MEP803a Battery Charging Issue

[Orpington13]

Good afternoon,

I have a MEP803a that won't charge its batteries. If I charge the batteries manually I am able to start and run the set but the ammeter on the panel never goes above 1.5A. When I shut the generator off I need to physically disconnect the battery closest to the alternator otherwise it will drain and be dead within a couple days. The other battery does not have this same issue.

I started by replacing the batteries to verify they wouldn't interfere with troubleshooting, so we're working with new batteries now. I then started troubleshooting with TM 9-6115-642-10 which instructed to check the battery charging fuse, the fan belt, and for any loose or damaged wires. Everything there seemed fine. After reading through these forums (thank you for all the great knowledge out there) I took a reading across the positive and negative terminals on the alternator while it was running and got around 22V. This seemed to point to a bad alternator or regulator so I ordered a new alternator and regulator and installed them. Now I am getting 24.6V across the terminals on the alternator, whereas I was expecting it to be around 27V based on what I have read. The wiring all seems to be correct, I don't see any obvious chaffing or shorts, and it seems unlikely that the new alternator or regulator is bad. Does anyone have any thoughts on what I should try next for troubleshooting?

Thanks in advance,
Adam

 

[Light in the Dark]

Assume nothing. Quality on a lot of stuff has gone down since March 2020. Here is who I used for my last round of alternator rebuilds:

Dave's Starter & Alternator
354 Mascoma St.
Lebanon, NH 03766
Attn: Jack Monohan
(603) 448-5314

 

[Guyfang]

Have you put the batteries on a quality charger?
Have you load tested the Batteries.

 

[Orpington13]

Have you put the batteries on a quality charger?
Have you load tested the Batteries.

I did have my local AutoZone charge the batteries and test them, and they said they're good.

 

[Guyfang]

The Battery charging alternator is hooked up like the picture?

 

[Orpington13]

The Battery charging alternator is hooked up like the picture?

Yes, I used that exact drawing as I was replacing the alternator to make sure the wiring was correct.

 

[Guyfang]

Do you have another thread concerning this gen set?

 

[Orpington13]

Do you have another thread concerning this gen set?

I do not, this is the first thread I have created. I have just read a lot of the other threads with people having similar issues to try to figure out what my problem is.

 

[Guyfang]

Lets start here. I know you said you have done some checking. But somehow we are missing something. So one or two steps at a time.

c . Using dead crank switch, crank engine for 15 seconds while observing multimeter. Voltage reading should not drop below 9.6 volts DC and should return to 12 volts DC when cranking has stopped.



Battery Charging Alternater Testing.

1. Set multimeter for DC volts and connect between positive and negative terminals of alternator.
2. Start and operate generator set at rated frequency and voltage. Multimeter reading should be between 27.5 and 29.5 VDC.
3. Shut down generator set.
4. Battery charging alternator is defective and requires replacement, if reading is other than above.

 

[Orpington13]

Lets start here. I know you said you have done some checking. But somehow we are missing something. So one or two steps at a time.

c . Using dead crank switch, crank engine for 15 seconds while observing multimeter. Voltage reading should not drop below 9.6 volts DC and should return to 12 volts DC when cranking has stopped.



Battery Charging Alternater Testing.

1. Set multimeter for DC volts and connect between positive and negative terminals of alternator.
2. Start and operate generator set at rated frequency and voltage. Multimeter reading should be between 27.5 and 29.5 VDC.
3. Shut down generator set.
4. Battery charging alternator is defective and requires replacement, if reading is other than above.

I just went out and ran those tests now. During the cold cranking the battery dropped from 12.5V to 11.5V and then returned to 12.5V once i stopped the crank.
I then started the engine and measured DC voltage across the positive and negative terminals on the alternator and got 24.7V. From what you say it sounds like my new alternator and regulator are as defective as the original.

 

[Guyfang]

Maybe not defective. Not adjusted right. And thats something I have no idea about. The army is a pluck and chuck user.

Can you post a picture of the alternator and the data plate on it?

 

[Orpington13]

Maybe not defective. Not adjusted right. And thats something I have no idea about. The army is a pluck and chuck user.

Can you post a picture of the alternator and the data plate on it?

Here are some pictures

 

[Guyfang]

Both the alternator and Regulator numbers are right. So when the wires are hooked up right, then it can only be the Battery charging alternator. On start up, every start up, the battery Charging Alternator should go up to 27-29 VDC, at least for a short time, before the Regulator "see's" that the battery is charged/getting charged, and lower the VDC accordingly.

 

[Orpington13]

Both the alternator and Regulator numbers are right. So when the wires are hooked up right, then it can only be the Battery charging alternator. On start up, every start up, the battery Charging Alternator should go up to 27-29 VDC, at least for a short time, before the Regulator "see's" that the battery is charged/getting charged, and lower the VDC accordingly.

How long does the voltage typically stay above 27V? Longer than the 30 seconds it takes me to open the door and get the voltmeter hooked up, or should I have someone watching the voltmeter immediately after I get it started?

 

[Guyfang]

It should take longer then that. But it never hurts to have someone else to start the set and you check voltage.

 

[kloppk]

FYI. On my 802 the voltage at the slave port is usually 28 volts after a minute of running. Upon starting it's about 27.7 volts before it climbs up.
The slave port is essentially connected to the batteries.

 

[nextalcupfan]

Good afternoon,

I have a MEP803a that won't charge its batteries. If I charge the batteries manually I am able to start and run the set but the ammeter on the panel never goes above 1.5A. When I shut the generator off I need to physically disconnect the battery closest to the alternator otherwise it will drain and be dead within a couple days. The other battery does not have this same issue.

Thanks in advance,
Adam

To me that sounds like the diodes are fused in the alternator, meaning a bad regulator.
Can you take the alternator somewhere to be tested?

 

[zarathustra]

the problem may not be in the alternator. --- it might be in a phantom "vampire" load that is outside of charging circuit. One way to check this is to shut down the generator when the batteries are fully charged and instead of disconnecting the battery cable, disconnect the alternator. If the batteries stay charged, then the alternator is the "vampire" that is draining the battery. If the batteries still go down then the "vampire" is not in the alternator, but elsewhere.

 

[Guyfang]

If the Alternator will not produce the required output voltage, (and it is not doing so IAW the alternator out put test) then the problem is not a "phantom vampire" problem. Its a problem that just might have to do with the voltage regulator.

And easier way to prove your "phantom vampire" idea, would be to simply disconnect the neg battery terminal and rub in on the Neg battery post. If you get sparks, that tells you there is a drain on the electrical system, (IF, everything is turned off and CB1 is popped out)

 

[Orpington13]

If the Alternator will not produce the required output voltage, (and it is not doing so IAW the alternator out put test) then the problem is not a "phantom vampire" problem. Its a problem that just might have to do with the voltage regulator.

And easier way to prove your "phantom vampire" idea, would be to simply disconnect the neg battery terminal and rub in on the Neg battery post. If you get sparks, that tells you there is a drain on the electrical system, (IF, everything is turned off and CB1 is popped out)

Thanks for the replies. I just had someone start the generator for me so I could immediately measure the voltage at the alternator and i got 25.3V, slightly higher than the other day but not where we want. I will try popping CB1 and see if I get sparks when running the negative post.