Ammeter reading on start up takes rev to start charge

B

Bradfordnj

Junior Member
Is it normal that my ammeter reads 0 on start up and that it takes revving the engine for it to start to charge. It almost acts like it takes some rpms to kick it over into a charge. After the rev at idle it keeps charging when at the slow rpm. I assume it's the cut out doing its job? Didn't really notice this for the first year and a half that I had the car..was not sure if this was a hint that the cut out was going bad or if this is the generator going bad.

By the way, long time user of the forum that has saved the day many times but this is my first post. I am the proud owner of a 29 model A Tudor and 23 model T roundabout!
 
Brad,


The theory from the Service Bulletins after the wiring change in November '29, upon engine cranking the ammeter needle will wiggle showing the points opening & closing.


So much for the theory, I've got the same lazy off the mark ammeter as you describe. Originals are almost 90 years old, so they are a little less attentive?


As you, I'll be reading other's thoughts.


JB
 
This is a wll known phenomenon with alternators, parcticucularly those wired for one-wire usage.
 
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Yes, car has a generator. I have a spare new cut out, and hope it’s that easy a fix. What makes people think it’s the Generstor? Is there a way to test the generator?
 
Simple check is to put a voltmeter ACROSS the cutout on startup. Proper operation is the cutout should close as soon as the engine starts. If not then take some voltage readings on both sides of the cutout in reference to ground. Possible the generator output isn't coming up until rpm is increasing. Let us know what the voltage readings are and we can help you.
 
Okay, I will try some readings. I am away until next weekend so I cannot check until then. Could you be very specific on how to conduct the readings? I am really hoping that it’s the cutout as I will be able to fix that quickly.
 
If the cutout has been messed with, the weak Genny output at low engine speeds may not be able to overcome the cutout spring to pull the contacts together. Like, if someone put in a stronger spring.?. But, more likely is that the Genny has too weak an output, which could be a brush issue, (best case), or an armature issue (worst case). Since you have a different cutout on hand, try that first, but then if nogo, get the Genny worked over.
 
Original ammeters without a bent needle, and with a drop of oil on each pivot point will work just fine. I only use original cutouts, and adjust the contacts to close between 6 and 6.5 volts. I've found some cutouts don't close until they see about 17 volts, and this takes more than 500 RPM. At 500 engine RPM, the generator should start producing enough voltage to charge the battery.
 
Good rebuilt generator, with Tom's EVR in place of the third brush, using a diode cutout after you snip the little wire on Tom's EVR, and an original ammeter...... can't beat this combo.

Do this and your troubles will be over. I've been running this and never looked back. Even keeps the battery fresh and it will last for years.
 
Use an analog voltmeter preferably. A good quality DVM will work as well. First put the Positive lead on chassis ground, make sure it makes a good connection, no paint. Then, before you start the engine, measure voltage on each side of the cutout. Should see battery voltage on the back of the cutout, 0 V on the front terminal. Now start the engine and idle, check voltage on each side of cutout. Now put the meter leads ACROSS the cutout, one lead on each side of the cutout and take a reading. Now run the rpm up to where you see deflection on the ammeter and hold it there (RPM). Again put the red lead back onto a good chassis ground and take reading again on both sides of the cutout. Write all readings down and report back. We can then give you an intelligent response to your question. All other is just simple guesses. I would definitely recommend AGAINST starting to replace parts until you diagnose.
 
eagle said:
Should see battery voltage on the back of the cutout, 0 V on the front terminal.
Click to expand...

Should see battery voltage on the wired (yellow/black) side of the cutout, 0V on the armature (generator) side.

Front/back is ambiguous, although most are connected as you say. My early 30 coupe, for instance, is the reverse. See Jim Schild's Shop Manual, p.148, for a pic.
 
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