Generator over-current

9

99sh

Member
I'm having a bit of a head-scratcher moment with my generator. With the 3rd brush set all the way up my ammeter is pegged and reading close to 20A. This is with no lights or any external accessories turned on.


Questioning the accuracy of the analog gauge, I took out my digital clamp style amp meter and placed it on the wire coming out of the starter. It was showing roughly 18 amps when I had the throttle set a bit.

I did double-check the 3rd brush to make sure it's at the upmost position. Also moved it down to see what would happen (got close to 28A).

I also measured resistance from the generator housing to vehicle chassis and got close to 0-0.1 ohms. Repeated this for both ends of the generator cover. This tells me the ground is good.


My cut-out is brand new and was just replaced with a Nu-Rex diode unit. I measured the voltage on both sides of the cutout. 8.1V on one and 8.9V on the other side.


Two questions:
1. What the heck is going on? :)

2. What are the consequences if I keep driving like this? Assuming a premature generator failure, battery boiling over


I need a larger pulley to slow this thing down!
 
What condition is the battery in?
Has it just started doing this?
Has the generator been worked on recently?
Anything else been done to the vehicle recently?
 
Last edited:
99sh said:
I'm having a bit of a head-scratcher moment with my generator. With the 3rd brush set all the way up my ammeter is pegged and reading close to 20A. This is with no lights or any external accessories turned on.


Questioning the accuracy of the analog gauge, I took out my digital clamp style amp meter and placed it on the wire coming out of the starter. It was showing roughly 18 amps when I had the throttle set a bit.

I did double-check the 3rd brush to make sure it's at the upmost position. Also moved it down to see what would happen (got close to 28A).

I also measured resistance from the generator housing to vehicle chassis and got close to 0-0.1 ohms. Repeated this for both ends of the generator cover. This tells me the ground is good.


My cut-out is brand new and was just replaced with a Nu-Rex diode unit. I measured the voltage on both sides of the cutout. 8.1V on one and 8.9V on the other side.


Two questions:
1. What the heck is going on? :)

2. What are the consequences if I keep driving like this? Assuming a premature generator failure, battery boiling over


I need a larger pulley to slow this thing down!
Click to expand...
could damage generator!!!! quickly melting solder in gen.
 
Like Katy said, the battery acts as a regulator in the charging system. Is it a known good battery? Are ALL your connections bright and tight: battery posts, starter switch post, chassis ground, junction box, cutout?
 
I recently purchased this vehicle, so don't have all the history.


  • Battery is new dated AUG-2024
  • I did recently changed the 3rd brush in the generator, it had broken
  • I just removed the battery to chassis copper ground bar and cleaned/sanded/scuffed it up well. Hoping this would do it, no change.
  • Measured resistance between battery positive terminal post to chassis, 0.1 ohms
  • Measured resistance between the two posts in terminals box, 0.1 ohms
  • Measured resistance between terminal post to cutout battery input at cutout, 0.1 ohms
  • Measured resistance between battery + terminal to Cutout post (the generator side on right) and measured 0.4 to 0.5 ohms. I can see this perhaps being a little high? Indicating generator to chassis is not well grounded?
 
Without examining the generator in person, its hard to say. When you installed the new 3rd brush, you may have moved the wires inside the generator to the point where the field wire is getting full power from the power brush. You may have a wire with missing or compromised insulation. Also, make sure no wires are rubbing on the armature. As a last resort, you could send it to me for repair or restoration.
Steve
 
Diode type cutouts have a forward voltage drop. They don't go to zero volts. 0.8V seems a little high but may be correct, I measured one about 5 years ago and it was 0.5V I think.
 
Diode drop increases with current, so it would be higher than usual.

I suspect short between field and output wires as ndnchf indicated.
 
The context of this whole post is the winning argument for conversion to an alternator. The alternator is self regulating of voltage & current. The alternator does not need periodic lubrication. No adjustment necessary other than an occasional belt tension. The battery will last longer. Lights will be brighter. Faster battery charging. Reduce overall Model A maintenance. Convert to an alternator. Saves you money! It is a reversible nut-&-bolt change for the purists. I recommend a NuRex alternator. Every Model A that is driven regularly should have an alternator.
 

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What is the expected resistance between the two wires in the generator? I did not visually see any indications of rubbing against armature but easy to take a measurement.
 
Between the 2 wires - none. The field brush wire goes through the fields to an internal ground. That should be 1 - 1.2 ohms. The wire from the power brush goes straight to the terminal post, so should be 0 ohms. It is insulated from the generator housing, so there should be no continuity to the housing. All these measurements are with the wires disconnected from the brushes, or with insulating paper put between the brushes and commutator.
Steve
 
ndnchf said:
Between the 2 wires - none. The field brush wire goes through the fields to an internal ground. That should be 1 - 1.2 ohms. The wire from the power brush goes straight to the terminal post, so should be 0 ohms. It is insulated from the generator housing, so there should be no continuity to the housing. All these measurements are with the wires disconnected from the brushes, or with insulating paper put between the brushes and commutator.
Steve
Click to expand...


Based on this my, last bullet in post #5 is the culprit, correct? I should have full isolation between the terminal post to structure.
 
Remove the cutout, and generator terminal post wire from the power brush. That will isolate the terminal post and wire. You should have 0 ohms from the terminal post to the flag terminal on the wire. Then being sure the flag terminal is not touching the body, check for continuity from the terminal post to the body. There should be none. If you show any continuity, the there is a breakdown of the terminal post insulator or the wire insulation.
 
In the Model T generator the whole brush assembly rotates so that all three brushes can be moved to adjust the current. Once the brush assembly is set in position, the third brush can be moved independently of the other two brushes to make adjustments for different driving condition, such as night versus day.
 
Check connections in the whole battery circuit including a fuse if it has one. Make sure all are serviceable. If you have a sensitive analog amp meter then put it in the line between the generator and the terminal box to check and adjust output. I'm curious as to what type of battery the car currently has. Any 6-volt wet or AGM lead acid type should be OK. Make sure it is positive ground for the system as intended. I wouldn't change polarity unless it is converted to 12-volts. Modern batteries that are other than lead acid are not recommended.
 
Quick update, I removed the generator from the car and did a full inspection, continuity and isolation test. Everything checked out the nominal. I took the generator to a local shop and had him test it out on the test bench. It was putting out about 16V, he then did a motoring test and commented that this generator is spinning really quickly. He's going to open it up and swap the fields with another unit and troubleshoot further. I'll report back here, but I'm glad my car is exonerated!
 
99sh said:
^ we talked that. But 16V? It shouldn't be putting that out even with the wrong fields
Click to expand...

Yes. Unregulated 6v generators often put out 8.5 - 9v. Unregulated 12v generators put out 16 - 18v. A voltage regulator would control the voltage. 3 brush generators are only partially regulated by the battery.
 
It may have been converted to a 12v generator in which case the armature would be different also. Have him check against another, if available.
 
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