12v conv. Cub not charging

BearGap

Member
1950 Farmall Cub with a 12v alternator conversion. It has worked well for years, but now is not charging. Alternator tests "good", and a replacement new alternator also fails to charge. 12v is present from the battery at the main 3-wire alternator lug and one of the other 2 pins, plus the second pin has 12v through a diode that is only on when the ignition is on. Battery is good, but after tractor starts voltage drops about 0.3v and the ammeter indicates about a 1 amp discharge. Fan belt os OK and is turning the alternator. What am I missing here? I had previously noticed the ammeter would "bounce" around a 10a charge in the past, and even saw that briefly during my testing when I first started the tractor, which is why I replaced the alternator since I assumed it must be a bad relay contact or something inside the alternator regulator. Is there another regulator or relay somewhere? This conversion was on the tractor when I bought it 25 years ago, but it looks pretty much like others I've seen.
 
Start with checking battery voltage when not running. You should have 12.5 plus if it is full charged. Then check battery voltage when it is running. You should have 13-14 volts. Also try the magnet test. Take something small like say a bottle cap off your beer bottle and see if it will stay on the back side of the alternator center area. If it will not stay then the alternator does not get excited like it has to and maybe the diode is bad.
 
Have you checked the ammeter to see if it is not an open circuit possible after all the yrs overcharging it just mite be an open circut
 
While an ammeter has certain advantages, when in doubt if a battery is being charged, I place a good quality voltmeter on the battery. If its full charged at rest and stabilized engine not running it should measure 12.6 volts (six cells each of 2.1 volts) subject to temperature. Then I fire up the tractor and at fast RPM IFFFFFFFFFFF the charging system is working battery voltage SHOULD RISE to at least 13 up to 14+ volts subject to the battery and the voltage regulation. Many times an alternator on an old tractor isnt spinning fast enough or the belt may be loose or slipping which causes poor or no charging. Many auto parts houses will test an alternator for free..

John T
 
Thanks for the replies. Yes, the battery reads 12.6 but drops to 12.3 when the engine is running. The ammeter seems to be working correctly. The alternator was tested at the local Advanced Auto place and reported 14v out and was determined to be good. I have also swapped it with a new one and get the same symptoms. The diode was replaced "just in case", but no change in symptoms and does test correctly in circuit. The voltage goes up (although seems to be losing about a volt somewhere through the system) on the lead that is hot from the ignition switch at the alternator. Frankly, everything checks good at the alternator except that it is not charging. A real puzzle.

Is there any issue with the ignition-sense lead being a volt lower than the lead from the battery?
 
Is there any issue with the ignition-sense lead being a volt lower than the lead from the battery?
Yes.

Normally a Delco 3 wire alternator has output lead going to ammeter and then to hot battery cable at the starter switch solenoid.

#1 (excite pin) goes through the diode to output side of ignition switch. Marked band on diode must go toward alternator.

#2 pin on alternator is the battery charge sensing point. On a tractor it often is connected to output post on alternator but can go anywhere on path from alternator to battery. Try jumpering #2 pin direct to output stud or back to hot battery post. Sounds like you have a high resistance connection somewhere in this line.
 
(quoted from post at 15:32:45 11/09/18) 1950 Farmall Cub with a 12v alternator conversion. It has worked well for years, but now is not charging. Alternator tests "good", and a replacement new alternator also fails to charge. 12v is present from the battery at the main 3-wire alternator lug and one of the other 2 pins, plus the second pin has 12v through a diode that is only on when the ignition is on. Battery is good, but after tractor starts voltage drops about 0.3v and the ammeter indicates about a 1 amp discharge. Fan belt os OK and is turning the alternator. What am I missing here? I had previously noticed the ammeter would "bounce" around a 10a charge in the past, and even saw that briefly during my testing when I first started the tractor, which is why I replaced the alternator since I assumed it must be a bad relay contact or something inside the alternator regulator. Is there another regulator or relay somewhere? This conversion was on the tractor when I bought it 25 years ago, but it looks pretty much like others I've seen.

Try this only if you can keep you fingers and other body parts out of moving parts while doing it with the engine running...

With the meter's (-) test lead on the (-) negative battery post,
use the (+) lead to check that voltages at the following points and post back with them.

Alternator "case"

Alternator's large output stud

Alternator terminal #1 ("Excite")

Alternator terminal #2 ("Voltage sense")

Post 'em and let's see what you've got going on!
 
Thanks. Yes, Pin #2 does connect directly to the output post on the alternator. Pin #1 has the diode band on the alternator side and I verified it has power at the connector when the ignition is on. There is a .6v drop across the diode, of course, and a bit more from all the old connections where were working for many years. The internittant charge indication that I got for a while suggests something, I suppose, but I just don't know what and it isn't charging at all now.

As an aside, a year ago the tractor was off to be split at a shop when the clutch bearing failed (and it is failing again now). When they split the tractor they failed to reconnect the leads to the alternator as one spade lug had come apart. I connected it back and the tractor ran, but would not turn off with the ignition, which is when I found there was no diode in the excite circuit, presumably it got dropped when the wire was pulled apart. I spliced one in and the tractor would start and shut down as expected. Before that, the ignition was continuing to be powered by the alternator when the key was off. Not sure if any of this is related to the current symptoms, but it is in the same area.
 
half a volt drop across a diode is normal. Which pin is a volt low?

What is voltage at pin 1 of plug with tractor running?

Can you turn the alternator pulley at all by hand with engine off?
 
Do you have the diode in backwards? marker band on diode toward alternator.

If engine would not shut down with no diode the alternator should be charging. With diode in backwards the alternator would not readily start up and hence engine would shut down.
 
Between the snowfall we had Saturday morning and forecast rain this week, I needed to use the Cub for several hours yesterday. I intended to just run on the battery since it was recently externally charged and the drain is not too bad when running. To my surprise, as I parked it back in the barn I noticed the ammeter was indicating a nice charging current of maybe 3 amps. My suspicion is that the 2-wire plug may be dirty, even though I measured power there when the ignition was on, and the contact with the alternator pins may be a bit flakey.

I'll take some measurements later, but now to see what is stopping the manure spreader chain and figure out when I can get the Cub clutch bearing fixed. Thanks for the replies.
 
"My suspicion is that the 2-wire plug may be dirty, even though I measured power there when the ignition was on, and the contact with the alternator pins may be a bit flakey."

Very possible. Pins on the plug can show voltage with switch on even if not contacting spade connectors on alternator.

Female side of those plug connectors are known to get loose. You can use a needle nose to gently tighten them or if any question spend three bucks for a new connector plug.
 

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