MEP-802A voltage readings not right.

Saunders · Jul 30, 2018

Trying to help a friend get his recent auction purchase MEP-802A going. Looks mint condition with only 14hrs showing, but several cover screws missing indicating someone has been inside for some reason or another.Hoping someone may have ran into this before or can point me to in the right direction...

With the switch behind panel set to 120/208v 3 Phase, we're getting around 25 volts on all three legs, and hertz meter working ok. Switching to 120/240v 1 Phase, we get L1=28v, L2=28v, L3=0 and no hertz reading on meter. Besides the voltage being too low, we shouldn't get any reading on L2, it should be on L3 should it not?. It's almost like some wires are crossed or something.

Troubleshooting thru the maintenance manual (http://www.liberatedmanuals.com/TM-9-6115-642-24.pdf) we went to section 2-27 AC Voltage Regulator. Step 2-27.2 h we get the 8VDC on terminal 5 ok. But on step J we get little to no voltage on terminals 7 & 8 where it should be 67.5 to 90 VAC. And step L little to no voltage on terminals 3 & 4 which should be 108 to 132 VAC.

Since we didn't get the voltage readings that were expected, I'm not sure what step to check next. The manual states if all these readings are correct, then the regulator is defective. We replaced it anyways with a known good one, and no change. Even tried the old one in another generator and it worked ok there. So we're certain the AC regulator is good.
It has the fuse upgrade on terminal 8 of voltage regulator and checks out ok. All wires nice and tight.

It'll be a week or two before we'll be able to get back out there (cabin in the sticks) and looking for some suggestions.

kloppk · Jul 31, 2018

Saunders said:
...Switching to 120/240v 1 Phase, we get L1=28v, L2=28v, L3=0 and no hertz reading on meter. Besides the voltage being too low, we shouldn't get any reading on L2, it should be on L3 should it not?. It's almost like some wires are crossed or something.

You are correct. In 120/240 single phase the AC output should be on L1 & L3 and not L1 & L2. The fact that you are getting no AC on L3 but get it on L2 points to a wiring issue. The Frequency meter is driven by L3. Since there is no AC there that explains why it stops working in 120/240 single phase.

I'd look at the wiring of S8 and K1 to determine the cause of the L2 L3 issue in single phase.

In 3 phase mode what voltage do you get at the convenience outlet if you hold S1 in the START position after the set is running?

DieselAddict · Jul 31, 2018

Let me channel my inner GuyFang.

Disconnect all the wires then reconnect them using the wiring info in the TMs. Ok, maybe not ALL the wires in the generator but the ones around S8 for sure. Since you have no idea what has been done to the set you will need to over-verify things. There could be many other errors laying in wait to bite you in the rear end.

There are tiny numbers on the wires themselves that correspond to the wiring diagrams in the TMs. To read many of them you'll have to disconnect the terminals to move them around.

What is the condition of the wiring harness? They are normally bundled together every inch or two with wire ties. Does it look like anything has been disturbed? Is everything still bundled neatly? Do the crimp connectors all match?

Saunders · Jul 31, 2018

kloppk said:
You are correct. In 120/240 single phase the AC output should be on L1 & L3 and not L1 & L2. The fact that you are getting no AC on L3 but get it on L2 points to a wiring issue. The Frequency meter is driven by L3. Since there is no AC there that explains why it stops working in 120/240 single phase.

I'd look at the wiring of S8 and K1 to determine the cause of the L2 L3 issue in single phase.

In 3 phase mode what voltage do you get at the convenience outlet if you hold S1 in the START position after the set is running?

Didn't think to check convenience outlet with Start held in. I'll make note of that when we're back out there. What will that likely tell us by trying that? I thought I read somewhere that holding it in too long can burn something up. If that's the case what is considered too long?

When we looked at all the wiring before, we didn't notice anything out of the ordinary, no disconnect or loose wires. However, we didn't trace anything back to see if any were mis-connected which I guess we'll have to do.

Saw a thread where one of the guys here was working on updating the wire diagrams awhile back and that the diagrams in the manual had some mistakes. Anyone know if that was ever posted?

DieselAddict · Jul 31, 2018

You should be able to get a reading within a few seconds. That is not long enough to hurt anything. Few meaning 5.

kloppk · Jul 31, 2018

Saunders said:
... What will that likely tell us by trying that?...

In 3 phase mode...
If you get ~120 VAC at the convenience outlet with S1 in the START position it will indicate a few things. What's likely good and what might be the issue(s) to look into.
If you don't get ~120 VAC at the convenience outlet with S1 in the START position will indicate a few other things. What's might be the other issue(s) to look into.

There is a great color coded wiring diagram but it only addressed the DC side of the genset.

Guyfang · Aug 1, 2018

Go back to post #3. Dieseladdict has it right. Start there. Here in the TM forum is the schematics you need to check the S8 wires. The portion concerning S8 is correct. Sooner or later we will get done with a newer version of the schematic. There is a chance that a wire to the output load terminals got switched, but my money would be on the S8. Take your time. Remove every wire. Take a break, get a glass of ice tea. Then put the wires back on. Check each and every one, twice, at least. Take your time. There also should be a metal plate with the schematics on the gen set. Do that right, and then you can test it again.

Saunders · Aug 2, 2018

Thanks guys for your insight. Looks like we got some detective work to do here. It'll be another week before we can make it back out there. Hoping its just a simple reversed wire connection and nothing shorted in the stator windings. I'll post the results of what we find.

Saunders · Aug 11, 2018

Well, we found three main wires from the K1 to S8 were on the wrong legs. Got that corrected and checked all other wire on S8 switch and K1. Now get voltage on the correct Legs.

However theyre still not right. On three phase, all three legs show around 20 volts. Single phase get 20 volts on L1 and L3 and hertz meter now working. With engine running, engage the start switch and we get 120 volts, let go and it goes back to 20 volts.

Anyone have an idea to check next? I saw another thread where someone had a similar issue and found their quad windings was bad. Disconnected wires 7 & 8 from A1 AC regulator and got no resistance reading. Reading thru the test procedures in manual section 4-10 for rotators and stators was a bit confusing. Was hoping there was an easier way to narrow this down.

Guyfang · Aug 12, 2018

Saunders said:
Well, we found three main wires from the K1 to S8 were on the wrong legs. Got that corrected and checked all other wire on S8 switch and K1. Now get voltage on the correct Legs.

However theyÂ’re still not right. On three phase, all three legs show around 20 volts. Single phase get 20 volts on L1 and L3 and hertz meter now working. With engine running, engage the start switch and we get 120 volts, let go and it goes back to 20 volts.

Anyone have an idea to check next? I saw another thread where someone had a similar issue and found their quad windings was bad. Disconnected wires 7 & 8 from A1 AC regulator and got no resistance reading. Reading thru the test procedures in manual section 4-10 for rotators and stators was a bit confusing. Was hoping there was an easier way to narrow this down.

If you get 120 volts, (where?) as long as you hold up the S1, your main gen is good. How much voltage do you get at pins 7&8 on the A1, when the S1 is held up, and how much when its released?

Guyfang · Aug 12, 2018

Something else occurred to me. We all know that the main gen for the 60 and 400 hertz models are different. BUT, how much? So I went and looked at the parts manual, and indeed, the pictures of the exciter stater, are different. BUT the parts numbers are the same. So I check the 802 manuals. Same thing. Then when I sat down and considered it, why should they be different? The main alternator wingdings and rotor are what give us the difference in hertz, nothing else. So without getting on a plane, flying over to you all to take a look see, I am going out on a limb by saying, they are the same, same. So if all else fails, and you need an exciter stater, you can also look at a 400 hertz main gen for a replacement part. It would be nice if someone here in the forum would look. Assuming will kick my butt every time. And I don't have one to look at. And my go to source of that kind of info is off line at this time. Just food for thought.

Saunders · Aug 12, 2018

Guyfang said:
If you get 120 volts, (where?) as long as you hold up the S1, your main gen is good. How much voltage do you get at pins 7&8 on the A1, when the S1 is held up, and how much when its released?

With the S1 held in, we got 120 volts off the the main leads (ground to L1 and L3 single phase). Also, got 120 volts at the convenience outlet with it held in as well and back to 20 volts when released, same thing in 3 phase mode. Didn't measure volts at 7&8, will try that next time I back out there, but we did measure ohms between leads 7&8 disconnected from AC regulator and got zero ohms (engine off).

If I read the manual correctly, we should get between .9053 to 1.224 ohms on the Q1 & Q2 leads which are the wires that go to pins 7&8 on the AC reg?

Guyfang · Aug 12, 2018

Correct, IF the wires, and the windings are not damaged.

Saunders · Aug 12, 2018

Guyfang said:
Correct, IF the wires, and the windings are not damaged.

Assuming the wires 7&8 are not broken, what would you suggest checking next? On my initial checks to test the voltage regulator, we weren't getting any voltage readings on 7&8, but that was with terminal 6 disconnected per procedure and the L0,L2, &L3 leads mis-connected from K1 to S8 which have now been corrected. Not sure if that would have any bearing on the 7&8 readings or not.

If I understood you right, if were getting 120 volts output when start switch is engaged, then the main generator is ok?

Guyfang · Aug 12, 2018

Yes, but the stater coils may not be. The exciter works. That's why it comes up to 120 volts as long as you manually excite it with S1. If you lose voltage, then it can be the A1 or the stater. If you have no ohm readings down to the stater, then perhaps the wires are bad, or the stater. Does the gen set have the quad circuit mod?

Normally you check the stater with a Meg-Ohm meter.

Saunders · Aug 12, 2018

Guyfang said:
Yes, but the stater coils may not be. The exciter works. That's why it comes up to 120 volts as long as you manually excite it with S1. If you lose voltage, then it can be the A1 or the stater. If you have no ohm readings down to the stater, then perhaps the wires are bad, or the stater. Does the gen set have the quad circuit mod?

Yes it does have the quad mod, fuse not blown. I'm thinking the A1 relay must be good since the AC interrupter light turns green and applies voltage to the main leads (although only 20 volts). Since the wires on it were connected wrong earlier, I'm wondering if the previous personnel repair attempt was to replace the A1.

If the stator is in fact bad, is that normally a replaceable part or are you better off trying to find another generator head?

Guyfang · Aug 12, 2018

Saunders said:
Guyfang said:

Yes, but the stater coils may not be. The exciter works. That's why it comes up to 120 volts as long as you manually excite it with S1. If you lose voltage, then it can be the A1 or the stater. If you have no ohm readings down to the stater, then perhaps the wires are bad, or the stater. Does the gen set have the quad circuit mod?

Yes it does have the quad mod, fuse not blown. I'm thinking the A1 relay, (do you mean the K1 relay? must be good since the AC interrupter, (the K1 has nothing to do with your problem) light turns green and applies voltage to the main leads (although only 20 volts). Since the wires on it were connected wrong earlier, I'm wondering if the previous personnel repair attempt was to replace the A1. (Have you checked the wires to the A1? to make sure they are hooked up right?)

If the stator is in fact bad, is that normally a replaceable part or are you better off trying to find another generator head, (its not the head, its the stater)?

Click to expand...

Yes, it is. You need to look at the parts manual. Re-read post #11.

Figure 24, Item 11, TM 9-6115-642-24P is the stater.

Saunders · Aug 12, 2018

Guyfang said:
Saunders said:

Yes, it is. You need to look at the parts manual. Re-read post #11.

Figure 24, Item 11, TM 9-6115-642-24P is the stater.

Click to expand...

Sorry I misread your post and got my A's and K's mixed up. Yes I meant K1 relay which is what I thought you were referencing earlier. I temporally forgot A1 is the voltage regulator. Not enough sleep.

We did check the wires on the K1, but didn't check the A1 wires to make sure they were correct too. I'll do that when I'm back out there.

When I stated generator head, I meant the whole generator as one unit (the assembly that attaches to the engine). Haven't looked or priced them yet, was curious if it's more cost effective to find a good used generator versus a replacement stator.

Guyfang · Aug 13, 2018

Like I stated, the 400 hertz main gen, is useless to you as a replacement. BUT the stater from the 400 hertz model should work in a 60 hertz main gen. You might be able to find someone in the forum who has a 400 hertz main gen, willing to sell just the stater.

Saunders · Aug 13, 2018

Guyfang said:
Like I stated, the 400 hertz main gen, is useless to you as a replacement. BUT the stater from the 400 hertz model should work in a 60 hertz main gen. You might be able to find someone in the forum who has a 400 hertz main gen, willing to sell just the stater.

Just to clarify, when you say you can use the stator from a 400 hz gen, you mean the "exciter stator" and not the main stator correct? If I understood right, in my case getting 120 volts on the legs with the start switch engaged indicates the exciter stator is likely ok?

I'll do some more troubleshooting this week and see if I can't get a few more details. Not knowing what all may have been done by the previous guy working on something always make it a bit more challenging.

MEP-802A voltage readings not right.

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