installing a alternator

This message is a reply to an archived post by hinton on May 12, 2010 at 09:37:08.
The original subject was "Re: installing a alternator".
Thanks for your work. I have been asking around to some of the local tractor machanics. They recommend wiring to the oil pressure switch not the ignition switch. Do you have any idea what they are saying?

Generally an alternator needs a source of battery voltage to excite the fields to start the charging process. It is common to also use the same terminal on the alternator to control a warning light by sending voltage out of the alternator when it is charging. With voltage being supplied by both the battery and the alternator on both sides of the warning light the bulb will not light. If the alternator does not provide power back through the wire the light will stay on showing a problem with the charging system. Automotive ignition switches have an accessory terminal on their switch and an ignition terminal which are separate electrically. The alternator is fed through the accessory terminal. Tractors being converted to alternators usually have just an ignition terminal. When both the ignition and the alternator are connected to that terminal the power coming back from the alternator will provide power to the ignition system so the engine will not shut off even if the ignition switch is off.

Adding a switched oil pressure sender to power the alternator excite circuit separates the ignition system from the alternator so the engine can be shut off as normal. The oil pressure switch is wired from the battery from somewhere different that the ignition side of the switch. When the engine starts and the oil pressure comes up enough to close the pressure switch contacts, power is fed to the alternator and it will start to charge. When the engine is shut down the oil pressure drops and the switch opens so the alternator will not discharge the battery through that wire.

I ain't no wizard, but I think the oil pressure sender wouldn't work too hot. It seems to me that the problem with alternators is the "backfed" voltage from the alternator supplies the ignition coil and keeps the engine running even after switching it off. Looks to me like you would switch off the ignition, but the oil pressure would still be up, thereby supplying voltage to the coil from the alternator.

The "idiot light" works as a current limiting device by not allowing enough current to pass the bulb and run the ignition. Put a larger bulb in the circuit and the engine will continue to run.

Respectively stated - not to start an argument.

Paul

Please excuse me Owen. After re-reading your post, I understand what you are saying and I agree with you.

Paul

Just to add, on tractors with only a magneto kill switch and no battery ignition switch, the oil pressure switch eliminates having to add another manual switch to excite the alternator, something to be forgotten on and discharge the battery. The oil pressure switch does this automatically with not input from the driver.

A lot depends on what tractor your working on. Yes if it has a mag the oil pressure switch works very well