Ignition circuit fuse burns

[lacamo]

Tractor: MF 275

The fuse in the ignition circuit blows several seconds after starting, probably once the alternator starts charging. (The tractor continues to run). The schematics indicate an excitation diode aligned so as to prevent the alternator charge going back through the ignition circuit: this is missing, which *might* explain why the fuse blows. However, I've had the tractor for 2 years and not seen this. I've disconnected everything else (lights, gauges, etc) and checked for shorts in the remaining cables. Any ideas ???

[BB_TX]

Some questions;
What is several seconds? 5? 10? 15?
How do you know when it blows?
What are the symptoms? Alternator light coming on?
A sudden problem not seen in 2 years indicates something new has failed, not an old problem just showing up.

[ByronBob]

I'm not an expert on this but it sounds like you've done some good homework on this already and you have a diode you haven't changed yet. Because the ignition only has so many things which can go wrong I would think changing the diode is a good idea. The alternator is the only thing which could put a load on the ignition so either the diode is bad or the switch itself.

[lacamo]

Actually, it's when the oil pressure switch closes and current is fed to the alternator to start charging, about 2-3 seconds. I just spoke with a mechanic who specializes in ignition systems and he told me that the diode is probably not necessary (he said an older JD he worked on had one, a newer one, same model, did not.). His opinion was that there was a short in the alternator, possibly a bad diode which can be repaired. Thanks for the help.

[lacamo]

Well, I got home and put a volt meter to the battery: 13.8V for 10 minutes. Then I read the output from the alternator: 14V. Guess that means she's charging OK. Maybe I will try that diode.

[BB_TX]

If you can scan your schematic and attach it, we might be able to make some more guesses.

[Soundguy]

On the gas powered jobs.. the diode was there to prevent power being backfed from P1 ( external excite ) to the ignition system.. thus keeping the tractor running.

Your oil sensor will disconnect the line once the engine is off.. and if it is a diesel there is no 'ignition' being maintained.. however.. if there is a fuel solenoid.. the alternator could keep the fuel solenoid energized.. preventing you from shutting off with the key switch.

A diode being bad in the alternator.. might be on the rectifier pack.. but that is for the output.. and that would manifest itself by the battery being drained back into the alternator after the engine was off, and then the battery would exentually die.. As far as I know.. the older alternator schemes.. and even current ones.. like the delco 10-si don't have an isolated P1.. so trhings like diodes, idiot lights, and isolated tabs on the ignition switch are used as isolation devices, allowing you to cut power to the ignition, and not have P1 supply that ignition power. This doesn't mean that something in the alternators VR couldn't have died.. anything is possible with electronics..

I'd at least pull the diode to check it.. if you can locate it.. or, if you are sure the wireing has not been modified to not need it.. then add it.

Soundguy

[BobReeves]

My logic says if it ran for two years trouble free without the diode. Something has changed/broke/burnt to cause the fuse to start blowing.

It would be a big help if we could see a schematic or wiring diagram.

[slowrev]

Seems like 14 Volts for a running alternator is about 1/2 volt low....

[lacamo]

I replaced the difficult-to-read ammeter with a spiffy new one -- can now see a nasty current spike 1-2 seconds after I connect the excitation line (oil pressure switch -- alternator).
Clearly, there's something nasty going on with this Delco 10SI, so I'm taking it to the local alternator expert tomorrow. Sure hope I can be back in the saddle soon: need to get that manure down before the grass gets to high.
I'll let y'all know what's up.