I could not find a 1955 GMC wiring diagram showing the separate control relay. I was intrigued because the 54 Chevy truvck with Hydramatic trans. does not have a separate control relay. (check 2nd pic) The closes I could come to a wiring diagram with the separate control relay was the 1953-54--55 Corvette. (1st pic)
I think they only used a separate control relay in vehicles with a automatic trans. and with a spring to start type switch. The neutral safety switch, ignition switch, and the solenoid on the starter are wired to the separate control relay.
I think the reason why they didn't use the separate control relay in the 54 Chevy truck with a Hydra-Matic trans is because they used a high current push to start button type switch..
The 1955 GMC 2nd series truck and the 1954-55 Corvette used a keyed ignition switch, which is not designed to handle high current.
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