raydurr Posted March 2 Share Posted March 2 Hello. I have an overcharging condition with my 1929 Buick charging system . I have adjusted the 3rd brush as far as possible yet the overcharging persist. At the moment I would rather not remove the generator for repair. Has anyone disabled the 3rd brush completely. I can clearly see the resistor. Will removing the resistor prevent the stepped up charging? Link to comment Share on other sites More sharing options...
Bloo Posted March 3 Share Posted March 3 (edited) As you may know, these systems are completely unregulated other than current (Amps), and current is controlled by the third brush. Overcharging (and undercharging) are just part of the game. The third brush has to be set fairly low because there is nothing to turn the charging rate down once the battery is full. The battery must dissipate the extra charge as heat. It also has to be pretty low so the generator wont overheat. I don't believe any of them were fan cooled yet in 1929. 1 hour ago, raydurr said: Has anyone disabled the 3rd brush completely. I can clearly see the resistor. Will removing the resistor prevent the stepped up charging? I believe if you do either of those things it will disable the generator completely. It should. If it doesn't it probably indicates deeper problems. I'm going to guess this is a Delco generator. If so, I don't have a manual that old in front of me, but here's how I think it works. Power for the field (the winding in the frame) comes from the third brush. At the other end of the winding there is resistor between the end of the winding and ground, and a thermal switch that normally shorts that end of the winding to ground, bypassing the resistor. In normal operation it charges to whatever current the third brush is set to. If the generator starts to overheat because the battery is full, or because the generator just isn't getting cooled enough, the thermal switch opens, putting the resistor in series with the field and turning the charging rate down. This probably happens a lot. It is a bit of a kludge, but it allows you to set the third brush a little higher than you ordinarily could, and that is a good thing. If you can't get the current (Amps) down to a normal level with the third brush adjustment, I think that indicates a problem in the generator, and you would probably have to take it off if you cant troubleshoot it from the outside. Something must be shorted. Speaking of the resistor, I have seen them blown to smithereens, probably from running with the battery disconnected. Never do that with a third brush generator. It can burn out the field coil, and fairly quickly. On the Delcos I have seen with bad resistors the field coil was fine so apparently that resistor gave its life to save the field coil. Edited March 3 by Bloo (see edit history) Link to comment Share on other sites More sharing options...
raydurr Posted March 3 Author Share Posted March 3 Bloo , my generator is a Delco 940M. Shown are brief descriptions of the 3rd brush and how it works. 1 Link to comment Share on other sites More sharing options...
Oldtech Posted March 3 Share Posted March 3 If you can't get the rate down I would be suspicious of the thermostat business. IF that isn't opening up it WILL charge too high. I don't really have any suggestions how to check it except that the points aren't stuck and the heater winding is good. Someone may have a document.... Link to comment Share on other sites More sharing options...
Mark Shaw Posted March 3 Share Posted March 3 (edited) I stand corrected; The cutout should protect the battery from discharging by disconnecting the battery from the generator when the generator is not charging. Edited March 4 by Mark Shaw (see edit history) Link to comment Share on other sites More sharing options...
Bloo Posted March 3 Share Posted March 3 Cutouts are not regulators, and only disconnect when the generator is not charging. If the cutout did open while charging, the smoke would come out of the generator. Third brush generators keep charging at whatever they are set at once the battery is full. The battery must dissipate the unwanted current as heat. About 6-8 Amps is a typical setting on a third brush system, but it depends on the driver and how much the accessories are used. The way the manual is worded is confusing. The currents listed sound like bench test currents, and third brush generators usually have such a test current listed in the manual, but this is to prove a generator good or bad, not to set one for everyday driving. I'd try setting it to 6 amps after the thermal switch opens, and see how it goes. I fear we are getting off on a tangent though, @raydurr's original post said moving the third brush did not help. I'm not sure how that is happening. I believe the third brush is on the positive side in this generator (but I don't have the book!). Moving the third brush as far away from the closest main brush as it will go should make the current too low. If that is not possible, then either the generator has to come off, or we need to figure out if the generator problem (a short?) can be cleared with the generator still on the car. 2 Link to comment Share on other sites More sharing options...
Brian_Heil Posted March 4 Share Posted March 4 (edited) Bloo, the 3rd brush is on an adjustable arched slide with a set screw to lock it in the position desired. The lowest I can get my 3rd brush 1923 to regulate to is ~8.5 amps down the road. I can adjust it up to 20 if needed. I run a halogen bulbed vintage spot light that consumes about 7 amps. Nice to have the added safety of a day time running lamp. It's quite bright. The remaining ~1.5 amps is tolerated by the battery well. My solution. Edited March 4 by Brian_Heil (see edit history) 3 Link to comment Share on other sites More sharing options...
Brian_Heil Posted March 4 Share Posted March 4 1 Link to comment Share on other sites More sharing options...
raydurr Posted March 4 Author Share Posted March 4 The manual describes most everything in terms of amps. My problems is high volts when operating. I can see 9.5-10 volts at driving speed. I checked actual volts after I started losing the lighting bulbs. I wanted to try to disable the 3rd brush as an attempt to lower the volts. 1 Link to comment Share on other sites More sharing options...
Bloo Posted March 4 Share Posted March 4 (edited) Volts don't mean much as there is no voltage regulation. You can't set voltage anyway. If you disable the third brush the voltage will be lower, but that will be because all charging stops. My advice would be to make sure the thermal switch is not stuck, and move the third brush as far away from the nearest main brush as it will go. Edited March 4 by Bloo (see edit history) 1 Link to comment Share on other sites More sharing options...
Brian_Heil Posted March 4 Share Posted March 4 Never had an over voltage issue. Can’t help. My expert is Jason Smith who runs Automotive Electrical Rebuilders He does excellent work on all the early starters/generators/horns/motors 3 Link to comment Share on other sites More sharing options...
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