LMTV died while driving

[brainsboy]

2001 M1078 3126B


Had the truck at work today and had been driving it off and on all day. Went to go home got up to about 40mph and the ABS light came on and the engine died like someone shut it off, costed off the road and it cranks but wont start. All the fuses are good, swapted some relays didnt make any difference. Turn the ignition off and back on and ABS light stays on. Wouldnt think that an ABS light would make the motor shut down, but im assuming they almost have to be related. Air pressure is good, Voltage is good and all new batteries. Any ideas where to start?

 

[GeneralDisorder]

2001 M1078 3126B


Had the truck at work today and had been driving it off and on all day. Went to go home got up to about 40mph and the ABS light came on and the engine died like someone shut it off, costed off the road and it cranks but wont start. All the fuses are good, swapted some relays didnt make any difference. Turn the ignition off and back on and ABS light stays on. Wouldnt think that an ABS light would make the motor shut down, but im assuming they almost have to be related. Air pressure is good, Voltage is good and all new batteries. Any ideas where to start?

ABS gets mad if there's a ground issue with the cab - have encountered that before. Check the ground strap behind the grill on the passenger side of the radiator tunnel. Check for voltage drop from the cab to the frame.

Connect a J1939 scan tool or laptop with CAT ET to the truck and see if the engine ECM is alive and talking and the bus is good (check if other modules respond).

There was a deprecated EMI/RFI filter box on that vintage of 3126b that was supposed to be disconnected and a patch harness installed in its place. The ignition switched power supply for the ECM goes through it and both those modules as well as the patch cable used to bypass it have been known to cause power supply problems to the ECM. It's located just outboard of the PS reservoir. Inspect and do a voltage drop test on the power coming into the ECM. Especially if the ECM does not talk to the scan tool/laptop.

And if you don't have a scan tool or laptop with CAT ET...... you just learned why you need one in the truck at all times. You can scan the ABS as well with the WABCO software or a good generic scan application like Dearborn DG Diagnostics, etc.

 

[fuzzytoaster]

Check for power to and through CB42 on you PDP. That's power to your ECM. The ABS has nothing to do with the engine ECM so you likely had a related issue that tripped both systems. Start with the basics and work your way up from the batteries. I've seen cables chaff and melt from the exhaust before causing issue while in transit.

 

[Ronmar]

Well they all communicate on the same data-bus…

 

[GeneralDisorder]

Well they all communicate on the same data-bus…

And the ABS will complain if it loses the J1939 bus. AFAIK it's the only module that seems to care enough to complain about it. I've seen burnt termination resistors bring down the bus on several trucks.

But the bus going down won't stop the engine from running. As long as the ECM has ignition power it's going to keep firing the injectors once it's running.

 

[brainsboy]

Check for power to and through CB42 on you PDP. That's power to your ECM. The ABS has nothing to do with the engine ECM so you likely had a related issue that tripped both systems. Start with the basics and work your way up from the batteries. I've seen cables chaff and melt from the exhaust before causing issue while in transit.

CB 42 has power, if I pull it the dash lights go off. How do I check power at the ECM, schematic here shows a power wire but my wires are not numbered

Did notice that when you turn on the power switch the ABS attempts to turn on for a second then shuts off

 

[GeneralDisorder]

3126B & 3126E Electrical Dwgs Electrical Schematics And Sensor Locations

3126B & 3126E Electrical Dwgs Electrical Schematics And Sensor Locations

forum.dieselrvclub.org

Check the two pin jumper harness used to bypass the EMI/RFI filter box near the ECM.

 

[hike]

The ABS cycles upon power up. When we were having trouble with the J1939 resistors at power up the ABS light would come on (as it always does), go off while checking the four spots around the truck, (rear, front, rear, front) and then come back on and stay on.

Our J1939 resistors were burning up due to a wiring issue in the CTIS harness.

Perhaps you have a harness issue to the CAT ECM that has also affected the J1939 harness? Checking power to the ECM at the jumper harness as @GeneralDisorder suggests is a good idea. And one of the two J1939 resistors is right there, too, (the other is underneath the PDP). Near the power steering reservoir and the fuel filters coming from the rear toward the ECM–

 

[brainsboy]

Not sure what jumper harness your talking about. I have two triangle connections and then two wires with black grommets and red plastic clips to keep from coming apart and traced one of those to the bottom of fuel filter. Says P39 other says P33. Also just found a wiring harness in there with a just a plastic connection in it with no wires on it. took the end off, its a jumper of some sort. disconnected and one line was 5 volts the other 0 volts

 

[GeneralDisorder]

Not sure what jumper harness your talking about. I have two triangle connections and then two wires with black grommets and red plastic clips to keep from coming apart and traced one of those to the bottom of fuel filter. Says P39 other says P33. Also just found a wiring harness in there with a just a plastic connection in it with no wires on it. took the end off, its a jumper of some sort. disconnected and one line was 5 volts the other 0 volts

The triangle connection you are calling a "jumper" is the J1939 termination resistor - one of the two hangs out near the ECM. The other is under the PDP. Put that back after verifying that it is at 120 Ohms. WIthout it the J1939 bus will not function. You can't read voltage on that without a scope - it's a communication bus not a DC circuit. When it's working correctly you should see about 2.4v to 2.6v across the two pins as the meter averages the coms signals which are 5v referenced.

The ECM is on the drivers side of the engine block. Near it there should be a box approximately 4" by 5" by 1" thick with two 2-pin round connections on it. It should NOT be plugged in - the military bypassed them with a jumper harness. It *was* a $2,000 EMI/RFI filter box. But the jumper harness the military used to bypass it is prone to corrosion where the wires are soldered to the pin solder cups on the back side. These wires are the ignition power circuits for the ECM - when the ECM loses those power feeds it shuts down.

You need a scan tool or laptop to check for coms with the ECM. The ECM is the gatekeeper here and without talking to it all you can do is verify power and ground and if those are good you can try another ECM. Beyond that you are guessing without consulting the gatekeeper.

And I mis-spoke above. The EMI/RFI box is INBOARD of the PS reservoir I think..... my truck doesn't have that I can't recall exactly where it's mounted. Someone posted a picture of one recently......

 

[hike]

Here is a photo of the area with some labeling and another of the box the jumper cuts out of the ECM circuit—

 

[GeneralDisorder]

Yep - that's the one. Check those connections. And NOT WITH A METER. You have to check with it plugged in and loaded. Digital Meter won't tell you $hit.

 

[brainsboy]

Thanks for the photos, So I dont have the box anymore but I do have the jumper wire. I unscrewed the jumper wires and ohm tested them, The jumper wire is good. Its maybe only about 6 inchs long. Reinstalled it and the ABS light went out and truck started. ABS came back on about 1 min later but truck still starts and runs so making progress. Amazing you guys knew to check that. Im a little gun shy to drive now, anything I can do with that line so im not stuck in traffic again. Can It be rewired or by passed?

 

[GeneralDisorder]

Thanks for the photos, So I dont have the box anymore but I do have the jumper wire. I unscrewed the jumper wires and ohm tested them, The jumper wire is good. Its maybe only about 6 inchs long. Reinstalled it and the ABS light went out and truck started. ABS came back on about 1 min later but truck still starts and runs so making progress. Amazing you guys knew to check that. Im a little gun shy to drive now, anything I can do with that line so im not stuck in traffic again. Can It be rewired or by passed?

Don't trust that cable! Ohm testing won't tell you $hit. Throw it away. Cut off the truck side connectors (throw them away) and permanently crimp the wires together. And yes that cable is your problem or the truck side connectors. The issues is in the back side of the connectors where they are soldered. You can't use a DMM the way you are using it. You MUST provide a load to the circuit. Ohm testing a non-resistor cable (IE: not a high voltage ignition wire) with a digital meter does not apply actual amperage to determine if the cable's connections are actually good and can perform work. It's sending a micro-amp current down the wire and trying to determine resistance - that setting is for things that are designed to be resistors not things that are designed to have zero effective resistance in a circuit.

And put the triangle plug back in - if that's removed the J1939 bus is down and the ABS module is probably upset about it. Thus the light after startup.

 

[hike]

And it is still possible that resistor and the R13 under the PDP are both blown which in our experience lit the ABS dash light. When you connect to the diagnostic port does J1939 and J1703 both connect and read?




Resistor plug under the PDP


Pulled the caps off two burnt out resistors. Truck would run, though diagnostics did not work and ABS light stayed on once the programmed start up ABS check routine was run—

 

[brainsboy]

I did what General Disorder recommended and cut the wire ends off and rewired it together. After doing this the engine runs and no ABS light anymore.

Interesting that it would run and still give me an ABS light with old harness but cutting it out fixed both issues. I did notice that the wire ends I cut off didnt have the ground shielding for about 6 inches on each side. I shortened the length of the wires soldered then shrink wrapped and reattached the grounding shielding all the way around.

 

[GeneralDisorder]

The wiring harness shielding is for EMI/RFI suppression and nothing is likely to interfere with the power supply circuits to the ECM..... it's really just a mil spec requirement for non-interference in case you happen to be driving near high-powered search radar arrays, etc (and more for the radar system, not the truck). It's unimportant in terms of civilian use. Nothing in the civilian world has that level of shielding on engine control wiring. Frankly it makes wiring harness repairs difficult and everyone's expectation of it surviving the apocalypse of a world ending nuclear exchange or solar flares is greatly exaggerated. Your truck's ability to run in such an event is unlikely to be the most important factor in your survival (which is also unlikely).

 

[profo]

Check for power to and through CB42 on you PDP. That's power to your ECM. The ABS has nothing to do with the engine ECM so you likely had a related issue that tripped both systems. Start with the basics and work your way up from the batteries. I've seen cables chaff and melt from the exhaust before causing issue while in transit.

Check for power to and through CB42 on you PDP

fuzzy what are you calling the pdp? passengerside dash panel?

I have been trying to figure my abs light for years I do have cat et , but not the wabsco software for abs , so been trying manual to figure out playing swap a part!

Marty

 

[fuzzytoaster]

Check for power to and through CB42 on you PDP

fuzzy what are you calling the pdp? passengerside dash panel?

I have been trying to figure my abs light for years I do have cat et , but not the wabsco software for abs , so been trying manual to figure out playing swap a part!

Marty

PDP - Power Distribution Panel ( inside the passenger dash board with relays and breakers).

You will need the Wabco toolbox to talk to the ABS. It's a COTS (commercial off the shelf) system so start with visually checking your sensor cables at the wheels, make sure your truck's voltages are correct (voltage spikes will cause it to go into a safe mode and shut down), and then try to talk to it.

 

[fuzzytoaster]

Here is a photo of the area with some labeling and another of the box the jumper cuts out of the ECM circuit—



View attachment 926483

Ah yes, the hidden yet infamous noise suppression box. It's not an official MWO but the commo guys install them when truck need a clean feed for their radio equipment onboard. I've had 3 of these units swell internally and cause a truck to crank no start. Nice find and no they're not in the schematics so you wont know until you visually check.