OK, what'd I do?

Yes, I can short out the two terminals and she will start. No prob.
So there is obviously some sort of safety cutout that is not working. Bit like the exhaust brake which has about 4 switches that must be in the correct position before it will work >

Gday Rod...
I am assuming it is a manual transmission?
If so, probably no safety cut out.
If the fault is intermittent, it may be a corroded or dirty connection in the start wiring.
I can't remember on a Hino, but the Japanese trucks generally have a good collection of plastic plug type connectors,
Have you had these apart and cleaned them?

Are you able to, WITH THE UNIT OUT OF GEAR AND PARK BRAKE APPLIED, WITH EVERY CARE.. able to have a competent assistant, hold the key to the start position while you wiggle the start loom at the starter end?
Fitting 'additional' equipment to a unit that has started for the last 30 years 'as is' doesn't seem like an answer.

Good luck with the fault finding, I've got my money on a crook connector,,

chocs

Hi chocs. I think you are probably right. Sometimes it works and sometimes it doesn't. Perhaps I will try to eliminate with a multi-metre.
Also....... someone told me about a "button" that is fitted to the frame near where the cab locking handles are and lo and behold, I found a button there. But it seems to be wired to a solenoid on the injector pump. Anyone know what that button is for?

Thanks to all for the inputs - Rod.

While you have the connectors apart to do a resistance check, you may just cure the problem.
On an Isuzu, the button on the cab lock down frame (rhs) is a starter. (with the key on)

I drove a FF177 and just can't remember.
Yours may well be a stopper?
When you get it running, see if it shuts it down.
But most likely a starter though, drawing its power from the solenoid and heading to the starter solenoid...

chocs

...pretty sure the ''button'' mentioned is a feature of a lot of Jap trucks...usually mounted around the left side of the cab lockdown hoop... and is in fact a remote starter for when the cab is tilted and the engine being worked on...

...the Peterbilt cabovers had the very same arrangement mounted to the gearshift remote tower way back in the early 1960's, but strangely it seems the Kenworths never had it...

...i'm not real sure how that set-up would go today, with the multifarious OH&S rules that infest and pervade all of our existence these days : : :

Early Isuzu buttons are on the right, same side as the fuel pump.
If Hino pumps are on the left, that would make sense.

chocs

heycharger take it from an electrician mate you need to forget the multimeter for this fault finding expadition what you need is a test lamp quicker to read and gives a much quicker result now for dumb ass cane cockys simple instructions

1 check the light globe is proper voltage been jap truck might be 24 volt and you might have 12 volt light globe
2 always test the tester on a known live supply one of the first things I learnt at trade school .No good with the earth clamp on the chassis but there is paint and crap in the way giving faulse readings so clamp it to the chassis and test to the battery lead on the starter motor
3 turn the key to try to start make sure it is in neutral and hand brake on and have the key held in the start position most times can stick a screw driver in hole in key and tie a piece of string off it or hang a weight to keep it in correct position
4 start testing at soliniod wire and work back wards towards the key when you have power ie light comes on you have past the fault and remeber rule number 2 always check tester in case the earth clamp gets knocked and gives a falsue reading

do the above and will find fault quickly if you wiggle the connectors and clear the fault it will only come back so be gentle you want find and fix not flange it and be broken down on the side of the road

hope Im not been to simple and am not trying to be smart but years of fixing lifts with all the relays it will always turn out to be something simple seeya and good luck

Paul

Latest.... The button on the chassis hoop, near the cab locking levers WILL START the engine. However, right now, if the key start won't start, then the button won't start. So I obviously have a bad contact that is common to both >.
More electrical tracking.

...have a good look at your chassis earthing.....also there may be more than one location including engine to chassis or cab to chassis etc...

...are you starting it with the hoop button and the cab tilted?...if so and is successful, try again with the cab lowered and see what happens....same for the key start ???

...simple stuff that may be the culprit....cheers mate

My diagnostic procedure is much like Mrsmackpaul, but the other way around. Start from the trigger terminal on the key switch and work from there. As hard as it might be, follow the wire and test at every connection. If your competent with one, use a multimeter. A test light will show you that power is there but it may not be enough power to pull the solenoid in. A multi will tell you how the voltage, and if your really tricky, you can check the resistance. Any more than a couple of ohms, and there's an issue.

My bet is the starter relay, good chance the key switch and button both work this relay. A wiring problem is usually more intermittent to begin with....