M1079a1p2 strange starting issue

[polk738]

Have an M1079a1p2 in my bay, me and my co-worker were working on it yesterday, it looked like something correlating with the cab disconnect switch in the lower left, and the main power switch on the dash, we were able to get it started and into my bay after a strange way of getting it started, we ended up finding all the remote switches were left up underneath the passenger wheelwell, after we flipped those down, it appeared to start fine, now this morning the starter will not engage, the remote switches weren't touched, but now it looks like the vehicle won't start unless you push the battery power switch in the dash and the starter switch at the same time? I also noticed the remote start switch under the wheel well will not work at all now. Any ideas?

 

[GeneralDisorder]

I have seen the manual battery disconnect switches fail - had one fail on a fairly new HEMTT not long ago. Stumped the guys at the base - TM did not include voltage drop tests (basically never does) so the troubleshooting tree would have never got them there. Old school troubleshooting by walking through the schematic revealed the problem quickly.

Also the cab controls and gauges act real funny if the cab ground isn't good - had an A1P2 that was only about 4 years old exhibit about a 2.5v drop between the cab ground at the PDP and the frame. Check the ground strap behind the grill on the passenger side of the radiator tunnel.

Finally I would check out the battery disconnect relay that is controlled by the disconnect rocker switch on the lower left and the disconnect switch under the cab. There's a PS magazine article on those - they are two relays in one unit and they are powered in both directions (open and close) but draw no power to remain in either state - thus performing the disconnect/reconnect function while drawing no power in either state. Doesn't really sound like this is the issue but it's something to look at and verify anyway.

https://www.ascrad.army.mil/News/Article/2992599/m1078a1-fmtv-culprit-for-cab-power-loss/

 

[Ronmar]

Well there is the remote disconnect, ign and start sw pax wheel well, and another disconnect in front of your knees when seated. Are they all in the right place?

do you have proper 12/24 voltage measured to chassis ground at the batts and thru the manual and remote disconnect terminals in the power box aft of the batteries, at the LBCD terminals and at the PDP 12 and 24 connections(relay K1 and K2 terminals?)?

what exactly goes click at ign on and what goes click at button push(relays in PDP, aux relay on drivers frame?

what do the above measured voltages look like while ign on and pushing the start button? Like General alluded to, voltage is not good untill tested good under load(load applied or trying to crank the starter).

bad LBCD connections are very common on these trucks also. Will distribute full voltage to the rest of the system, but like a kink in a garden hose the pressure(voltage) will fall flat when you try and flow current thru the kink/bad connection…