I have a 78 f100 with a 351m. I have a 4 post relay on it. I have the battery cable on the left post, then there's the starter cable on the right side. Nothing is connected to the two front posts. When I turn the key, nothing happens. It doesn't even click. I'm assuming the relay is the problem? Thanks for the help

 

Look closely at the two small terminals on the front of the solenoid. One is marked "I" and the other is marked "S".

I= Ignition

S=Start

.. I gets voltage with the ignition on. S gets voltage when start (cranked).

Review the wiring diagrams and identify the wires in the area which wires those are. You have a volt meter, right?

 

Looks like this? Maybe this will help. Make sure there is clean, unpainted metal where the bracket attaches, this is the ground point connection.

 

There should be two 16 gauge wires, one brown and one red w/ blue trace.
The brown goes to the "I" terminal, the red w/ blue trace goes to the "S" terminal.

 

I Hate those relays, always had bad luck with them. Use caution to not turn the copper thread posts!

Newer design starter relay like this Echlin ST404 which also has the same labeled lugs as the old.

 

So I figured out that my problem is someone had snipped the red "S" wire. I need to replace that, then hopefully it should crank. Thanks for the info everyone.

 

Sorry to bring up an old thread, but I'm not sure how to go about fixing that wire. Can I just solder in a short piece? I only need about an inch or two. Would this affect anything? Am I better off to replace the whole wire? It seems like a hassle to replace because it is taped together to a bunch more cables and I'd have to take all that apart to get to it. Thanks

 

As long as the wire has power when the key is in the start position I would repair it. Solder in a short piece it shouldn't be a problem if you know how to make the repair. You don't have to have the end connector which is a push on. You can get a ring terminal and a small nut that matches the threads on the solenoid post. Or get a barrel connector that fits tight on the post and crimp or solder it on.

 

Thanks for the info. I honestly don't know a thing about electricity/wiring. How do I check if the wire has power? Also, I've never actually soldered before, do the double sided crimp connections work as good?

 

Yes a butt connector crimp will work. They aren't the best for reliability but they are the most convenient as long as you have the right crimper tool. If you want to solder watch some youtube videos on it. Lots of good instructions on it.

To test you will need an assistant to hold the key in the start position and use a multi meter to check for voltage at the wire or at minimum use a 12 volt test light. On a multi meter you ground the ground lead somewhere and hold the wire to the positive lead and read dc voltage. It should be 12V +. With a test light you ground the ground clamp and the probe is put on the wire in question and the light bulb on the tool will light up when current is supplied to the wire being probed.

A test will confirm if its worth repairing the wire or replacing the whole thing or digging deeper into the issue. It will also tell you if it is the right wire to use if it works.

If you are just getting into owning and repairing these old trucks and want to work on them yourself to save money and learn, there are a few tools beyond the basic tool set that I have found much use for and are worth buying.

The nice to haves are a vacuum gauge, mighty vac vacuum pump, multi-meter, stethoscope, timing light with advance adjustment, and I suppose wiring tools like a soldering iron or at minimum a wire stripper/crimper. probably many more depending on what you need to do but these are often used in troubleshooting

 

Here's a couple of videos to help you out learn some basics of using the Multimeter and Testlight.

I would splice in a new wire and for simplicity sakes, I would use that heat shrink solder tube. The heat shrink solder tube has a bit of solder to make a really decent connection and the heat shrink tube itself helps keep water and other elements out from the wires. another video about it:
Use a heat shrink tube on the ring terminal or bullet connector at the relay side. I'm kinda **** about keeping water out of my wiring, cos of corrosion. Nothing worse than being stuck somewhere cos of bad wiring.

I go a little overboard with my wiring, by adding extra layers of protection, like silicon wrap or electrical tape and then an outside layer of hockey cloth tape for abrasion resistance.

 

So I used a butt connection to attach a small length of wire. Before I added the wire, I checked it with a voltmeter to make sure I was getting power. I was. Now when I check it, it's seems as though I have no power now. Also, will the starter still crank without being connected to the coil? I don't know how to hook up the coil or what it does. Thanks

 

As for your dilemma with the voltage reading after the butt connection, perhaps the connection has not been made and you need to do it again. If it is the wire that goes to the S position on the solenoid, you will only have power to it with the key in start position. Yes, you can make the starter crank without the solenoid, but the design of the solenoid is to be able to turn on and turn off the starter safely. Using a butt connector will work just fine as long as it is crimped correctly.

 

 

Just to let everyone know, I used butt connections to fix that wire, then had to replace the relay, and now my starter cranks. Thanks everyone for all the helpful info. I really appreciate it.