Manual says don't disconnect battery with engine running. Why not?

[stuckmotor]

I ran my MF 261 for a couple of years with a cheap replacement battery terminal clamp that occasionally slipped loose from the positive cable. I simply reattached it several times and the tractor cranked right up. Now it only cranks if I use the battery out of my truck. While prepairing to trouble shoot the problem I ran across a note in the repair manual saying not to disconnect the battery with the tractor running and think that, the cable working loose from the clamp with the tractor running did effectively that. So, my question is : How can disconnecting the battery with the tractor running harm the charging system or can it? Maybe the caution was in the manual to keep me away from the spinning fan? The MF 261 is a 1997 model with few eletronics that I know of.

 

[Oldtractorfixer]

I always heard it blows the diodes in the alternator. No where for the current to go. A short time swap may not do it but running for any length of time and at higher rpm will.

 

[RNeumann]

The battery is like the holding tank in the water system. Without it the alternator would cycle on off very rapidly until failure. On the tractor side all the electrical equipment would see low amps and low or no voltage while the alternator constantly adjusted.
Much like the water example- when you turned the faucet on a pump would have to kick on which would cause a delay. Then the pump is way to big so it just cycles on and off so it didn't over pressure.

 

[RandyT]

Disconnecting the battery while it is running causes voltage spikes in the system, and electronics which include the diode in the regulator don't like voltage spikes. Good example is on the newer lawnmower engines with EFI There has been cases where people would connect jump packs to start the mower, and then disconnect and it would instantly cook the ECU which is around $800. They started putting diodes in the electric clutch wiring because turning off the electric clutch will cause it to act like a generator, and cause voltage spikes, and has lead to cooked ECUs I know of one case on a Kohler engine were they had the ECU troubleshooting software connected and turned off the clutch and cooked the ECU and the technicians laptop computer.

 

[Oldtink]

Here is some great info on how alternators work Pirate4x4.Com - The largest off roading and 4x4 website in the world.
Surges would be the demon in doing so. There is a good chance it would ruin the voltage regulator in the alternator and any associated electronics in the vehicle.

 

[stuckmotor]

Many thanks to everyone who responded to my question. I hate being told not to do something without being given a reason.
Stuck

 

[MultiMow]

I'm assuming it has an alternator, not a generator.

An alternator is only designed to keep the battery full... not for direct load supply.

 

[3930dave]

With modern electronics on-board, yanking the Batt cable can be like playing Russian Roulette...

Automotive grade electronics should be designed to withstand a decent amount of surge energy....let's call it something like 40 volts. One of the reasons for this is a "load-dump" scenario, even with wiring intact. Flat battery in a vehicle, gets boosted, car drives away - alternator is putting out high current to bring battery up fast - while it is rare, it is possible for a battery to suddenly Open Circuit - resulting in a significant system-wide voltage spike.

The small engine problems listed here sound like either incomplete designs, or these weaknesses had been "cost-optimized" in on purpose. Inductive clamping diodes are always important, even a very small relay can create enough of a voltage spike to reset a nearby micro-controller, if left un-clamped.

I'm overdue on worrying about paint on my olde 3930, but failures described here remind me to appreciate it for what it is.....

Rgds, D.

 

[AxleHub]

Think of a gate on a fence. Today's computer chips and electronics boards etc. can be highly durable when that gate is opening and closing the way its designed - but they are incredibly fragile when that gate is forced in the other direction. Often times the whole concept of surge protection is based on stopping power from further transferring in the wrong direction - that is done by "blowing the circular path". In other words save and protect the most important components by self sacrificing the less important components.

On another issue of the same topic - lets say you remove the positive cable while you are leaning over the unit and have your hand or a tool you are holding - in contact with metal that is grounded. That sends alternator amperage thru you to the frame - not a good plan.

 

[ishiboo]

Think of a gate on a fence. Today's computer chips and electronics boards etc. can be highly durable when that gate is opening and closing the way its designed - but they are incredibly fragile when that gate is forced in the other direction. Often times the whole concept of surge protection is based on stopping power from further transferring in the wrong direction - that is done by "blowing the circular path". In other words save and protect the most important components by self sacrificing the less important components.

On another issue of the same topic - lets say you remove the positive cable while you are leaning over the unit and have your hand or a tool you are holding - in contact with metal that is grounded. That sends alternator amperage thru you to the frame - not a good plan.

As mentioned, it's the diodes in the alternator.

This is very common in boats which have a battery switch with multiple batteries to select. "Break before make" switches disconnect one battery and reconnect the second later as you turn the knob, killing alternator diodes.