At first, imported products were used for the electrical components in the Model GA truck and the Model AA passenger car produced by Toyoda Automatic Loom Works Automotive Department.1 Kiichiro Toyoda explained the reason for this as follows:

For a while, we used imported products because, in order to examine the good and bad points of our products, we had no choice but to employ items with which we were familiar, rather than adopting new things at that time. For example, we used foreign products for components such as carburetors, plugs and oil brakes.... For the time being, how good Toyota's cars could be was unclear. However, once our automobiles left Toyota, we had to take full responsibility for any faults. Not being able to escape responsibility for faults in our own produced parts of the automobile by blaming other parts of the vehicle was the most important thing for gaining self-confidence in our products.2

Investigation and research into electrical components was commenced under the direction of Kiichiro when the Model G1 truck was launched at the end of 1935.3 The electrical engineers researching motors for spinning frames in the spinning and weaving design department started by analyzing and sketching the Delco Remy products used in the A engine for the Model G1 truck, including generators, starters, distributors and ignition coils. At the time, the production of electrical components in Japan was quite advanced.4 Nevertheless, Kiichiro decided that there would be in-house development of electrical components because he thought that it would be better for the company to obtain and develop its own electrical technology through independent efforts.

Based on the drawings made in the spinning and weaving design department, production of prototype electrical components commenced in a corner of the research plant and the first prototype generators were completed in October 1936. Around that time, in September 1936, Toyoda Automatic Loom Works was designated as a licensed company under the Automotive Manufacturing Industries Law, and was required to use Japanese product components after 1938. For that reason, the in-house development of electrical components was expedited but, as it was considered that such efforts would not be sufficient, orders were also placed with a specialized manufacturing company.

In pursuing this course, after experimenting with samples from six electronics manufacturers5, it was decided to place orders with Hitachi Works, Ltd. Electrical components produced by Hitachi6 were similar to the Delco Remy products in the drawings made by Toyoda Automatic Loom Works, with dimensions were almost the same, apart from a small degree of error arising from the conversion from inches to millimeters.

In November 1936, a 600 square-meter electrical components plant was completed on the west side of the engine plant7, and trial production was commenced in January the following year. Procuring materials was a problem in the manufacturing of the prototypes. It was very hard to find manufacturers that would accept orders for small quantities of special-sized electrical wires and Bakelite (phenol resin) molded components. Also, distributor condensers were produced in Japan but, as there was no locally-made condenser paper, imported products were used. Insulation layer paper for ignition coils was manufactured from Japanese paper soaked with insulating varnish, and enameled wire insulating varnish for secondary coils was developed using special quick heat-drying varnish made with tung oil to avoid wire breakage and insulation failure.

The prototype products were installed in engines in the neighboring engine plant and trial running tests took place. However, various malfunctions occurred. These included a generator flying apart due to an output shortage and the centrifugal force of the commutator, damage to starter gears, shafts, springs and other components, ignition coil burnout and wire breakage, and a defective distributor advance. Despite repeated failures, improvements continued, and by around July 1937 the company was able to produce electrical components that could be utilized.8

In November 1938, Toyota Motor Co., Ltd. was relocated to the Koromo Plant, and the electrical components plant was moved to the Kariya Plant, which was previously the automobile assembly plant. The components plant then operated at that location together with the remaining truck body plant and the radiator plant that had been transferred from the Tokyo Shibaura Plant. As the copper, brass and solder materials used in radiators were common to electrical components, the electrical components plant was also put in charge of producing radiators.9

In February 1943, preparations began for the transfer of the electrical components plant to the Kariya-kita Plant (the Kariya components plant)10 which was leased from Chuo Spinning Company. However, circumstances completely transformed, and Toyota Motor Co., Ltd., which had been proceeding with preparations for producing air-cooled aircraft engines at the Koromo Plant since the start of the year, started to produce them at the Kariya Plant. After that decision was made, the radiator plant was transferred from the Kariya Plant to the Koromo Plant in September 1943 and, in October, the machine tools for aircraft engine production were all brought from the Koromo Plant to the Kariya Plant. Finally, the electrical components plant was transferred to Chuo Spinning's Kariya-minami Plant11 in November of that year.

After the war, in October 1946, Toyota Motor Co., Ltd. created the Spinning and Weaving Department, and resumed spinning and weaving operations at the Kariya-minami Plant. Later, following an examination of the splitting off of the Spinning and Weaving Department, the electrical components plant was moved from the Kariya-minami Plant to the Kariya-kita Plant in October 1948. At the same time, the radiator plant was transferred from the Koromo Plant to the Kariya-kita Plant, and both plants were together renamed the Electrical Components Plant. The Electrical Components Plant was then spun off from Toyota Motor Co., Ltd. on December 16, 1949, and Nippondenso Co., Ltd. (currently Denso Corporation) was established.