LMTV M1078A1R - Battery Disconnect Reset Issue

[fuzzytoaster]

I have a 2006 M1078A1R that is having an odd issue related to the battery disconnect box. This truck is current gen so no TM help. Batteries are fresh, wired correctly, and voltages checked. Turning the battery disconnect to "on" will result in a satisfying click for power. This click sounds like my other two M1078A1R trucks, nothing abnormal. The relay will eventually reset itself without a pattern or cause as shown by the video. Removing the cover and inspecting everything looks clean and almost factory new. I've had power to the dash at one point but now flipping the toggle switch for dash power gives me a second of power then causes the relay to trip and it resets randomly.

Has anyone had this issue? This would be good documentation as more of these trucks come out.

https://youtu.be/Px8kH-SUjUY

 

[coachgeo]

Gremlin .... its always the Gremlins.... well of course not when it is the Trolls

 

[Ronmar]

Check and clean all the connections associated with the polarity protection device, as well as all your battery connections. It might be that enough current is flowing to start the power-up process but as soon as you try and pull any current thru a bad connection, the voltage falls flat and resets the process...

Had a similar problem on my A0. Powered on Ok, until I pressed the start button. Then it went clunk and relays chattered as a bad connection on the polarity box refused to allow enough current to pass to power the circuitry properly...

 

[fuzzytoaster]

Check and clean all the connections associated with the polarity protection device, as well as all your battery connections. It might be that enough current is flowing to start the power-up process but as soon as you try and pull any current thru a bad connection, the voltage falls flat and resets the process...

Had a similar problem on my A0. Powered on Ok, until I pressed the start button. Then it went clunk and relays chattered as a bad connection on the polarity box refused to allow enough current to pass to power the circuitry properly...

That's my next step actually. Trace the issue from the batteries on up.

 

[Suprman]

Unplug the mil connector on the polarity box see if symptoms change.

 

[fuzzytoaster]

Unplug the mil connector on the polarity box see if symptoms change.

Pulled it and cleaned all contacts for sale measure. Power is good now, no reset issue. Now..click in the dash no crank issue. Onwards and upwards!

 

[Suprman]

Without the connection you will probably get a charge fault on the dashboard. I have seen alot of the polarity boxes go bad like that.

 

[coachgeo]

Pulled it and cleaned all contacts for sale measure. Power is good now, no reset issue. Now..click in the dash no crank issue. Onwards and upwards!

now check the relay and fuses for the start button.... that may not have liked the intermittent power surges and/or trying to suck the high resistance power thru them.

 

[Ronmar]

First check I would make is that you are getting 12V and 24V power to X1 and X2 in the PDP. Then I would insure that the same power then passes on to X3 and X4 once the main sw is turned on.

The A1 did away with the dedicated crank lockout relay(k24 prevented starter engagement with oil pressure sw open/above 15psi). Instead it combined this function with the alternator excitation control. When you turn on the main sw relay K11 must energize to complete the circuit that allows the start switches to activate the start relay and crank the engine... Once the engine starts and the oil pressure sw opens at 15PSI, K11 de-energizes to disable the starter and excite the alternator...

This way actually makes more sense than the way they did it in the A0 as you can have a failed K11 and still start the truck, which will allow the alternator to try and come online while you are cranking the engine which can be very hard on the alternator/regulator...

 

[tennmogger]

Ronmar, you deserve the SS exemplary service award for circuit analysis on the FMTV vehicles! Thank you!

 

[fuzzytoaster]

First check I would make is that you are getting 12V and 24V power to X1 and X2 in the PDP. Then I would insure that the same power then passes on to X3 and X4 once the main sw is turned on.

The A1 did away with the dedicated crank lockout relay(k24 prevented starter engagement with oil pressure sw open/above 15psi). Instead it combined this function with the alternator excitation control. When you turn on the main sw relay K11 must energize to complete the circuit that allows the start switches to activate the start relay and crank the engine... Once the engine starts and the oil pressure sw opens at 15PSI, K11 de-energizes to disable the starter and excite the alternator...

This way actually makes more sense than the way they did it in the A0 as you can have a failed K11 and still start the truck, which will allow the alternator to try and come online while you are cranking the engine which can be very hard on the alternator/regulator...

I'm concluding your info is for the earlier generation A1 trucks with the CAT 3126 up to 2005. The truck I have is 2006 which has the CAT C7 and WEC 4/5 so it's a complete different setup. I do have a digital copy of the early A1 generation TM's but they've been no help on the issue due to the changes they made. Here's photos of the PDP for comparison for A1 trucks. The only other A1 PDP would for the rebuilt Oshkosh units but we'll get to that when Uncle Sam is tired of playing with those toys.

A1 w/ 3126 (Early Gen)


A1 w/ C7 (Later Gen)

 

[Suprman]

I am pretty sure the c7 dash is an electronic bus and is controlled by the engine computer. I have seen a lot of ground issues in a wide variety of trucks. The main ground goes thru a current shunt so current draw can be measured for diagnostics. A little bit of corrosion and you get a bad ground. I had previously thought there was a separate starter ground from the batts but this does not seem to be the case. You can check the ground with a multimeter between the battery negative and a good ground on the power panel if you measure voltage between the two then the ground is bad.

 

[Ronmar]

The A1 was not all that different from the A0. Based on the relay count the A1R probably still isn't all that different.

If you can shoot some closeup pics of the module covers so I can identify the relays I may be able to help. On a legible pic I found of module 2, I noted that relay numbers and names are still the same.

Also are there any relays under the dash in the vicinity of the trans control unit? If so there may be a neutral start relay there...

 

[fuzzytoaster]

The A1 was not all that different from the A0. Based on the relay count the A1R probably still isn't all that different.

If you can shoot some closeup pics of the module covers so I can identify the relays I may be able to help. On a legible pic I found of module 2, I noted that relay numbers and names are still the same.

Also are there any relays under the dash in the vicinity of the trans control unit? If so there may be a neutral start relay there...

Overview


Enlarged by Module - 1/2/3/4


There's no relays or related controls under or behind the PDP modules, it's a rather spacious design. The transmission controller is mounted underneath like all the trucks but that's it.

I dug into the dash more today and started cross comparing voltages from this truck and my running truck (identical trucks). I have good power to the PDP modules and have checked voltages to/through all fuses to ensure they're not tripping. The "K" numbers don't all match up with previous PDP's, we know the K19 and K1 are usual culprits for a no start situation, and K19 is the Start Inhibit but K1 does not exist here. The lowest K# is K7 that is listed.

I traced the starter button wires back to Module 2 where they enter into the Module on the J204 plug, confirmed no voltage coming out of the Module. I have a NOS Module #2 and swapped it out to eliminate any potential for corrosion etc. I ohm'd for continuity on the Start Inhibit Relay K19 and to the connector and all is good there. Something is tripping the Start Inhibit relay..


Major Component Locations & Numbers:

Module 1 is lights only.

Module 2 - Start inhibit relay K19
XMSN ECU/CAT ECU fuse CB79 (23.9v)

Module 3 - Start disable relay K40
Ignition fuse CB45 (23.9v)
Transmission ECU fuse CB43 (23.9v)

Module 4 - Neutral start relay K26
Alt excite relay K11
Gauge/start/alt fuse CB77 (23.9v)

 

[Ronmar]

Looking at module 4, I see K11(alt excite) and K26 Neutral start. I would compare the operation of those 2 relays first with the operating truck. What do they do when you turn on main power, turn on ignition and try to start. I would expect both to energize with ignition turned on(and trans in neutral). K11 should de-energize after the engine reaches 15PSI oil pressure. in the A1 this brings the alt online AND disabled the starter so you cannot crank the starter accidentally while the engine is running. If they kept it the same in the A1R, if K11 doesn't energize the starter may not operate. The start circuit of course passes thru the neutral start relay which should cycle with trans gear selection.

On the module 3 pic I noticed K40 “start disable” the A0 used a standalone starter lockout relay that operated in concert with the K11 alt excite relay to disable the starter after the engine has oil pressure(and you release the start button). They got rid of it in the A1, perhaps they brought it back in the A1R... I would also compare the operation of K40 between the working and non working trucks...

I think the start inhibit relay should allow you to crank the engine without the engine starting for testing and maintenance.

hope this helps, good luck.

 

[Suprman]

In the first gen truck troubleshooting for no start it has you ohm out one of the oil sensors. If it’s open then you don’t get a complete circuit to crank.

 

[Third From Texas]

I'm curious about how this turned out. My A1R (which was running fine) is showing 12.5v-12.7v on each *wet* cell 6TMF batteries. If they were Hawkers I would say they are suspect, but being wet I suspect they are alright. Load tests all show good and they are wired correctly.

The other day I went to start it and "click" followed by a "No Charge" indicator.

Working from the battery box, things appear normal and test out thru the disconnect sw. I've pulled and tested the individual relays from the dash. Now I'm going back and starting along from the disconnect sw, checking grounds, etc.

 

[Ronmar]

I'm curious about how this turned out. My A1R (which was running fine) is showing 12.5v-12.7v on each *wet* cell 6TMF batteries. If they were Hawkers I would say they are suspect, but being wet I suspect they are alright. Load tests all show good and they are wired correctly.

The other day I went to start it and "click" followed by a "No Charge" indicator.

Working from the battery box, things appear normal and test out thru the disconnect sw. I've pulled and tested the individual relays from the dash. Now I'm going back and starting along from the disconnect sw, checking grounds, etc.

If the batts have been setting a bit, that is not a bad static voltge.

 

[fuzzytoaster]

I disconnected the top plug on the reverse polarity box and it stopped acting up for the time being. Unfortunately I'm dealing with another major issue on this one but the first issue hasn't returned. The M1088A1R had an intermittent no charge light issue but only after it warmed up and the engine was restarted. I cleaned the ground and it hasn't acted up since. I wish I had more info to give (and TMs).

 

[Suprman]

Ground under the power panel?