Go BottomStrange 790 electrical problem
 | deeremd
Join Date: Jul 2003 Posts: 18 maryland 
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2004-08-05 92757
A friend of mine sent me this message below concerning an electrical problem he's having. I've never heard of this particular issue, nor had his dealer. Does anybody have any ideas??
"The John Deere Is dead!!!! It keeps blowing the 3 amp fuse to the fuel solinid but it goes thru the pto switch and the seat switch. The Local jd dealer says that jd's web info says that its only happened 1 other time but they could not give me any specifics."
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 | Murf
Join Date: Dec 1999 Posts: 7249 Toronto Area, Ontario, Canada 
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2004-08-05 92765
I think the key to the mystery is that it goes through the two safety switches. If somewhere in all that electrical wandering there is an intermitant short a 3 amp fuse will lead a very short life indeed.
I would look for anywhere the wire rubs a sharp edge, a misaligned switch, etc., and don't just glance, get the wire in hand a nd roll it around examing every side of every inch.
The only other possibility is that the solenoid is defective and overloading the supply line tyo it but that's a pretty remotre possibility.
Best of luck. ....
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| TomG
Join Date: Feb 2002 Posts: 5406 Upper Ottawa Valley
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2004-08-06 92845
Murf's suggestions are good. I might disconnect the safety switches and jumper them to take the switches themselves out of the circuit and then jiggle the harnesses to see if I could get the fuse to blow. In addition, I'm not sure how close JD engineering places the solenoid start-up surge to the fuse size. Surge from a marginal solenoid might be enough to blow it.
In all cases I'd use an approved solution. You might check if a larger or slow-blow type fuse might be approved. NH went to a larger fuse for a similar problem. ....
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| eddie
Join Date: Posts: 1
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2004-08-07 92949
I had a almost the same thing happen to my 790. I burnt up 2 fuel shut off sellinoids. It wasnt the switch under the seat or the one for the pto, it was the relay that they both connect to. I love my 790 and crazy about the #7 backhoe. ....
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| deeremd
Join Date: Jul 2003 Posts: 18 maryland
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2004-08-09 93009
My buddy was finally able to figure out what the problem was -- his battery had an internal short in it. Once he replaced the battery (and the fuses again), it worked fine.
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| TomG
Join Date: Feb 2002 Posts: 5406 Upper Ottawa Valley
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2004-08-10 93103
Good it's fixed and I'll remember the explanation. Usually a battery short would be associated with low voltage/current rather than blowing fuses.
Many solenoids have pull-in and hold-in windings, and the current draw decreases in the hold-in state. I guess if a battery didn't have sufficient power to completely close a solenoid it could stay in a pull-in state and might blow fuses even with a doggy battery. At least that's some theory and it's a bette thought than a problem that's still there waiting for the worst time reoccur. ....
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| Murf
Join Date: Dec 1999 Posts: 7249 Toronto Area, Ontario, Canada
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2004-08-10 93106
Actually Tom since the relationship between current, voltage and resistance is a variable one best described as a triangle, when one of the three 'sides' of the triangle changes, at least one other side has to change to keep it a triangle (connected). In this case for instance, a shorted cell in the battery would mean lower voltage than the 12v design, since the resistance of the solenoid is fixed, the only thing that can change is the current. As Voltage drops the current raises proportionately if the resistance is unchanged. If you lose one cell you are down by about 12%, that doesn't sound like much, but if that short drags the other cells down to lower voltages and the battery is putting out only say 9v, that is a 25% drop in voltage. If the normal peak load in that circuit is 2.5 amps and it goes up by 25% it is over the 3 amp fuse capacity and it blows the fuse.
I have seen starters burned up from this effect, particularly when people are trying to light up big diesels equipped with multiple batteries in the winter. The current can get way beyond normal, and when combined with the load it can do in a good starter surprisingly fast.
Best of luck. ....
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| TomG
Join Date: Feb 2002 Posts: 5406 Upper Ottawa Valley
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2004-08-11 93211
Ohm's Law for current is I = E/R. If voltage decreases and resistance is unchanged then current has to decrease. My point however was that resistance, or the more general term impedance, may not be fixed.
For example, in series wound DC motors the impedance changes with rpm and it is minimum at 0 rpm. That is why they can burn up due to low voltage if they never come up to speed. ....
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