Starter disconnect switch

[Harleyd315]

Got a MEP002 and checked and adjusted the starter lock out switch, works as it should with a .35 gap when the machine is running. The problem is the starter will still engage. I took apart the connection by the oil filter and I have continuity to grnd on both wires going back to the control panel. Took the plug off the control panel and checked the wires are not shorted between the panel and diconnect switch. I checked the wire I think is the problem and it goes back to K3. It has continuity to ground while it is diconnected from K3. My eyes aren't to good with all those little wire numbers so if anyone has some ideas please give me some advice where to look. Thanks

 

[RichardR]

Harley,

I had the same problem and it almost drove me crazy --maybe it DID drive me crazy. The problem turned out to be a pop rivet (one of those holding the unit ID plate on the OUTSIDE of the control box) grounding a lead on the back of the AC voltage regulator card INSIDE the control box. When the unit had been reset and the ID plate removed/replaced with new pop rivets, the new pop rivets extended into the inside of the control box too far and touched the back of the AC voltage regulator card.

The solution is to remove the AC voltage regulator card, trim down the offending pop rivet(s), and replace the AC voltage regulator card. Also, the card should have at least 2 nuts on the backs of the card mounting screws to provide some stand-off between the back of the card and the inside surface of the control box and/or the pop rivets. One nut doesn't provide enough stand-off to keep the back of the card from touching the inside surface (ground) of the control box.

It's a weak design if I have ever seen one! Or maybe they didn't design it that way and a real stand-off was originally intended but it got left out at some point during maintenance. I can say that ONE nut doesn't provide enough stand-off, and TWO nuts is barely adequate. Then a long pop rivet can bridge any gap.

All in all, the symptoms are just as confusing as you say. The problem appears to be in the DC circuitry that runs the starter solenoid and the relays, but the real problem is the stray ground on the AC voltage regulator card. Enough to drive you CRAZY!!

Hope this helps,
Richard

 

[Harleyd315]

Thanks for the help,I'll be checking that today. I used to work on heavy equipment and one of my many tasks was to trouble shoot some of the electrical problems on CAT equipment. They had there quirks but the electrical schematic was understandable. The schematic for these genset leaves me scratching my head every time I look at them. What you say makes a lot of sense as I was poking around with the ohm meter and had continuity to ground on a lot more things than I do on my other machines inside the control panel. I don't know about your machine but mine won't start with the lockout switch diconnected. But if I hook it back up and start the machine I am able to engage the starter with the main switch, so something is shorting after the machine is running. Thanks again I will let you know how it turns out.

 

[RichardR]

Harley,

You are about to enter the 400 Hz Zone, where things are not as they appear, or as you THINK they appear. You have already stumbled upon one of the inexplicable differences between 60 Hz control boxes and 400 Hz control boxes. You should remember that this has already driven me CRAZY, so don't be drawn into my psychological whirlpool of confusion. I'll try to explain what's going on with your control box.

You already know, of course, that there are at least 3 slightly different MEPs with many interchangeable parts: MEP-002A (60 Hz), MEP-003A (60 Hz) and MEP-112A (400 Hz). I believe that the control boxes for the 002A and 003A are identical and completely interchangeable. However, the control box for the 112A is slightly different. In addition to the frequency transducer and Hz meter, there is a small WIRING DIFFERENCE that you wouldn't notice unless you suspect that you have a 400 Hz control box, or that you stumble upon during troubleshooting (as you have already done, whether you realize it yet or not).

The key is that you disconnected the starter cutout switch at the Cannon connector by the oil filter, verified all the wiring as OK, and you still had ground at terminal X2 of K3. That's impossible according to the 002A, 003A schematic, right? But it is exactly what can happen in a 112A control box with a short to ground on the back of the AC voltage regulator card! Look at Terminal Block 4, Terminal 3. Do you see 3 wires connected to TB4-3? If so, you have wiring for a 400 Hz control box. The 60 Hz wiring, and probably the SCHEMATIC you are looking at, has only 2 wires on TB4-3.

What's it all mean and how did it get that way? Well, I don't really know, but my guess is that somewhere along the long maintenance and revision road that all these machines have traveled, the 60 Hz and 400 Hz control box designs were merged and differences were reduced to the essential minimums. The essential minimums are the Hz transducer and meter, and (perhaps) the AC voltage regulator card. Unfortunately, the TMs (even through many revisions) never caught up with all this. My guess is that the maintenance folks freely interchanged 60 and 400 Hz parts (except for the Hz transducer and meter) and called it good. Storeman can verify that most of his 60 Hz machines contain 400 Hz AC voltage regulator cards, as do most of my machines, and they all work fine. However, the TMs call out different AC voltage regulator cards for 60 and 400 HZ.

The slight wiring difference between 60 and 400 Hz is shown ONLY ON THE SCHEMATICS you and I use for troubleshooting, which leads us to LOTS OF CONFUSION and has driven me completely CRAZY!

Hope this helps instead of leading to more confusion,
Richard

 

[Isaac-1]

How do you tell a 400 hz voltage regulator from a 60 hz?
Ike

 

[RichardR]

Ike,
They have different part numbers either silk-screened or etched into the copper traces. And they are laid out different, although they are functionally very similar. The TM for the MEP-003A, 112A shows both and identifies which is which.
Richard

 

[Speddmon]

You guys should also know that if you try to run your 003a with a 400 Hz regulator, the VR will not last long. They are interchangeable in the 002's due to the smaller field in the 5KW head. But the card will not stand up to the increased current draw of the 10 KW head.

 

[storeman]

By golly, I think I'm starting to understand the anomalies I see. Awesome! Thanks to all!

BTW: sold an 002a to a member today. A VERY pleasant visit and lunch. Glad to meet you Will and wish you the best.
Jerry

 

[Harleyd315]

Didn't get any time today but tomorrow I'm diving in head first on this. So if I understand this correctly the 400 hz VR is ok as long as it doesn't ground on the side of the control box? Or is there a better way to fix this problem? My machine is freshly painted and has an Onan Tag on the side of the blower sheet metal. I assume the engine was changed and some work was done to the unit but no Letterkenny rebuild tag is on the machine. It does have a new ID plate on it, with fresh rivets. Hope thats the culprit!!!!All the gauges look new but the hour meter was not changed out with over 2,000 hours on it. All the wiring harnesses are new white wire with the yellow part # tags on them. Somebody may have put the wrong control box on the unit a 400 Hz instead of a 60 Hz box? Thanks again

 

[RichardR]

Harley,

My experience is that either a 400 Hz or a 60 Hz control box will work OK if there is no stray ground on the back of the VR board from the new pop rivets. YMMV.

The difficulty you are having in diagnosing the problem is that the schematic you are working from does not correspond exactly to the box you apparently have. There is just that subtle difference in the wiring terminating on TB4-3.

Hope you fix it without further trouble,
Richard