Oil & Fuel Finally fixed 4400 no start and it wasn't the solenoid or thermistor

[MacAttack]

For the last 10 years or so my 4400 has occasionally had the crank but no start problem.
I was buying thermistors several at a time. I would just replace the thermistor and magically the tractor would start. I probably replaced over a dozen of the dang things as well as a solenoid. A while back I soldered a pigtail into where the thermistor went and brought it out to the front of the fuse panel. Made it a lot easier. Yesterday it wouldn't start....again!
I got out my crimpers and changed the thermistor, turned the key and Nothing! Doh!
Now the manual would have you test the pto switch connector, transmission neutral switch, the diode block?, seat switch, eng run relay and finally the fuel solenoid connection. Most of all this entails a fair amount of disassembly or crawling under the machine.
I'm getting old and lazy so I skipped everything on the list and went to the solenoid. No power on either circuit. It's always had at least one leg with power. I checked the run relay and no power. Now off come the dash panels and check the key switch. Good except I get an intermittent power at the run sol. Pulled the fuse panel out looking for a broken wire.
Then I notice the power comes and goes when I touch the diode block. It looks like a relay. After all the thermistors I believe the diode block was the problem all along. It tested bad (intermittently). The diodes were all good but there was a broken weld in a terminal. $20 at JD dealer.

 

[beenthere]

Mac
Can you be more specific about this "diode block"? A schematic of the circuit would help, as well as a pic of the location too, and a part # . Thanks, as it should help others (maybe even me someday ).

 

[MacAttack]

Sorry for the delay Beenthere. I have pics and diagrams but I'm having problems with my file sharing account.
I have all the info but no way to show it.
It's really simple to check though. In the fuse panel there are 3 vertical rows of relays.
The center top isn't a relay it's the diode block.
Take the run relay(bottom right row)out and check for power at the #86 terminal in the panel. It's the very top one as you look at the relay socket. Test probe there when you turn the key to ON you should get power.
Power comes from the diode block at this point.
If there is no power there the diode block needs to be examined and tested for continuity with a volt ohm meter.
There are 9 terminals on the block. On the old style block it has a circuit diagram right on it. These are diodes so they should have continuity in one direction and not the other. If any of the circuits in the diagram have no continuity going both ways(switching the test leads pos. neg.) the block needs to be replaced.
On mine the first terminals I tested were bad.
I could flex the block and get continuity. I popped the cover off and all the diodes were good. They had broken connections to the terminal blades.
The diode block is a creative way of eliminating some wiring.
The neutral safety switch, start relay, PTO switch, seat switch, parking brake switch, 2 safety relays all go through or are connected to the diode block.

Sorry for the long rant.
"And I know that this post is useless without pictures".
Try Googling "JD4400 electrical diagrams". There are some pics.

Beenthere. I saw some of your diagram postings there. Good ones.