Ford 3910 stumped by electrical fault

[duncan_hardy]

I am struggling with an electrical problem on a Ford 3910 (diesel) that I can't figure out.

Symptoms: when you turn the key switch ON the dash lights up. At this point if you apply any electrical load (eg trying to start it, use the lights, indicators or horn) the panel lights go out and everything is dead. It stays dead for 5 mins or so then the dash magically relights but if you apply any load it goes dead again. This cycle can be repeated endlessly.

At the suggestion of "experts" I have replaced:
- the battery
- the ignition switch
- the flasher relay
No difference, so having wasted money on experts I'm trying to trace the fault myself.

I tried disconnecting the instrument cluster and alternator and pulled all the fuses. If I re-enable any circuit like (say) the side lights and try using it - it all goes dead.

I have established that normally there are 12.75v at the main power feed on the back of the ignition switch. When it goes dead there are 0v. So I tried connecting the switch directly to the battery. Everything then works. So assuming there was something wrong in the supply side, I pulled the main loom connector off the solenoid and connected it directly to the battery. That doesn't work, same "goes dead" issue.

So I appear to have narrowed it down to a fault in the power supply from the solenoid to the ignition switch. I would have thought this is a direct connection but as this tractor has a cab its route is hidden and I can't find a wiring diagram.

Anyone got any bright ideas? Is there a direct power connection from solenoid to switch or does it come via something else like a fuse or relay? If I were to rig a direct connection to the switch from the battery (through a 25A fuse) is that likely to break anything?

 

[kthompson]

Often the simple is the issue, even if you replaced the battery it still could be glazed over in the battery clamps, could be battery cable connection to starter if I followed your post correctly. I would begin with removing each of the positive battery cable connections and cleaning them all the way to the starter and fuse and on to the switch. Based upon your wiring direct to the battery it would seem the negative cable is not the issue but some where the positive system has a poor connection. I had issue with clamps on battery cables once that about stumped me as they looked good but had oxidation heavy enough to prevent good connection from time to time. You also could have a broken wire or damaged inside a connection, maybe poor crimp or such.

 

[RickB]

Bad battery cable connection or a bad cable. Not limited to the positive cable, check both.

 

[sawtooth]

Also check where wire connects to alternator. Mine did similar thing and ended up being bad connection at that point. I think the other guys are correct though as it sounds like bad connection somewhere.

 

[SPYDERLK]

You can check the battery cables with a voltmeter. Put a lead probe on each end of the cable. Power off the meter reads 0. Power on it should still read very near 0. If it reads 12 theres a problem in that cable.

 

[DLTTrekkers]

Check ground cable at bat and body !

 

[guyw]

You can check the battery cables with a voltmeter. Put a lead probe on each end of the cable. Power off the meter reads 0. Power on it should still read very near 0. If it reads 12 theres a problem in that cable.

Also, as has been found many times in other threads, a battery cable can look good using the multimeter test when there is no load and still be bad when under load (internal corrosion, strands broken, bad crimp etc).

Guy.

 

[SPYDERLK]

Also, as has been found many times in other threads, a battery cable can look good using the multimeter test when there is no load and still be bad when under load (internal corrosion, strands broken, bad crimp etc).

Guy.

Yes. - - Thats why you use the volt function to test the voltage drop on the cable under a load, rather than an ohm setting to test its quiescent resistance.