12v Generator Not Self-Exciting

RTR

Well-known Member
I have a 1975 Farmall 140 tractor and it has the original 12v generator with voltage regulator setup. I have installed a new voltage regulator and had the generator rebuilt. The system charges well, but for some reason it stopped self-exciting and charging on its own. If I take a jumper wire and jump the BAT and GEN terminals on the regulator, it will charge with no problem until I stop the engine on the tractor.

What is my problem and how can I fix it so I do not have to jump those terminals each time I crank the tractor?
 
Try this... before starting, use a jumper wire to ground the "F" terminal at the generator, then start and note if it "excites".

(Only run it that way as a short test.)

If it DOES excite that would point to a voltage regulator problem.

If it does NOT excite, likely there's a damaged/partial shorted armature or field winding that works well enough to charge but not well enough to begin to charge with the very low residual magnetism left in the field polepieces.

When you polarize it, you bump up the residual magnetism a bit and the generator can then start the charging process.

One thing I wonder about... if the field "shoe" retaining screws are not simply darned tight and/or there's rust between the pole shoes and the generator's "belly", it may affect self-exciting.
 
That jumper is the way to polarize a type-A generator, when engine not running. If you are jumpering with engine running, then I suspect that generator is just fine and you still have a regulator problem. To verify, disconnect the wires from generator Armature terminal and Field terminal.. Then connect Field to Ground and have jumper ready to connect Armature terminal to battery post as soon as you start tractor and watch voltage. If voltage rises over the next minute with engine at speed, then generator is working & VR is the problem.
 
RTR, welllllllll if all was well, once the genny starts producing sufficient the Cutout Relay, which is essentially between the VR's BAT and GEN/ARM terminal, closes so the output can get to and charge the battery yet otherwise its open/disconnected.

HOWEVER you have to manually make the BAT to GEN/ARM connection after which you say it charges !!!! But I wonder how much or how well its actually charging. Maybe a few amps when it should be higher ?????

When the BAT is jumped and she's charging try to temporarily dead ground the Gennys FLD post and see if she charges MORE amps then ????????????? If so,I suspect either the VR isn't well grounded (insure that) orrrrrrrrrrrr the VR is at fault. Next before you start it dead ground the FLD and see if it will start charging and the cutout relay closes on its own no manual jump required ???????? If so I again suspect a poor VR ground or the VR is at fault.

If you look at my Troubleshooting Procedure under the MOTOR TEST (two part test) it can help find if the Field windings or something else may have a problem.

When any genny or other charging system work is performed I always polarize the genny BEFORE start up just to be safe.

Work through my Troubleshooting Procedure linked below, including the Motor test ?..

John T
John Ts Troubleshooting
 
I am assuming that you placed the jumper and removed the jumper before starting the tractor.

If you left the jumper on while the tractor is running then it may have a bad regulator or generator. Use JohnT's guide to sort it out.

If you removed the jumper prior to starting the tractor and it then charges look for a high resistance connection between the regulator and the field terminal, poor ground at the regulator, or a bad regulator.

However if you ground the field terminal prior to starting the tractor and without first using the jumper and it still does not charge then look at the generator for the problem.

A shorted armature or field can cause those symptoms you are describing i.e. needing to repolarize the generator prior to starting, but more than likely it is either worn brushes or weak brush springs allowing for higher than normal resistance. If the generator is in a position where you can get at the brushes safely while running, try pushing on the brushes with a small wooden dowel rod. If it starts to charge then remove the generator and fix the brushes.
 

This is the procedure I followed; but it is important to Note that I DID NOT leave the Field grounded like it says on that side note in the attachment. I didn t have an alligator clip for that. All of this is done while the tractor is running. I ll try polarizing while it is shut off and go from there like you said to.
 

Attachment procedure.

mvphoto45253.gif
 
Screw the procedure, do what I told you to identify generator or regulator. SIMPLE.
 
(quoted from post at 17:24:51 11/14/19) Screw the procedure, do what I told you to identify generator or regulator. SIMPLE.

Sounds good. I m going to check it here In A couple hours when I get off work.
 

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