003A developed sudden starting circuit short 24v to ground

[reybo]

Everything has been top notch with this mil gen. Run it for routine test every few months and it's been A-1.


Suddenly it isn't.


This weekend when I tried to start it, as I hit start there was a spark where the ground wire is close to the trailer bed, and thereafter a dead short between the 24v batteries and ground. (I had just checked and these were fully charged batteries.)


The ground wire was instantly -very- hot and the starter barely had enough current to turn over due to the short. It turned slowly but didn't spin.


The ground wire is only heated when the starter is engaged, not otherwise.


This unit is in a protected, covered area where neither rain nor wind can get to it. So it appears something to, from, or in the starter solenoid suddenly shorted.


Unless the generator is running, the batteries in this unit are disconnected from the motor by a knife switch. They are charged between runs by a 24v solar panel which has kept them charged very well. The knife switch has been open since the last test until being closed for today's test.


There's one other point. There is a small seepage of diesel fuel from the on-board tank. Very slight; no loss shown on the fuel gauge in 8 months. Just a wet trailer bed. But could that leak be above the starter or solenoid, and have soaked the windings?


Anyone had experience with this?

 

[rustystud]

Do you have rodents around your area ? Rats have been known to chew through wires and cause shorts when you shoot voltage through them. Did you check all the battery cables ? You also could have a bad spot on the sheathing of a positive cable and it's touching a ground ( like the frame). You also could have fried the solenoid.

 

[Keith_J]

Probably threw a winding on the rotor of the starter. Usually happens near the commutator pads, just one break can cause the starter to have crazy high draw.

Best bet is to take it to a dedicated starter repair shop, they have a growler and the know-how to diagnose.

As far as the tank leaking, probably bacterial sludge from water caused a rust-out. These tanks need annual draining and cleaning. Coating with the POR product or equal (polyurethane products) slows this damage and makes cleanup easier.

 

[gimpyrobb]

If your starter is bad, I might have one.

 

[reybo]

If your starter is bad, I might have one.

Thanks. I hope to be able to work on it this week.

 

[PeterD]

1. The ground wire was instantly -very- hot and the starter barely had enough current to turn over due to the short. It turned slowly but didn't spin.


2. The ground wire is only heated when the starter is engaged, not otherwise.

1. Where does the wire get hot? At the end (say where it connects to the frame?) or in the middle some distance from the connectors?

2. This does indicate either the starter or the ground cable has failed.


(diesel on the starter or wires would not normally cause a problem...)

 

[reybo]

1. Where does the wire get hot? At the end (say where it connects to the frame?) or in the middle some distance from the connectors?

2. This does indicate either the starter or the ground cable has failed.

(diesel on the starter or wires would not normally cause a problem...)

Can't say with assurance where the wire gets hottest. I only tested (touched!) the wire under the control panel and about 3' beyond. Both very hot in just a few seconds. Seemed like a dead short when the starter is engaged.

It's the suddenness that's perplexing. There was no sign of anything amiss before this. So I'll be looking for rodents as a possibility. Mice rather than rats. I've had them chew up an air filter on a classic car by crawling in through the air intake. Needed to stuff wire screening in to put a stop to it.

 

[Chainbreaker]

Can't say with assurance where the wire gets hottest. I only tested (touched!) the wire under the control panel and about 3' beyond. Both very hot in just a few seconds. Seemed like a dead short when the starter is engaged.

You say the wire going under the control panel. Are you talking about the wires that come off the starter and go to the 2-pin slave connector (jump start connector)? If so, it would be very plausible that you could be talking about the + slave cable which dips low and can touch the metal frame of your trailer. With enough time and vibration it could have rubbed off the outer insulation. Then having the inner insulation come in contact with diesel (you indicated your trailer bed has some diesel on it) might have caused the insulation to further degrade enough to now cause a dead short to ground. Since you have it on a battery cut out knife switch it was not a problem until you went to start it and activated current through those slave cables. Just my theory FWIW.

 

[reybo]

You say the wire going under the control panel. Are you talking about the wires that come off the starter and go to the 2-pin slave connector (jump start connector)? If so, it would be very plausible that you could be talking about the + slave cable which dips low and can touch the metal frame of your trailer. With enough time and vibration it could have rubbed off the outer insulation. Then having the inner insulation come in contact with diesel (you indicated your trailer bed has some diesel on it) might have caused the insulation to further degrade enough to now cause a dead short to ground. Since you have it on a battery cut out knife switch it was not a problem until you went to start it and activated current through those slave cables. Just my theory FWIW.

Haven't had a chance to look into this yet, so I'm no help. All I can add is that the ground is bare wire, uninsulated. It was a great surprise to see it spark when in proximity to the trailer floor. This is the main ground, the one connecting to the rod driven underground. I would not have expected a voltage difference between the ground and the trailer chassis, considering that a few feet from the spark, the ground has an eye bolted to the chassis.

 

[Isaac-1]

If it is the wires to the slave jump starting jack on the front, you can simply disconnect them on the far end, they are either hard wired to the battery post or the positive is to the solenoid at the starter. I have seen both ways, not sure which one is "correct" and which was a retrofit. I give this advice as I know you don't have any other 24V military equipment that you would want to connect for jump starting over the slave connector.

Ike

 

[Chainbreaker]

OK, so your talking the chassis/terminal gnd wiring to gnd rod connection point where you saw the spark. If your trailer chassis is suddenly energized with +24V it is going to find that gnd! I agree with Isaac. If you don't plan on using the Slave Jack, disconnect it regardless (at least the positive terminal at solenoid or battery). That long run of positive wiring going to the slave connector is just another failure point that could eventually wear through and short 24V to gnd if it hasn't already in your situation. I'm sure the Military made good use of the slave connector whenever remote conditions required a jump with no immediate battery replacements/chargers available. However, in civilian use it is not needed unless you have other compatible military equipment and plan to use it. Keep us posted to what you find once you further investigate.

 

[reybo]

If it is the wires to the slave jump starting jack on the front, you can simply disconnect them on the far end, they are either hard wired to the battery post or the positive is to the solenoid at the starter. I have seen both ways, not sure which one is "correct" and which was a retrofit. I give this advice as I know you don't have any other 24V military equipment that you would want to connect for jump starting over the slave connector.

Ike

Thanks, Ike and Chainbreaker. I'll look at that. Haven't noticed any unusual wiring off the battery terminals, but there might be off the hot starter or solenoid post. I've got the TM for this. It's not showing what would be termed a slave jump.

 

[Chainbreaker]

It's not showing what would be termed a slave jump.

It's the "Slave Receptacle" used for a jump start situation. It is located below your throttle control adjustment knob. You either have a 2-pin or 1-pin model. Follow the wire(s) back and look to see if there are any rubbed/chewed or bare spots along the positive lead. The + lead should run below control box and make its way back to the starter solenoid (or perhaps + battery terminal) where you can disconnect it to test to determine if it makes a difference. You might want to remove the slave receptacle completely since it is not being utilized in your situation. Eliminates the possibility of the + wire ever having the opportunity to short to gnd.

 

[reybo]

It's the "Slave Receptacle" used for a jump start situation. It is located below your throttle control adjustment knob. You either have a 2-pin or 1-pin model. Follow the wire(s) back and look to see if there are any rubbed/chewed or bare spots along the positive lead. The + lead should run below control box and make its way back to the starter solenoid (or perhaps + battery terminal) where you can disconnect it to test to determine if it makes a difference. You might want to remove the slave receptacle completely since it is not being utilized in your situation. Eliminates the possibility of the + wire ever having the opportunity to short to gnd.

Still haven't had a hands-on chance to look yet. The TM doesn't mention the slave receptacle in the text or even in the drawing that shows it. The first schematic shows it in a dotted box (optional?) as J15.

So what is the slave receptacle for: A) 24v output to jump start something with low batteries, B) 24v input to jump start this gen when its batteries are low, or C) both?

And yes, the Tesla-worthy jumping spark came from the vicinity of that receptacle.

 

[Jimc]

both

 

[reybo]

both

Thanks.

Spent a few minutes looking and feeling. Nothing obviously amiss. No chewed insulation or busted cable. Behind/below the slave is where the stranded steel ground wire begins.

 

[gimpyrobb]

both

I'm gonna have to disagree. Its there to start the genny only. The genny only has like a 5amp 24v generator on it, so it would be useless for a truck. Yes the batterys could supply enough current, but it would take a LONG time to recover.

Lets not go putting bad ideas in people's heads!

 

[Chainbreaker]

Just for further clarification - In your earlier post #9 you indicated that your trailer/genset is grounded via a driven gnd rod. Just to understand your usage...is your generator connected to your house (via an inlet box?) or is it being used as a stand-alone power unit used to power something else?

 

[Isaac-1]

He bought it for household emergency backup a year or two ago to power emergency medical devices, etc, Which I think he has connected to a transfer switch via a plug in receptacle.

Ike

 

[PeterD]

The concept of grounding a generator set is often not fully understood by users.

Grounding had no meaning on the low voltage (battery) 24 volt DC system. It doesn't use ground and simply achieves its power relative to the battery's negative terminal.

Grounding is used to reference the generator set's output with regard to established power systems. It is used to create a safe environment where there is little or no possibility that the generator's ground or neutral might be inadvertently attached to a potential to ground. When this happens, the generator's frame and structure becomes electrified with reference to the ground (earth). At this point, while standing on the ground were one to touch the generator it is possible to receive an electrical shock, with the subsequent risk of injury or death. I strongly recommend you double ground: use ground rod(s), and as well bond to the facilities ground as well. This eliminates any possible ground resistance issues.