M1079 electrical help please.

[K10A]

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.

 

[Ronmar]

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...

 

[K10A]

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

 

[Ronmar]

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

Good Luck!

 

[K10A]

Thank you ! Hopefully we make progress tomorrow

 

[K10A]

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.

 

[Ronmar]

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...

 

[K10A]

Awesome, thank you. So what should my next be if I continue to troubleshoot this monster? Is any of the potentially affected wiring under the cab? He is also having trouble with the hydraulic system. He thinks it needs bled?

 

[Ronmar]

Awesome, thank you. So what should my next be if I continue to troubleshoot this monster? Is any of the potentially affected wiring under the cab? He is also having trouble with the hydraulic system. He thinks it needs bled?

Under or behind it… the polarity box is below the cab latch best accessed by lowering the spare tire… the main truck ground strap is on the front end of the starter motor over to the drivers frame. That is easiest access from under the truck… Basically you measure voltage as you apply load and fix the bad connections that cause voltage drop when load is applied… start at the batteries, measure the voltage then turn on the ign and measure again… low drop, move to the power panel test points and do it again…

 

[K10A]

Thank you! The spare is off and the rack is down so hopefully that will provide access.
Also how big of a deal are the battery types? He has “regular pickup truck “ size batteries. Is that not goi mg to cut it? Do they have to be 24v or anything special?

 

[Ronmar]

4ea of anything will work, 2 of anything in series is probably better in there, as long as they are properly charged... the 6T batts they originally specd for these trucks are too large for the 60A/40A alt they used... Cat specifies a pair of group 31 batteries for the power equipment that uses these engines... i run a pair of 31's, but a pair of 27's would also be OK. many run 4EA group 24 automotive batteries... the key is that they both or all must be fully charged... the truck has a 12v vampire load from the transmission so the gnd-12v battery is often discharged. no 12V, no 24v ign relay, nothing happens...

The first elec issue I had on my truck was a bad electric connection back at the polarity box. ign would turn on but as soon as i pushed the start button things would start to click,click, click as the poor connection caused voltage drop under load...

The PPD is a heat-sync looking module in where the spare is located with 4 stud terminals that have 6 large wires attached. 12 and 24 from the batteries and alternator connected to the 12 and 24v battery terminals, and 12 and 24 to the cab hooked to the 12 and 24 load terminals. do one stud at a time so you don't mix-up wiring, clean everything to bare metal and re-assemble. there is a diagram on the side of the case. Adjacent to it is an instrument shunt with a large cable attached to each large terminal. clean those also as that is the main ground lead from batteries to starter motor case and on to the drivers frame and cab via a braided straps...

 

[K10A]

Thank you.

The small display box on the dash “ idiot light” I guess was unplugged when he bought the truck. Does that mean something is possibly bypassed or does that not have anything to do with starting? We plugged it in but it did not light up.

 

[Ronmar]

well it needs power to light. it being plugged or not can affect relay K11 operation, but your other descriptions lead me to believe you have some power supply issues to resolve first...

 

[NDT]

The box with square red lights is the tire pressure controller and has nothing to do with starting.

 

[Ronmar]

The box with square red lights is the tire pressure controller and has nothing to do with starting.

oh yea, i guess he could have been talking about the CTIS module below the park brake control... I was referring to the main idiot light panel in the dash in front of the steering wheel..

if the OP was talking about CTIS, leave it unplugged, that's an issue for a later date...

 

[NDT]

I’ve found that lots of times when dealing with FMTVs that have had civilian batteries installed, people get the cables mixed up. Check all that first.

 

[K10A]

I’ve found that lots of times when dealing with FMTVs that have had civilian batteries installed, people get the cables mixed up. Check all that first.

Is there a diagram available?

 

[K10A]

oh yea, i guess he could have been talking about the CTIS module below the park brake control... I was referring to the main idiot light panel in the dash in front of the steering wheel..

if the OP was talking about CTIS, leave it unplugged, that's an issue for a later date...

It’s the idiot light panel in the top of the dash. The ctis was labeled and plugged in. I will suggest to unplug it if he wants me to come troubleshoot it again.

 

[NDT]

Is there a diagram available?

Post #28 and 34

FMTV Battery hook up diagram | Page 2 | SteelSoldiers

 

[K10A]

Th

Post #28 and 34

FMTV Battery hook up diagram | Page 2 | SteelSoldiers

Thank you again for all the help everyone!