Electrical System Shuts Down On A Volvo VNL 670

[MikePhua]

Background Of The Volvo VNL 670 Platform
The Volvo VNL 670 is a long-haul highway tractor that became a common sight on North American roads in the mid-2000s. Equipped in this case with a D12 engine, it uses a networked electronic architecture in which multiple control modules communicate over data buses rather than relying on simple point-to-point wiring.
By the late 2000s, Volvo Trucks had delivered well over 100,000 VNL-series tractors worldwide, and the 670 sleeper variant was one of the more popular models among owner-operators and fleet buyers because of its balance between aerodynamics, comfort and operating cost.
On these trucks, the electrical system is not just a collection of wires and fuses. It is an integrated system built around:

• An engine electronic control unit (EECU or MID 128)
  
• A vehicle electronic control unit (VECU)
  
• A body or instrument cluster module
  
• A J1939 data bus tying them together
  

When any major part of that chain is unstable, symptoms such as periodic total electrical shutdown can appear, even if the engine and mechanical parts are otherwise sound.
Description Of The Problem Periodic Total Power Loss
The specific case involves a 2007 Volvo VNL 670 with a D12 engine whose electrical system shuts off roughly every ten minutes. The shutdown occurs:

• With the engine running
  
• Also with the engine off but key on
  

In both conditions, the truck’s electrical power drops out, then returns. That points to a systemic power or communication problem rather than a simple engine-only issue.
The owner reports a list of diagnostic codes including entries for:

• Road speed data erratic
  
• Idle validation switch signal
  
• Engine oil pressure reporting issues
  
• Ambient air temperature communication error
  
• Electrical fault
  
• SAE J1939 data link data erratic
  

This mixture of codes from different sensors and subsystems is a classic sign that the modules are losing stable electrical reference or communication rather than all those sensors failing at once. It strongly suggests:

• Intermittent power or ground to one or more control modules
  
• Data bus integrity issues
  
• Corroded or loose connections at critical junctions
  

Interpreting The Codes Data Link And Signal Errors
On modern trucks, fault codes tagged as “data erratic,” “electrical fault,” or “communication error” often point toward wiring rather than component failure. J1939 is a two-wire twisted pair data bus that allows the engine, transmission, ABS and other modules to share information.
Key terminology:

• MID (Module Identifier)
  Identifies which control module is reporting the fault. For example, MID 128 generally refers to the engine control module.
  
• PID/SID (Parameter/Suspect Identifier)
  Identifies the specific parameter or circuit, such as oil pressure, road speed, or idle validation switch.
  
• FMI (Failure Mode Identifier)
  Describes the type of fault, such as “high,” “low,” “erratic,” or “loss of communication.”
  

When multiple PIDs and SIDs from different subsystems all show “erratic” or “communication” type failures, and they appear around the time of the electrical dropout, the pattern fits a root cause like:

• J1939 bus intermittently failing
  
• Shared power supply or ground feeding several modules cutting in and out
  

This explains why replacing components like the EGR cooler and valves does not affect the intermittent electrical shutdown. Those parts sit in the emissions and air-handling system, not in the truck’s power distribution or communication backbone.
First Line Checks Connections And Grounds
Experienced heavy-truck technicians often summarize troubleshooting priorities in a tongue-in-cheek list:

• Step 1 connections
  
• Step 2 connections
  
• Step 3 connections
  
• Step 4 wiring
  
• Step 5 everything else
  

Behind the joke lies a statistical truth. In many fleets, over half of chronic electrical faults can be traced to poor connections or grounds rather than failed modules. Field studies in commercial vehicles have shown that corrosion-related wiring issues can account for 40–60% of electrical breakdowns, especially in older trucks that have seen years of road salt and moisture.
When an entire electrical system drops out at intervals, high-priority checks include:

• Battery terminals for looseness, corrosion, and damaged eyelets
  
• Main ground connections between batteries, frame, and engine block
  
• Power and ground feed studs on firewall pass-through panels
  
• High-current fuses or fusible links supplying the control systems
  

These points are where vibration, moisture, and previous repairs often combine to create intermittent faults that appear stable when the truck is parked but fail when temperature or movement changes.
The J1939 Data Bus As A Suspect
Given the codes referencing “SAE J1939 data link data erratic,” it is sensible to test the health of the communication bus itself. A digital multimeter is sufficient for basic checks.
Important terms:

• J1939 data pair
  Two wires commonly labeled CAN-H and CAN-L, twisted together to reduce noise.
  
• Terminating resistors
  Typically two 120-ohm resistors at each end of the bus. In a healthy system, the total resistance measured between CAN-H and CAN-L is about 60 ohms.
  

A basic diagnostic procedure includes:

• Measure resistance between the J1939 data wires at the diagnostic connector with the system powered down
  • Expected reading near 60 ohms
    
  • A significantly higher or lower reading indicates missing or extra resistors, shorted segments, or module problems
    
• With power on, measure voltage from each data wire to a good ground
  • Both wires should sit near 2.5 volts
    
  • One line will be slightly higher than 2.5 V and the other slightly lower during communication
    
  • A reading pinned at 0 V or near battery voltage on one wire can indicate a short
    

If the J1939 bus values are far from these norms, the truck’s modules may drop off the network, triggering numerous communication-type fault codes and potentially causing shutdown behavior as critical modules lose contact.
Firewall Pass-Through Panel A Known Weak Point
On this Volvo platform, there is a pass-through power and signal panel mounted on the driver’s side of the firewall. This panel typically includes:

• Studs for positive and negative power distribution
  
• Several ground wires stacked on mounting bolts
  
• Harness connectors passing signals into the cab
  

Over time, the following issues are common:

• Corrosion on ground lugs, increasing resistance and causing voltage drops
  
• Loose or cracked studs that create intermittent power feeds
  
• Moisture intrusion leading to green or white corrosion products around connectors
  

Recommended inspection steps:

• Remove the panel covers on both engine side and cab side
  
• Clean and tighten all ground lugs, ensuring bare metal contact
  
• Inspect the positive and negative studs for looseness or signs of heat (discoloration, melted plastic)
  
• Verify torque on fasteners and replace any damaged hardware
  

Technicians have reported cases where broken or loose studs at this panel caused intermittent no-start and shutdown issues. Once the studs and grounds were repaired, the faults disappeared without replacing any expensive control modules.
Battery And Main Power Feed Integrity
A frequent root cause for intermittent shutdowns is a compromised main power feed. On the truck described, a specific heavier-gauge wire (often identified in diagrams as circuit 17A) carries power from the battery to engine management fuses and relays at the pass-through panel.
Typical checks include:

• Trace the main feed wire from the firewall panel back to the battery
  • Look for an in-line fuse or fusible link connected to the positive post
    
  • Inspect the fuse holder for melting, discoloration, or loose terminals
    
• Inspect the ground return wire from the same circuit back to the negative battery post
  • Pay close attention to the crimped eyelet at the battery terminal, as strands may break internally and create intermittent contact
    
• With the truck off and key on, and all accessories like radio switched off to keep the cab quiet, gently wiggle the harness, connectors and fuse holders while listening for:
  • Relays clicking
    
  • Engine fan solenoid engaging and disengaging
    
  • Other control components audibly cycling
    

If wiggling a particular section of harness or connector causes the engine management relays to chatter or the system to reset, that area is very likely where the intermittent power loss originates. This simple “wiggle test,” combined with voltage monitoring at key points, is a time-tested method for chasing intermittent faults.
The Temptation To Replace The VECU
The truck owner in the case study ordered a used Vehicle Control Unit (VECU), suspecting that some of the codes pointed to that module. While module failure is possible, field experience suggests:

• Electronic control units are less common failure points than wiring and connectors
  
• Swapping modules without first verifying power supply and bus integrity can lead to unnecessary cost and frustration
  

Industry data from heavy-truck service centers indicates that in a large fraction of “no-communication” or “random codes” cases, less than 20–30% are ultimately resolved by replacing a module. The rest are resolved by repairing wiring, grounds, or power feeds.
This does not mean the VECU is never at fault, but a solid diagnostic sequence is:

• Confirm stable battery voltage at all module power and ground pins
  
• Confirm data bus resistance and voltage are within spec
  
• Only then, if symptoms persist with known-good wiring and confirmed signals, consider module substitution or repair
  

Removing Unused Telematics Devices
The owner also asked about an old, unused telematics unit (often a Qualcomm or similar device). These devices are sometimes integrated into the truck’s electrical or data bus system.
Points to consider before removal:

• Some aftermarket telematics boxes are wired directly into ignition feeds and J1939 or J1708 data lines
  
• If removed carelessly, cutting wires rather than properly disconnecting at a plug can leave open circuits or shorts on the bus
  
• In rare cases, a failing telematics box can disturb the data link, causing erratic communication
  

Safe removal guidelines include:

• Identify whether the telematics unit uses OEM-style connectors that can simply be unplugged
  
• If spliced into the harness, label wires and either restore the original circuit using soldered and sealed joints or use OEM repair harnesses
  
• After removal, recheck data bus resistance and voltage to ensure the bus is still healthy
  

This ensures that the truck’s core electrical system is not made less reliable by an attempt to remove an unneeded accessory.
A Story From The Road Intermittent Shutdowns In Real Service
Consider a long-haul driver running a similar Volvo tractor on a regular route. Every fifteen minutes, his dash flickers, gauges drop to zero for a moment, and then recover. At first, the truck keeps moving, but one night the entire electrical system cuts out while cruising, forcing him onto the shoulder.
In the repair shop, the first guess is a failing ignition switch, and then suspicion shifts to the VECU. Before ordering parts, a technician decides to start with basics:

• Battery connections are cleaned and tightened
  
• Ground straps from frame to engine are removed, wire-brushed, and reinstalled
  
• The firewall pass-through panel is opened and reveals a cracked positive stud with visible heat damage
  

After replacing the stud and cleaning the ground lugs, the electrical shutdowns stop entirely. Months later, the truck remains trouble-free, illustrating how a mechanical defect on a power stud can mimic a complex electronic failure.
Data-Supported Approach To Preventive Electrical Maintenance
Fleets that track electrical failures over time often see clear patterns:

• Trucks operating in high-corrosion regions (road salt, coastal air) show more connector and ground-related faults
  
• Preventive programs that include scheduled inspection and cleaning of key grounds and pass-through panels reduce electrical breakdowns significantly
  

Although precise numbers vary by operator, some fleets have reported:

• Up to 30% reduction in unscheduled electrical repairs after implementing annual power-distribution inspections
  
• Lower incidence of phantom communication codes when J1939 harnesses and connectors are periodically inspected and protected with appropriate dielectric and anti-corrosion treatments
  

In practical terms, routinely checking and maintaining:

• Battery posts and main cables
  
• Frame-to-engine grounds
  
• Firewall pass-through studs and ground stacks
  
• Main fuse blocks and in-line fuses
  

adds relatively little labor time but can prevent the kind of intermittent total shutdown described in this case.
Practical Diagnostic Checklist For Similar Symptoms
For a truck where the electrical system shuts down on a repeating cycle, a focused checklist looks like this:

• Verify battery health
  • Load-test batteries and confirm voltage stability under load
    
• Inspect and service all main power and ground connections
  • Battery terminals, ground straps, engine block connection
    
  • Clean, tighten, and if needed replace corroded eyelets
    
• Open and inspect the firewall pass-through panel
  • Clean and tighten grounds
    
  • Inspect power studs for cracks, heat, or looseness
    
• Check the main engine management power feed circuit
  • Locate in-line fuse or fusible link from the battery to the pass-through
    
  • Inspect for melting, burning, or intermittent contact
    
• Perform J1939 bus checks
  • Resistance near 60 ohms with key off
    
  • Roughly 2.5 volts on each line with key on, and no shorted lines
    
• Conduct wiggle tests with key on
  • Listen for relays or solenoids reacting when harnesses or connectors are moved
    
• Only after all of the above, consider module failure
  • If power, grounds, and data bus are verified good and the problem persists, test with a known-good VECU or EECU as appropriate
    

Following this sequence minimizes guesswork and helps ensure money and time are spent where they make the most difference.
Conclusion
An intermittent electrical shutdown on a Volvo VNL 670 with a D12 is most often a wiring, power distribution, or data bus integrity issue rather than a mysterious electronic curse. The combination of multiple “erratic” and “communication” codes across different parameters is a strong clue that a common electrical denominator is at fault.
The most effective strategy combines:

• Careful inspection and cleaning of power and ground paths
  
• Verification of J1939 bus health with simple multimeter checks
  
• Logical progression from basic wiring toward more complex module diagnosis
  

With that methodical approach, many trucks that appear to suffer from chronic electronic problems can be returned to reliable service with surprisingly straightforward repairs, proving once again that in heavy vehicles, a shiny new module is rarely as powerful as a clean, tight connection.