M1079 Won’t Start.

[FlyCessna185]

I have a 1998 M1079 and I’m having issues with not starting. I got it from GovPlanet. I put new batteries in. I seem to have some power but not all power. Lights, Horn, Wipers, Fuel and Voltage Gauges Work. The Starter button, Trans Screen and Low Air Alarm don’t work. Does anyone have any ideas what I might need to chase? Thanks.

Link is to the video of the issue.

https://youtube.com/shorts/oFrRmd1qNJ0?feature=share

 

[Mullaney]

I have a 1998 M1079 and I’m having issues with not starting. I got it from GovPlanet. I put new batteries in. I seem to have some power but not all power. Lights, Horn, Wipers, Fuel and Voltage Gauges Work. The Starter button, Trans Screen and Low Air Alarm don’t work. Does anyone have any ideas what I might need to chase? Thanks.

Link is to the video of the issue.

https://youtube.com/shorts/oFrRmd1qNJ0?feature=share

.
Under the spare tire. The battery wires run up toward the cab into the Polarity Protection Box first... That is a good place to investigate. Disconnect those new batteries, then remove the 12v and 24v and grounds and clean that box up.

Might also be useful to look at the fuse box in front of the passenger's seat. Missing breakers could be a problem. Dirty contacts on that board is a problem too. Corrosion there from exposure to rain blown into a window that was down for who knows how long will drive you crazy.

Batteries. What did you replace yours with? 4 pieces like the factory - or just two? Many discussions about that here offering many suggestions that two are better than the Factory four.

 

[Suprman]

What year truck? If gp tried to jump start it they could have damaged the trans tcm. No trans display power equals no start from push button.

 

[Ronmar]

No trans display, no neutral start circuit. As mentioned clean the polarity box connections and shunt connections between spare and air filter.

CB 79 should power the trans controller, but it also gets 12v from another breaker(will have to.look it up). Ypu need those to power the trans which when in neutral will give you permission(a circuit) to enable the start circuit...

 

[FlyCessna185]

.
Under the spare tire. The battery wires run up toward the cab into the Polarity Protection Box first... That is a good place to investigate. Disconnect those new batteries, then remove the 12v and 24v and grounds and clean that box up.

Might also be useful to look at the fuse box in front of the passenger's seat. Missing breakers could be a problem. Dirty contacts on that board is a problem too. Corrosion there from exposure to rain blown into a window that was down for who knows how long will drive you crazy.

Batteries. What did you replace yours with? 4 pieces like the factory - or just two? Many discussions about that here offering many suggestions that two are better than the Factory four.

I put 4 new batteries in. The same type that comes with the truck. The truck was also missing the alternator I also put a new one of those on. I’ll check the polarity box. It looks like all the fuses and relays are in. I’ll clean that as well. There’s power coming to the board I checked that.

The truck came from Texas. It seems to have half of West Texas in the cab.

I also forgot to mention the hydraulic pump that operates the cab lift, spare tire, and suspension. Doesn’t work. It doesn’t make a sound either. I haven’t checked power to that.

 

[FlyCessna185]

No trans display, no neutral start circuit. As mentioned clean the polarity box connections and shunt connections between spare and air filter.

CB 79 should power the trans controller, but it also gets 12v from another breaker(will have to.look it up). Ypu need those to power the trans which when in neutral will give you permission(a circuit) to enable the start circuit...

I will check that. Is there anyway to check the trans controller or would I have to get a replacement?

 

[Suprman]

What year truck. Gotta see if the tcm is getting power. The cab lift is air powered hydraulic

 

[TomTime]

I have a 1998 M1079 and I’m having issues with not starting. I got it from GovPlanet. I put new batteries in. I seem to have some power but not all power. Lights, Horn, Wipers, Fuel and Voltage Gauges Work. The Starter button, Trans Screen and Low Air Alarm don’t work. Does anyone have any ideas what I might need to chase? Thanks.

Link is to the video of the issue.

https://youtube.com/shorts/oFrRmd1qNJ0?feature=share

 

[Suprman]

wtec3 tcm probably toast from GP trying to jump start the truck with bad batteries or no batterie. Happens all the time. I have the replacements on hand.

 

[Ronmar]

Drop to two batteries, your very expensive alt will thank you. The truck doesn’t need 4, and the alternator is really undersized in the dual volt configuration to charge them properly.

CB79 feeds 24v IGN power to the TCU. Yours could be either wtec2 or 3 TCU. A 2 has a wider green LED display. A 3 has a narrower 2 digit red(I think) display. My 98 m1079 chassis was a wtec2. If it is a 2 it will have a Vehicle Interface Module(VIM), a black waterproof box mounted up under the passenger dash behind the kick-panel. The VIM gets 12V battery power via CB35. That is the 12v vampire load on these trucks that kills the 0-12 batteries When they set for long periods

The VIM also has fuses in it for the 24V ign power and 12V battery power that passes thru it to reach the TCU. It must have both as the TCU uses that battery 12 to power/control the neutral start relay located in the VIM.

 

[Ronmar]

Here, I condensed the important bits out of the mil schematics for the A0 start circuit. you will need this eventually, everybody does

 

[FlyCessna185]

What year truck. Gotta see if the tcm is getting power. The cab lift is air powered hydraulic

1998 M1079 Van Body