Starter Relay and How to troubleshoot starting issues

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How to connect the starter relay and how to troubleshoot a car that does not start

Starter Relay Diagram

A. How the starter relay works


This article covers only starting issues related to the starter, ie the starter/engine will not turn, but push starting the car works. 

It is on the example of my 64 Mustang, but should be valid for for all Classic Mustangs and other American Muscle Cars from the same period work similar.


Here’s the easy schematic on starter solenoid there are 4 connectors.

Starter Relay Diagram

The two outer ones (BIG cables go there),
and the two inner ones (small cables go there).

The inner one's are labelled with “S” and “I”

Most people call this a Solenoid, I prefer the term Starter Relay, as that describes it better. As well there is a Solenoid in the starter that will push the spindle out once you start and this way the two of them can be easily confused.


Now when you call this thing a starter relay it's easier to understand how it works.

For starting you want the starter to get 12V+ at the starter. A lot of amps, hence a heavy gauge cable.
It is the one connected to the relay on the right hand side.
Now if we hadn't got the relay we would have to route those big things to the ignition switch and from there to the starter. No one wants that for obvious reasons (as well as cost on production).
Hence the idea of a relay: The relay is an electronic switch and some of them can switch BIG power. The starter relay is one of them.
If you get a tiny bit of 12V+ to a plug at the starter relay it will connect the big 12V cable from battery directly to the starter.
This connector for activating this switch is the “S” port (S for start).


How do the 12V get there?
Basically there's 12V+ going to your ignition switch. When you turn the ignition switch to start it will power a little cable connected to the switch with 12V+ that connects to this “S” port. Once we have 12V there the starter will get it's power directly from battery.

left big connector: battery plus
right big connector: starter (there is only one connector on the starter). It get''''s it''''s minus/ground from the engineblock.
small connector “S”: Ignition switch “S” connector (goes through firewall connector)

The 4th connector (“I” for ignition) has only a simple purpose:
Factory coil is getting 7.5V during run. This is not enough for starting the engine (works fine once started, but problematic _during_ the starting process).
So when you start the engine the “I” port gets 12V+ as well which leads to coil+.

Only low amps needed hence a thin cable.
This is not needed for the starter to be able to turn.

Here’s the starting troubleshooting process. It's very easy to follow once you've the above said understood.