MEP-004a DC wiring problems

Just got in a mint 004a with 27 hours after a tier 2 reset in 2006. Everything worked perfect, then one of my guys was power washing the bottom of the set, under the motor and accidentally touched the positive cable on the starter to ground with the metal power washer hose. Now I have no start (no power to the starter) and the fault indicators do not work. Checked fuses and all are good. Looking at the schematics, it looks like there is R13 Shunt that could be damaged inside the special relay box. Wanted to get some others opinions before taking the box out.

I would start with load testing the batteries, a cheap or marginal batteries can go bad after a direct short. Next I would check the battery cable connections and cables going to the ground and the big lug on the starter terminal. Make sure you disconnect the battery cables from the batteries and make sure they are clean. 95% of the time a bad battery connection is on the positive post. A good way to check for bad connection is to attach one lead of a multimeter set to volts to the positive 24 V battery post, not on the battery cable. And attach the other end to the large post on the starter, then turn the switch on and try to start it, if your multimeter reads 24 V then there is a bad connection between those two points. Do the same thing with the negative battery post going to ground and the second lead on a good ground on the engine block, and turn your switch on and try to start it and if your multimeter reads 24 V there's a bad connection between your multimeter leads. Do the same thing with the battery crossover cable and check that. Just make sure you put your multimeter on the battery post itself and not the battery cable.

jamawieb said:

Just got in a mint 004a with 27 hours after a tier 2 reset in 2006. Everything worked perfect, then one of my guys was power washing the bottom of the set, under the motor and accidentally touched the positive cable on the starter to ground with the metal power washer hose. Now I have no start (no power to the starter) and the fault indicators do not work. Checked fuses and all are good. Looking at the schematics, it looks like there is R13 Shunt that could be damaged inside the special relay box. Wanted to get some others opinions before taking the box out.

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Do the panel lights work? Have you reset the CB1. Sometimes you have to pull it out, then push it back in. That's because it sometime doesn't always pop completely out.

You should be able to test the R13 without taking out the Special Relay box, (S/P box) by removing the J37 canon plug from the S9 speed switch, and inserting one probe of your multimeter, (+) set for VDC into the pin F of the plug. The other probe to ground. You should get 24 volts. If not. the fuseable link in the R13 is burnt through. How long was the Press washer want stuck on the starter?

I hope this is right! The schematics are not real readable, and my 65 year old eyes are not infallible.

Or just remove the 6 mounting bolts to the S/P box and the canon plugs. Then two screws, (I think) on top. The S/P box then comes apart. The shunt is next to the A5 card. If the link is burnt through, you need a new one. This is a seldom seen fault, but I have seen it a few times.

I should mention, that I can hit the battle short on and the fuel pumps activate, then when I go to run on the start switch, the fuel pumps activate and the gauges move into position, so I'm getting power. Guy I did reset CB1 thinking the same thing.
So I pulled the special relay box and I could hear something moving around inside. When I opened it up, I found K5 relay sitting on the bottom, not in the holder. The inside of the box is prestine and everything looks new but K5 hold down screws are missing. Looking at the schematic, this could be the issue. Maybe the arch did not effect anything and moving the generator around made K5 fall out resulting in the problem. I have not reinstalled the S/R box yet because I wanted to get everyone's opinion before I reinstall.

jamawieb said:

I should mention, that I can hit the battle short on and the fuel pumps activate, then when I go to run on the start switch, the fuel pumps activate and the gauges move into position, so I'm getting power. Guy I did reset CB1 thinking the same thing.
So I pulled the special relay box and I could hear something moving around inside. When I opened it up, I found K5 relay sitting on the bottom, not in the holder. The inside of the box is prestine and everything looks new but K5 hold down screws are missing. Looking at the schematic, this could be the issue. Maybe the arch did not effect anything and moving the generator around made K5 fall out resulting in the problem. I have not reinstalled the S/R box yet because I wanted to get everyone's opinion before I reinstall.

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My bet is, you found the problem.

Looks like to me, that the K5, field flash relay and the K3 all need to work so the L4, (starter) can engage. So yes, I bet that's the problem.

If you cant find the screws for K5, just take one out of the other relays. One is good enough.

When ever I tested a S/P box, I never even closed it up. Toss it in, screw in one mounting bolt and run the C-Plugs down. test it and then do it right. If it doesn't work, You do not need to go through the whole nine yards of taking it completely apart again.

Guyfang said:

Looks like to me, that the K5, field flash relay and the K3 all need to work so the L4, (starter) can engage. So yes, I bet that's the problem.

If you cant find the screws for K5, just take one out of the other relays. One is good enough.

When ever I tested a S/P box, I never even closed it up. Toss it in, screw in one mounting bolt and run the C-Plugs down. test it and then do it right. If it doesn't work, You do not need to go through the whole nine yards of taking it completely apart again.

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Thanks Guy, I'll put back in tomorrow and give an update.

The schematic is hard to read. Bad copy. It's best to look at both the 004A and 005A schematic to find the "best" copy. The circuit where K5 and K3 are in line, is hand drawn in the schematic. In the 70's, the gov was too cheap to spend money on fixing the problem, and in the 80's-90's figured if I hadn't bothered anyone much, why fix it now.

Ok so I put the relay back in and install the special relay box. Now the fault indicator lights work but still no start.

Going back to square 1...Are you 100% sure the batteries didn't take a hit with the direct short to gnd? Have you performed KennyO's suggested battery diagnostics in post #2?

Yes, 100% sure. Load tested each battery. Can also jump the starter and the it starts right up.

jamawieb said:

Yes, 100% sure. Load tested each battery. Can also jump the starter and the it starts right up.

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OK, didn't understand that you could now jump the starter and it would start and run OK. So...its truly a problem somewhere in the control circuit for starter activation. Sounds like its time to pull out the schematics and verify everything in that circuit.

I will look a bit later, but anytime I could not get one to start, the first thing I did was reset the over speed switch, and if that didn't help, removed the over speed switch C-Plug, and stuck a paper clip into pins A&B. Then tried to start it. THEN when that didn't work, started troubleshooting like normal. You did look at the shunt?

Guy, you're right again! I double checked all the relays and resistors in the special relay box and reinstalled. Still no start. Took the speed control harness off and jumped AB, started right up. So I took the speed switch apart last night and found the housing was to far apart so the reset button couldn't reach the micro switch. I'll reinstall today and report back. Thanks GUY!

Attached is an excellent write up on the rebuild of the speed switch.

No attachment.

The "Bad" thing about the S9 Speed switch is that unless it actually over speeds, you get no idiot light. Since you have no idiot light, no one looks at, or resets the S9. Back in the 70's we had so many problems with the S9, we all got to pressing the reset button whenever we were fooling around, or checking oil level. And we all carried a paper clip in our pockets. Just for that reason. Now a days, the (solid state) S9 is much more robust. Almost no problems. The NSN in the TM is not the newest NSN for the switch. Glad you got that problem fixed. Sounds like you need to take a bit of time and check screws and stuff. The A4 in the control cube is also well known for the relays falling out, IF they have been replaced or someone tested them. As it comes from the box, the A4 and A5 is covered with a sealant that will not allow the screws to loosen up.