M1079 electrical help please.

Hi Everyone, I am heading to go try and help a fella get his m1079? Box type truck running. He states he has new batteries and some small wires for start interlock were rodent chewed but repaired. Still has a no start condition. Some electrical works and some doesn’t. He has been living in the box as a camper and I think all that electrical works but they are now selling the property and need to get the truck running to relocate and sell the truck. I downloaded the electrical schematics. He says he has a big manual for it but he is not good with electrical. . Any suggestions are appreciated ! Thanks in advance.

well it helps to know the year/model? is it an A0 or an A1?

With no crank, The first thing I always look for are what relays energize when the ignition is turned on, and what relays energize when the button is pushed.

With ignition on K11 and k24 i think should energize, along with the low oil idiot light in the dash. k11 and k24 are controlled by the same 15PSI low oil switch that controls that idiot light. The light and those relays being energized enables the start circuit.

When you push the button the start relay in the dash should energize. That relay in turn energizes the large current aux start relay down on the drivers frame rail near the power steering reservoir. the aux start relay energizes the starter solenoid on top of the starter to crank the engine...

If it is an A0 i can get you a simplified start circuit diagram later when I get home...

Awesome, thank you ! I can try and have him text me a pic of a data plate to determine the year and model.
The post states a 1998 M1079

Well 98 would be an A0 with the cat 3116. Here is a 1 page start circuit drawing that might help.

Good Luck!

Thank you ! Hopefully we make progress tomorrow

So I’m in the truck and I have no idiot lights, that panel was unplugged and the truck ran down when he bought it. No power at start inhibit k19 relay no power at cranking lockout relay 24. 12 and 24 volt at ignition relay K2. No power at voltage gauge. Power at ignition switch.

Well the source of all the ign power is K02. They use 12V via the ignition switch to energize the coil of K02. the contacts of K02 switch/provide 24V ignition power to the rest of the system. if that isn't working, nothing is going to work

First rule of using voltage checks to troubleshoot, is that voltage is only good when tested under load... resistance resists current flow, not voltage... So like a kink in a garden hose a really crappy connection will pass full voltage/pressure to a closed nozzle and the voltage won't fall on its ear until you open the nozzle and try an flow some current thru the kink/high resistance connection...

A good check is to look at the voltage at the 3 test points on the left side of the panel(gnd, 12 and 24v) while you turn on the switch... they should be ~1/2 volt below the voltage measured at the battery and should not drop much when you turn on the switch.

bad connections are common on these at the polarity protection device located between spare and air filter. Since it is out in the weather they degrade/corrode frequently. and of course all the grounds are trash right from the factory...