Many new replacement units are coming supplied with solenoid control relays. These are the small solenoids mounted closely to the starter solenoid. The 39MT series Delco come with the control relay mounted to the starter housing.
When replacing an older model that does not have the solenoid control relay installed, or if the control relay is removed, wiring it may become confusing. Following is the wiring diagram for the common solenoid control relays installed on Delco 39MT insulated ground starters. Other models such as the 38MT, Mitsubishi, etc. have similar relays some using case grounded starters and relays that do not have dedicated ground wires.
The control relay is powered from the main starter solenoid battery post. When the control relay gets a start signal from the ignition switch, power is transferred through to the main starter solenoid switch post. Both the control relay and main starter solenoid are grounded back to the starter ground post in the back cap.
The main reason for using a solenoid control relay is that the signal to the main solenoid switch post is much stronger. The relay is mounted close to the battery source (main solenoid battery post) and the switch it is feeding (main solenoid switch post) ensuring there is as little drop in power from wiring as possible.
The standard switch circuit may become quite long, routing from the battery through a fuse panel and the key switch and eventually ends up many feet long. This distance as well as connections in between may lead to a weak signal when it finally gets down to the starter switch post. The main starter solenoid not only transfers power through to the starter, but also physically engages the starter drive requiring a strong, solid signal. Use of a control relay ensures that the main starter solenoid is getting the strong signal it needs, and since the control relay does not physically move anything, it can function on a weaker signal. This prolongs the life of the main starter solenoid.
The solenoid control relay does not have to be mounted to the starter, but the closer it can get the better.
This wiring diagram has been expanded in Blog Post 3/15/17 showing OCP wiring.
*This is a basic wiring guide and will not cover every application or scenario. Always use caution when modifying a system. While Smith Co Electric deems this information accurate, we are not liable for problems arising from use of this information.