintain charge or over 13 to get any charge at all into the battery (unless it is already flat). From all the comments on this I am assuming that the standard harness takes the feed to the voltmeter off the end of a wire with a load on it so it is reading lower than what the battery is really getting. Hopefully the OP will put a meter on his battery terminals and tell us what he gets.
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Old 07-11-2018, 07:58 PM
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Originally Posted by Aliboy View Post
Don't want to pick a fight about it but you would have to explain that one to me. You put amps into a battery by raising the voltage difference between the battery voltage and the alternator voltage. That is why the standard alternator specs usually show around 14.4v for bulk charge and around 13.2v for float charge. If the alternator voltage is lower or equal to the battery voltage you are not putting amps into the battery. That would also mean that your battery was supporting the engine needs without getting charge until the battery dropped to the 12.0v alternator level which is actually an almost fully flat battery voltage level. .Take the battery out of the system (alternator wouldn't work unless self exciting somehow) and yes you could power pumps, engine etc at 12.0v and produce amps for that, but once you have a battery hard connected to the alternator you are 12.7/12.8v to maintain charge or over 13 to get any charge at all into the battery (unless it is already flat). From all the comments on this I am assuming that the standard harness takes the feed to the voltmeter off the end of a wire with a load on it so it is reading lower than what the battery is really getting. Hopefully the OP will put a meter on his battery terminals and tell us what he gets.
The main thing to remember is that on a running vehicle, the regulator is seeing an active electrical system under load, not just a battery at rest. It’s not really a comparable scenario to charging a battery. In essence after startup, or after any big surge load, if you are watching a meter it will look like a battery on a charger. The rest of the time it is going up and down based on the needs of the system.

The voltage set point on an alternator is the voltage at which it quits putting out. Basically drops back to idle output of around 10-20 amps at that level. Any vehicle electrical system will be easily using that much, so they never really cut completely out. For instance, if the setpoint is 13.8, which is the common setpoint for GM units, if the regulator sees any voltage below that, it will increasingly bump up output amperage. The lower the voltage drops, the higher the amperage gets, to the design limit of the unit. It is much easier to demonstrate on a test bench than to explain, but that’s the basics. Different families of units use different regulation methods. Some act differently than others, and have bigger voltage swings. GM’s stuff (Delco/Remy) tend to have a bigger swing than others. BTW, alternators and starters are my day job. Family has been in the business since 1989.

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Old 07-11-2018, 08:12 PM
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I follow that OK thanks. How does a system with a 12.7v battery stay charged if the voltage is only 12v at cruise rpms as the OP is seeing? That's the bit I can't work out as everything I know tells me that the battery is discharging if it is at 12.7 and the circuit voltage across the loads is 12. I assume I am missing something obvious, but can't see what.
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Old 07-11-2018, 08:32 PM
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Originally Posted by Aliboy View Post
I follow that OK thanks. How does a system with a 12.7v battery stay charged if the voltage is only 12v at cruise rpms as the OP is seeing? That's the bit I can't work out as everything I know tells me that the battery is discharging if it is at 12.7 and the circuit voltage across the loads is 12. I assume I am missing something obvious, but can't see what.
batteries are nominal 12V. Close works. A system can run slightly below nominal and stay there for a long period of time. Essentially the battery is just sitting there at and the alternator is providing the needs of the system. Heat buildup can become problematic over time, but electrically all is well.
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Old 07-12-2018, 06:25 AM
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Originally Posted by autobaun70 View Post


batteries are nominal 12V. Close works. A system can run slightly below nominal and stay there for a long period of time. Essentially the battery is just sitting there at and the alternator is providing the needs of the system. Heat buildup can become problematic over time, but electrically all is well.
Is there a voltage sensitive function in the simple integral regulators like there is in modern vehicles which measure "battery temperature" ?
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Old 07-12-2018, 06:31 AM
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Originally Posted by billinstuart View Post
Is there a voltage sensitive function in the simple integral regulators like there is in modern vehicles which measure "battery temperature" ?
In some there are. Not specifically looking at temp, but modulating in a more complex manner than traditional units. When I say heat buildup can be an issue, specifically talking about the Alternator. Regulation is all over the place these days. Chrysler hasn't run internal regulation since 1987 (with a few exceptions). Ford is for the most part tying theirs back to the ECM for something or another. We don't run much Ford product, so haven't looked too deep into it. I do know that you have to have a special converter module to test them or you will smoke the regulator.

I laugh every time a walk in comes in and says the guy at the parts store reported they had a bad regulator......on a Chrysler product. I would never, under any circumstance allow a parts store bench test an alternator. They simply don't know what they are doing, and are bound to screw it up in the process. Of course that may be the point.
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Old 07-12-2018, 06:44 AM
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Originally Posted by autobaun70 View Post
Of course that may be the point.
That is the point! The machine says the part is bad. Kaching$$! Like all the fancy readouts and print-outs on alignment equipment. Sell sell sell. (keeps you in business!)

I'd bet at least 50-75% of parts sold are unnecessary.
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Old 07-12-2018, 06:53 AM
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Originally Posted by billinstuart View Post
That is the point! The machine says the part is bad. Kaching$$! Like all the fancy readouts and print-outs on alignment equipment. Sell sell sell. (keeps you in business!)

I'd bet at least 50-75% of parts sold are unnecessary.
We see that in our warranty analysis. Our customers of course aren't capable of analyzing failures, so that falls on us. Roughly 75% of parts that come back are either well beyond the warranty period, or are good, just have greasy finger prints on them. They got installed, were not the issue, so they store just turns it in as a warranty. I fully understand that you can't sell a dirty unit, but explaining this scenario to every fresh out of school "quality manager" that walks in our door from a customer gets old. By the time they grasp the concept, they move on and get replaced by another kid that is wet behind the ears.
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Old 07-12-2018, 07:02 AM
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Originally Posted by autobaun70 View Post
We see that in our warranty analysis. Our customers of course aren't capable of analyzing failures, so that falls on us. Roughly 75% of parts that come back are either well beyond the warranty period, or are good, just have greasy finger prints on them. They got installed, were not the issue, so they store just turns it in as a warranty. I fully understand that you can't sell a dirty unit, but explaining this scenario to every fresh out of school "quality manager" that walks in our door from a customer gets old. By the time they grasp the concept, they move on and get replaced by another kid that is wet behind the ears.
It's called "throw parts at it until something sticks". Look at the replies on here..some desk jockey had a neighbor with a "similar" problem, and did such-and-such. No one (very few) understands the process of HOW stuff works. OBDII is a great example..most people think it tells the tech the problem.."o2 sensor reading wrong" must be the sensor...throw a sensor at it. No, it is PROBABLY telling you something is going on in the engine to create the problem. That's where further diagnosis is required...the HARD part.
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Old 07-12-2018, 07:08 AM
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Speaking of auto parts stores... my Ford Focus needed an alternator. The clerk threw the box on the counter for about $160. I know I have to undo a motor mount to get it in the car and only wanted to do it once so I told him to put it on the machine to test it. It failed. Two days later they had the replacement in. I had them put the replacement on the test machine -- it wouldn't even spin up. Went to a local starter/alternator rebuiler and got a good one for $85.
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Old 07-12-2018, 08:07 AM
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Originally Posted by jeffnick View Post
Speaking of auto parts stores... my Ford Focus needed an alternator. The clerk threw the box on the counter for about $160. I know I have to undo a motor mount to get it in the car and only wanted to do it once so I told him to put it on the machine to test it. It failed. Two days later they had the replacement in. I had them put the replacement on the test machine -- it wouldn't even spin up. Went to a local starter/alternator rebuiler and got a good one for $85.
He likely smoked the regulator. You have to use a special adapter on the tester to simulate the regulator talking to the ECM. Most parts store clerks are morons, and don't know how to do this, or even that it's necessary.

Did you go to converse or Pacolet for your replacement?
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