Dmch alternator issue

[Michael]

He already checked (replaced) the battery.....

You can't make one sentence posts like that in the shadow of a David T reply!

[Bitsyncmaster]

Here is an interesting problem with battery/alternator. Yesterday I was in a boat with an Mercuriser outdrive. The engine would cut (die) if the power tilt button was moved and the volt gauge was pegged over 18 volts. Well turned out the battery cable fell off (boats take a beating) and the engine was running fine off the alternator (Mercuriser uses a GM engine and parts). I have not found out if the alternator is good yet but since there was no load on the alternator it probably showed that high voltage seeing peak rectified voltage.

By the way this 67 year old man could not get up on a slalom ski. Don't understand why I did it all the time 30 years ago.

[Ron]

You can't make one sentence posts like that in the shadow of a David T reply!

Sorry, ironically, I was trying to set an example and be moderate, but evidently went too far. Let me try again:

A bad battery can make a good alternator look bad and a bad alternator can make a good battery look bad. They affect each other and they really should both be tested if a problem is suspected.

The battery's job is to store energy (for starting the engine later). And the alternator's job is to replenish the battery and provide an energy source for the vehicle's electrical system. Given the battery is new and starts the vehicle, and that the alternator is also new, but showing a voltage higher than the limit you say it is supposed to (below), check the fucking alternator.

The voltage regulator is supposed to limit the voltage to around 14.7 volts and if it is much above that you start to suspect the regulator, or possibly the battery or the connections between them.

14.9 is greater than 14.7, the limit -- Check the fucking alternator.

If you were cranking the life out of the battery and the car finally starts....

Evidently, the battery is doing it's job, and more. Check the fucking alternator.

the output voltage of the alternator can be high initially but should start to come down once you get some of the charge back into the battery.

The fans were running, so the vehicle was running long enough get some of the charge back into the battery (actually replenish), but the multi-meter showed it was being overcharged. So, check the fucking alternator.

Another possibility is you have one or more blown diodes in the rectifier bridge. That can be very bad for the battery if the alternator is outputting AC.

A "blown" diode does not conduct electricity. The output would be low instead. Contrary to a blown diode, a shorted diode allows current to flow in both directions, instead of one, which could cause the alternator to output AC. Either way, check the fucking alternator.

Bottom line, if you think you have voltage problems you test the alternator AND the battery and all of the connections between them.

If you have (high) voltage problems, it's most likely the voltage regulator (which is inside the alternator.) Check the fucking alternator.

If the battery is over 5 years old it should probably be replaced.

He already checked (replaced) the battery.....

Happy Labor Day

[Michael]

LOL...that was funny in a "Tommy Boy, get yourself a new map" kind of way.

[DMCMW Dave]

Sorry, ironically, I was trying to set an example and be moderate, but evidently went too far. Let me try again:

The battery's job is to store energy (for starting the engine later). And the alternator's job is to replenish the battery and provide an energy source for the vehicle's electrical system. Given the battery is new and starts the vehicle, and that the alternator is also new, but showing a voltage higher than the limit you say it is supposed to (below), check the fucking alternator.


14.9 is greater than 14.7, the limit -- Check the fucking alternator.

Evidently, the battery is doing it's job, and more. Check the fucking alternator.

The fans were running, so the vehicle was running long enough get some of the charge back into the battery (actually replenish), but the multi-meter showed it was being overcharged. So, check the fucking alternator.


A "blown" diode does not conduct electricity. The output would be low instead. Contrary to a blown diode, a shorted diode allows current to flow in both directions, instead of one, which could cause the alternator to output AC. Either way, check the fucking alternator.

If you have (high) voltage problems, it's most likely the voltage regulator (which is inside the alternator.) Check the fucking alternator.

He already checked (replaced) the battery.....

Happy Labor Day

Where is the thumbs-up button on this thing!

[David T]

Here is an interesting problem with battery/alternator. Yesterday I was in a boat with an Mercuriser outdrive. The engine would cut (die) if the power tilt button was moved and the volt gauge was pegged over 18 volts. Well turned out the battery cable fell off (boats take a beating) and the engine was running fine off the alternator (Mercuriser uses a GM engine and parts). I have not found out if the alternator is good yet but since there was no load on the alternator it probably showed that high voltage seeing peak rectified voltage.

A problem with the battery or the connections between the alternator and the battery can cause the voltage to soar. A bad voltage regulator can do it as can bad diodes. A high DC voltage can also be a sign of bad diodes and you are actually reading the DC component of a lot of AC being impressed on the electrical system. New batteries can be bad or fail. My point is any time you have to test the alternator you should also be testing the battery and vice-versa. If you do have bad diodes the battery is going to fail prematurely, they do not live long on a diet of AC. They also do not do well with too much voltage, it cooks them. Especially the new ones that aren't vented and the gell type. Yes, start with the alternator but check the battery and the connections also.

[Ron]

Here is an interesting problem with battery/alternator. Yesterday I was in a boat with an Mercuriser outdrive. The engine would cut (die) if the power tilt button was moved and the volt gauge was pegged over 18 volts. Well turned out the battery cable fell off (boats take a beating) and the engine was running fine off the alternator (Mercuriser uses a GM engine and parts). I have not found out if the alternator is good yet but since there was no load on the alternator it probably showed that high voltage seeing peak rectified voltage.

Pure plagiarism.

A problem with the battery or the connections between the alternator and the battery can cause the voltage to soar. A bad voltage regulator can do it as can bad diodes. A high DC voltage can also be a sign of bad diodes and you are actually reading the DC component of a lot of AC being impressed on the electrical system. New batteries can be bad or fail. My point is any time you have to test the alternator you should also be testing the battery and vice-versa. If you do have bad diodes the battery is going to fail prematurely, they do not live long on a diet of AC. They also do not do well with too much voltage, it cooks them. Especially the new ones that aren't vented and the gell type. Yes, start with the alternator but check the battery and the connections also.

A problem with the connections between the alternator and the battery would not allow him to crank the life out of the battery...

Changing the word "blown" to "bad" does not work... A "bad" diode is either open or shorted (to some degree, aka "leaks").

===================

Good alternator:
Normal.jpg

Bad Alternators:
open.jpg
Open Diode


shorted.jpg
Shorted Diode

Note both have a lower output!

===================

If you have an open circuit between the alternator's output and battery, then the system voltage will spike (as with Dave M's boat). But the battery is not going to be overcharged -- It's disconnected!

If all of the diodes were open ("bad"), it would not cause the battery to "fail prematurely" because there would be no output.