Only able to jump start the 40

[ColoraDOOM]

So, like so many others, my first post is a problem.
Hello everyone!
Last summer I bought my first 40. My parents had a 62 that I grew up in, but rusted apart (thanks salty VT!)
Anyway, My FJ40 is a 1974, the engine is out of a 78 as best I can tell-no smog stuff because 75 is the cutoff for emissions here. The alternator is internally regulated, which is making the wiring harder and easier at the same time.
The problem: I can only start the truck "Agnew" when being jumped. I've been lurking through various alternator and battery threads, but cant seem to find my problem.
First solution was a new battery, which solved it months back (dead battery-far too old)
Now it's back. I've replaced the battery cables, assuming they were corroded, added a cut off switch on the neg post, and tried to get going again. My battery is showing 13+ volts. As far as I can tell, the alternator is producing power, I can disconnect the battery after turning the truck on and still run/drive around town. Right now all that happens when I put the key to start, something in the engine bay goes click once, then nothing. No starter "wirring", just a click. As soon as I wire up an additional battery to jump, Agnew will roar to life immediately, without hesitation (unless I've been miserably flooding the engine).

Any ideas on where to troubleshoot next would be great. This is very confusing.
Sean

 

[antFJ]

the alt may produce enough power to run the truck but not enough to charge your battery. What is the voltage of your battery when the truck is running. It should be around 13.5-14.0 - it needs to be higher than the voltage of the battery when the truck is off in order for it to charge.

That click is mostly likely the starter solenoid. The battery is not strong enough to power the starter in its current state.

 

[Ian Macdonald]

My 4Runner does the same thing. Cleaning the copper contacts inside the solenoid does the trick.

 

[JungleBiker]

I had a truck that did this once--I kept putting bigger and bigger batteries in it to get it to start (I was young and had too much money, I guess)--in the end it turned out that the starter was seizing up. Once I put in a new starter, the problem was solved.

 

[DougAustinTx]

Never unhook the battery on an alternator equipped car when the engine is running. You risk destroying the diodes and the regulator.

If you really want to troubleshoot the issue, make sure the truck is in Neutral with the parking brake on. Then run a hot wire from the Pos terminal of the battery straight down to the solenoid terminal on the starter. If it starts, then the problem is in the wiring from the battery to the ignition switch, or possibly from the switch down to the starter solenoid. This will eliminate the alternator and battery as the culprit.

 

[sggoat]

Never unhook the battery on an alternator equipped car when the engine is running. You risk destroying the diodes and the regulator.

If you really want to troubleshoot the issue, make sure the truck is in Neutral with the parking brake on. Then run a hot wire from the Pos terminal of the battery straight down to the solenoid terminal on the starter. If it starts, then the problem is in the wiring from the battery to the ignition switch, or possibly from the switch down to the starter solenoid. This will eliminate the alternator and battery as the culprit.[/QU

Never unhook the battery on an alternator equipped car when the engine is running. You risk destroying the diodes and the regulator.

If you really want to troubleshoot the issue, make sure the truck is in Neutral with the parking brake on. Then run a hot wire from the Pos terminal of the battery straight down to the solenoid terminal on the starter. If it starts, then the problem is in the wiring from the battery to the ignition switch, or possibly from the switch down to the starter solenoid. This will eliminate the alternator and battery as the culprit.

This would parallel the existing batt feed to the starter, correct? Seems like this would only tell you if the original batt cable was bad somewhere.( I'm assuming here you would run the jumper wire, then try to start with the ignition as normal, is this right?)
If this is an OEM starter equipped engine, it will have the full power required starter(not like a GR starter)--there might be an issue with the resistor that bleeds some voltage to the boost firing circuit voltage during startup.
I would lean towards what Antfj & Ian say-the click sounds very much like a bad contact in the starter solenoid-easy enough to open up and clean--

 

[DougAustinTx]

I was talking about the small wire going to the starter SOLENOID terminal (on my '79 it is a black wire with yellow stripe). Not the heavy battery wire. Unplug that smaller wire and use a jumper from the battery positive terminal to momentarily touch it to the terminal you just pulled the wire off of. The engine should turn over. If the ignition key is in the ON position at the time, the truck will start.

 

[ColoraDOOM]

Thanks everyone. I'll be checking into the solenoid after work today. I'm learning that the previous owner really did a hack job wiring this thing.