Need Help Starting 2003 A1 M1078 - Addison, Texas

[Aselk]

I need some electrical help in determining what is going on with my LMTV. I bought a while ago from auction, replaces many items, truck turned over, heard a noise from behind the cab and then, it stopped turning over. replaced the batteries, replaced complete fuse,/relay panel from passenger department, replaced 12v/24v distribution box behind the cab, and still does not turn over. Need some help. Ken

 

[NDT]

What is going on in the relay box next to the battery box? Are the relays coming on? There is one relay for 12 volts and one for 24 volts.

 

[Ronmar]

What NDT said, whats going click when you turn on the main switch and what goes click when you push the button. You also need to measure the voltage at the power test points in the power panel WHILE you are pushing the start button(voltage isn’t good until tested under load)…

 

[Aselk]

What is going on in the relay box next to the battery box? Are the relays coming on? There is one relay for 12 volts and one for 24 volts.

I had someone come help me and battery disconnect model 3106 was rusty and looked bad, so we took out and bypassed. He tested it and it was bad. I ordered a new one from MWM today. Could that be the problem? After bypassing the truck cranked, but did not start.

I also replaced the engine ecm as well after this was determined to by bad.

 

[Aselk]

What NDT said, whats going click when you turn on the main switch and what goes click when you push the button. You also need to measure the voltage at the power test points in the power panel WHILE you are pushing the start button(voltage isn’t good until tested under load)…

Thank you. I'll check this out as well.

 

[fuzzytoaster]

@Ronmar @NDT

The current issue is when the power switch is turned on we have power to everything (like normal) but it quickly fades then everything shuts off. It's a 1-2 second power fade. Toggling the power switch doesn't reset the truck for several minutes (no power). Truck will not crank for the moment it has power.

Steps taken for initial no crank no start: no power to dash at all

1. Verify PDP relays and CB's - all good
2. Check battery voltages - new batteries, voltages verified good
3. Traced power from batteries finding remote shutoff relay faulty - bypassed unit
4. LCBD faulty - unit began to smoke with power on, top plug was disconnected
5. Power traced to PDP/board - verified good voltage to 12v and 24v terminals on board
6. Truck cranked but no start. No exhaust fumes.
7. Plugged into truck through J1939 port on kickpanel - no communication from ECM
8. Unplugged ECM and connected directly to unit with Laptop and CAT ET - ECM would not detect/communicate. Unit diagnosed "brain dead"

Here's where things digressed: back to "no crank no start" with intermittent power to dash

1. Aselk installed ECM from Holt Cat - unit would not crank (Aselk please verify if it had intermittent power issue at this time)
2. Traced voltages to PDP - voltage is good for brief moment before it drops out. 0v on 12v and low voltage on 24v ( forgot the actual voltage, like 8v-ish)
3. Aselk Replaced LCDB with good used unit from @Suprman (we brain stormed it) - no change
4. Inspected grounds and used jumper cable to run additional test grounds - no change
5. I loaned Aselk a NOS PDP to swap relays or the whole unit in for a test *fingers crossed*

Now we are here.

 

[coachgeo]

@Ronmar @NDT

The current issue is when the power switch is turned on we have power to everything (like normal) but it quickly fades then everything shuts off. It's a 1-2 second power fade. Toggling the power switch doesn't reset the truck for several minutes (no power). Truck will not crank for the moment it has power.

Steps taken for initial no crank no start: no power to dash at all

1. Verify PDP relays and CB's - all good
2. Check battery voltages - new batteries, voltages verified good
3. Traced power from batteries finding remote shutoff relay faulty - bypassed unit
4. LCBD faulty - unit began to smoke with power on, top plug was disconnected
5. Power traced to PDP/board - verified good voltage to 12v and 24v terminals on board
6. Truck cranked but no start. No exhaust fumes.
7. Plugged into truck through J1939 port on kickpanel - no communication from ECM
8. Unplugged ECM and connected directly to unit with Laptop and CAT ET - ECM would not detect/communicate. Unit diagnosed "brain dead"

Here's where things digressed: back to "no crank no start" with intermittent power to dash

1. Aselk installed ECM from Holt Cat - unit would not crank (Aselk please verify if it had intermittent power issue at this time)
2. Traced voltages to PDP - voltage is good for brief moment before it drops out. 0v on 12v and low voltage on 24v ( forgot the actual voltage, like 8v-ish)
3. Aselk Replaced LCDB with good used unit from @Suprman (we brain stormed it) - no change
4. Inspected grounds and used jumper cable to run additional test grounds - no change
5. I loaned Aselk a NOS PDP to swap relays or the whole unit in for a test *fingers crossed*

Now we are here.

in all this have you replaced the battery cables between batts?

 

[NDT]

Agree, this sounds like cable terminal ends, what can happen is corrosion occurs between the copper strands and the crimped on connector, leading to intermittent weird behavior. Get your voltmeter, starting at the batteries, work your way towards the pdp until you find voltage dropping off. Turn on all loads such as heater and lights and ignition.

 

[Ronmar]

Well like I said earlier, voltage isn’t good until tested under load. A high resistance connection will pass full voltage with no load applied to it(resistance resists current flow not voltage). But the voltage will quickly fall on its face as soon as you try and draw any current. Bad connections or cables will cause this.

If you have a voltage drop-off when loaded/turned on, you need to work back toward the source measuring the voltage with the same load applied/circuit turned on until the voltage is good. The issue lies between the good and bad test points…

 

[Suprman]

The main ground goes thru that current shunt they go bad often. Polarity box another common culprit