This week we are blessed to have a very instructive article written by Ron Patterson and Bob Cascisa.
Over the years I have had the opportunity to help many with Model T electrical system problems. I discovered that many Model T Fords were incorrectly wired. In some cases those cars were being driven and the owner thought everything was working correctly. I quickly ceased believing anything anyone told about their car and developed some simple tests that will validate if the wiring is correct so one could rely on the dash Ammeter to provide reliable data when looking for problems.
I suggest you first read the article “Model T Ford Battery Charging System” to fully understand how the charging system operates. This article can be found
Here in the Model T Ford Fix website (modeltfordfix.com).
The basic function of the Model T Ford Charging System is to maintain the Battery at a proper level of charge to provide sufficient electrical power for the starting motor, ignition and headlamps while providing an Ammeter indication the system is working properly. The Ammeter indicates the net power in the electrical system. If it indicates a Charge, the generator is providing sufficient power to meet the electrical demands (Ignition and lights) and to keep the Battery charged. If it indicates a Discharge, the Generator is not providing enough power to carry the load and keep the Battery charged. A dead or weak Battery will result by driving the car with the Ammeter continually showing a discharge condition.
When working on Model T wiring it is important to be sure you are using a correct wiring schematic. Many commonly available wiring schematics are incorrect, particularly in the charge/discharge circuit wiring. Additionally, many reproduction dash Ammeters do not have their polarity clearly marked + or – on the terminals and as a result is commonly wired incorrectly. These wiring errors can allow the Ammeter to show only discharge current and no charge current or vice versa. I recommend you use the wiring schematic in Figure 1. This wiring schematic is electrically correct for the starter-generator equipped Model T.
Here is an easy way for anyone with limited electrical knowledge to functionally check the charging system and correct any trouble.