M1078 no crank blows fuse

[AlaskanLMTV]

Good morning from Alaska!
I've acquired a 2000 M1078A1, learning as I go... after completing the services, filters and all of the fluids since the Army drained everything, this machine pops the CB77 fuse every time I flip the power switch.
Fuse block and cover appear to match, the blown CB77 fuse that came with this machine was a 20a. I've swapped out all of the resettable fuses that failed with standard blade fuses. the fuse box cover says the CB77 should be a 10a. that had me a little concerned right away...

so far, we've isolated the starter relay, completely removed it and still pops the fuse. if I jump the relay, the starter engages and spins the motor but won't start since the Cat computer isnt on.
I've isolated the air heater grid, still pops the fuse. And we've isolated and bypassed the Pac Brake solenoid, still pops the fuse.
Next, I downloaded the manuals, but I'm running in to a wall... I think the TM manual I found is for an older unit, we went through the electrical steps til it calls for testing K24 relay. My LMTV doesn't have a K24 relay, unless there is another panel that I haven't found yet.

Any ideas?

 

[Ronmar]

Pull relay K11(alt excite) and K26(neutral start) and see if it still pops CB77. If it doesn't pop 77, re-install one relay at a time and see which one pops CB77...

need a direction to follow. My money is on K11 providing the short circuit path

 

[GeneralDisorder]

You need the A1 schematics:

FMTV Schematics A 0 A 1 R : Stewart and Stevenson : Free Download, Borrow, and Streaming : Internet Archive

Family of Medium Tactical Vehicles wiring schematics. A0, A1, A1R WTEC III, A1R Gen IV

archive.org

 

[Suprman]

Maybe shorted trans tcm. Behind passenger kick panel. 3 plugs just unplug them

 

[Ronmar]

My reason for betting on K11 being the path that causes the short, is that when K11 energizes with the ignition sw, it provided power from CB77 to the start switches. One of those switches is down on the passenger frame rail/out in the weather. If the wiring has been damaged or the sw has developed an internal short to ground, that could be a very likely source of the issue…

 

[AlaskanLMTV]

My reason for betting on K11 being the path that causes the short, is that when K11 energizes with the ignition sw, it provided power from CB77 to the start switches. One of those switches is down on the passenger frame rail/out in the weather. If the wiring has been damaged or the sw has developed an internal short to ground, that could be a very likely source of the issue…

K11 removed did not correct the CB77 fuse popping. I went ahead and pulled every fuse and relay on the 24v side of the fuse block EXCEPT the CB77, flipped the master and immediately popped the fuse. Next, I reinstalled all of the fuses and relays in correct order, and started pulling connectors off of the back of the fuse block. The very far left hand connector disconnected, master on and didn't pop the fuse.
I think my next step is to identify all of the lay line numbers and track each circuit that they go to. Yay...........

 

[AlaskanLMTV]

Maybe shorted trans tcm. Behind passenger kick panel. 3 plugs just unplug them

Unplugged all three TCM connectors, no change. The transmission display comes alive when I put power to the system, displays vehicle is in Neutral. Some of the lights illuminate on the dash, the Transmission light comes on and quickly goes out. I'm thinking the transmission is talking, it appears to be at least electrically communicating. Wont know till I can get the engine to fire and then top off the remaining fluids, and then push a button and see what happens!!

 

[Ronmar]

Well never was much of a gambler

Ok, if K11 and K26 removed did not stop 77 from tripping, there are only a few other places that power goes, and all of them are in the drivers dash.

The alarm buzzer in the dash, which is controlled by grounds provided by the low air switches or the troop call sw, the 12/24v volt switch near the volt meter, the lamp test sw and all the gauges. CB77 is lower right on this drawing, CB77 output to dash circuits drawn upwards from that point.

i suspect you have a chaffed/shorted wire between the power panel and the main dash panel devices it powers.

i would pull the steering wheel to lift out the dash for a proper looksee… you can disconnect the buzzer, gauge and lamp test switches easy enough to eliminate those devices. You will probably have to unplug all of the gauges individually to isolate them

Good Luck.

 

[AlaskanLMTV]

Well never was much of a gambler

Ok, if K11 and K26 removed did not stop 77 from tripping, there are only a few other places that power goes, and all of them are in the drivers dash.

The alarm buzzer in the dash, which is controlled by grounds provided by the low air switches or the troop call sw, the 12/24v volt switch near the volt meter, the lamp test sw and all the gauges. CB77 is lower right on this drawing, CB77 output to dash circuits drawn upwards from that point.

i suspect you have a chaffed/shorted wire between the power panel and the main dash panel devices it powers.

i would pull the steering wheel to lift out the dash for a proper looksee… you can disconnect the buzzer, gauge and lamp test switches easy enough to eliminate those devices. You will probably have to unplug all of the gauges individually to isolate them

Good Luck.



View attachment 930486

Thank you!! this is immensely helpful direction to chase, I was going down a much darker rabbit hole.

I suppose its never to late to tell you, I'm functionally electrically dumb. LOL!! But, I have a nice multimeter and I'm tenacious enough to learn this one way or another!!!!

Next challenge, I have family in town for next week and then I go back to work for a month. So, please please please stay tuned, when I get home and can play with this thing again, I promise I will update!

 

[GeneralDisorder]

Guys do like to disconnect the alarm buzzers - could be one of the wires wasn't properly secured and insulated.

 

[Keith Knight]

Guys do like to disconnect the alarm buzzers - could be one of the wires wasn't properly secured and insulated.

After reading all of this that’s what I was thinking.

 

[AlaskanLMTV]

Good chilly afternoon from Alaska!

Back from my 3 week hitch in the arctic, playing with this thing in the driveway again.

We've pulled the steering wheel and looked at the wiring behind the dash, nothing looks terribly out of order. I pulled the connector J301 from the back of the PDP and it won't pop the CB77 fuse. Next, I pulled terminals from that connector one by one, and proceeded to pop another dozen fuses. there is only one terminal that I can't remove, has two wires: 1546-a and 1546-b.
then I slid back over to the driver seat and tried manually looking for the other end of those 2 wires, no luck. Now I'm paging through the EWD, trying to keep from going cross eyed. The termial is very solid in the connector, the PDP side kinda looks like it has been hot. And since its the only one I can't isolate, I'm kinda leaning toward something downstream of those two wires.

 

[AlaskanLMTV]

slight update!!

I got to the end of the wires that lead to the volt gauge. Disconnected the volt gauge and CB77 holds now. Everything on the dash (except the volt gauge) come to life and things are starting to click and buzz!! Still not getting a Start Signal... power on turns everything on, big red button doesn't do anything.

Here's to failing successfully!!

 

[Ronmar]

Do you get an oil pressure idiot light when you turn on the ign? K26 and K11 both must energize when you turn on the ign in order to enable the start pushbutton as K11 provides the power to the button when it is energized. K26 is controlled by the transmission neutral start signal. It provides power for K11. K11 is controlled by the 15PSI low oil pressure sw down on the engine block. That switch provides the path to ground for both K11 and the oil idiot light, so no idiot light no crank

once the engine starts, the 15PSI sw removes the ground path, which turns off the idiot light and de-energizes K11. this disables the starter and shifts that starter power to the alternator to bring it online…