I've just switched my 79 F150 to a Cummins 4bt diesel, and am having issues getting it started.

There are only two wires needed to run the engine, aside from the ground strap, and the positive cable to the starter. They are the fuel solenoid open wire, and the starter solenoid wire. My fuel solenoid wire is working fine, but my starter solenoid wire (which goes to the solenoid on the starter) often doesn't have the juice needed to fully trip the solenoid.

I have the starter solenoid wired to the red with blue stripe wire that used to connect to the S terminal on my fender mounted starter solenoid. I turn my key and the solenoid clicks, but 9/10 times the starter doesn't spin. When I touch the solenoid wire to the POS battery terminal, the starter works right every time, so it must be the wire.

Is there another easily accessible hot in START ONLY wire I can try?

Thanks, Alex

 

Hopefully I can get it right this time

RED with BLUE stripe should be the only hot-in-START (never hot otherwise) signal coming from the ignition switch. However, this is beside the point as you're not looking in the right place. If the solenoid clicks at all, the coil side of the starter solenoid and everything upstream (including the hot-in-START signal from the switch) are completely fine. The single-solid "click" you hear is the solenoid engaging due to a trigger signal (in this case, the RED with BLUE stripe wire).

If the solenoid clicks but the starter doesn't spin, then something is wrong with the starter, the solenoid, or cable in between. I'm hesitant to believe that bypassing the switch and jumping the battery to the 'S' post works 100% of the time. This is either coincidence, something is not being communicated properly, or something else is going on. What the starter does or does not do is isolated from whatever trigger is given to the solenoid (which is the purpose of the device in the first place). Getting a click at all eliminates any concern with what's called the "coil" side of the solenoid. The problem you've described is part of the "contact" side of the solenoid. Jumping from the battery isn't giving the solenoid more "juice." The click is the sound of the contacts hitting. They either hit or they don't.

 

There could be a slight voltage drop in the wiring that needs correcting . Check grounds , and check actual voltage at the solenoid/relay when it will not start and only clicks .

 

Which starter are you using? I dont know about the Ford diesel starter, but I know that the solenoid on the Dodge starter, if that's what you used, does the double duty of moving the gear out into mesh AND closing a set of contacts to close the current to the motor.

I recently had some starter issues with mine that slowly built up to a complete failure to actuate in any manner; When I pulled the starter for inspection, the entire plunger on the solenoid had rusted enough to bind inside the coil. That may be worth a shot. If you pull that out, be sure to inspect the contacts and replace as needed. They arc with some pretty serious current every time you start and stop that motor, go through a lot of wear and can cause some strange phantom issues when they start thinking about taking a vacation.

If youre worried about the wire, verify voltage at the terminal; if you have less than full voltage, it's time to liberate that wire from the bundle and inspect it.

 

FMC, I hear what you're saying, but my results just don't agree, there's something more complicated going on. With the key, I sometimes get a click at the starter and no start, sometimes no click, but when I simply contact the solenoid wire to the POS terminal, it ALWAYS starts, first try.

I'm not sure what starter I've got. I could definitely see a poor solenoid contact being part of the issue. Maybe the thicker wire to the POS terminal causes the solenoid to switch more firmly than the weak signal from the S wire and key, resulting in a better electrical connection?

As of yesterday though, the solenoid sometimes doesn't even click.

 

if i understand you are using the start signal to the factory solenoid, ran to your new starter solenoid? Check the stuff that would cause your stock eng not to start. Neutral start safety switch? clutch interlock?

 

There's nothing complicated about it. It's either coincidence, or something isn't being communicated or described properly. Anytime I'm asked to believe something that doesn't make sense, there's always more going on than what I'm being told. There's an explanation for everything and there is no room for mystery, especially with 12-volt automotive circuits.

See what I mean about there always being more to the story? Your first post said that turning the key makes the solenoid click. Now, actually sometimes the solenoid doesn't click at all. This changes things. You probably have a bad solenoid to begin with. Not some convoluted problem about weak wiring from the ignition switch.

Have a friend hold the key in START while the solenoid is completely hooked up, and measure the voltage on all three posts of the solenoid. Report your results. For the 'S' post, you can get at the exposed nut beneath the wire boot. To be absolutely clear, your next post must consist of three numbers.

 

Until it dies , at an intersection , in rush hour traffic , in a rain storm , with an emergency vehicle coming ...... now get out , pop the hood , jump the solenoid . Could be worse ....

 

just make sure it is not still in gear when you do it.

ask me how i know....