When multiple telematics or Fleet Management System (FMS) devices are connected to the same vehicle, they can generate interference on the Controller Area Network (CAN) line used for tachograph data. This interference can interrupt tachograph downloads and reduce the reliability of automated data collection. This issue typically originates from third-party devices connected to the vehicle’s FMS module rather than from poor wiring at the tachograph head.
You may see one or more of the following indicators of CAN-line interference:
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Tachograph downloads stop or fail intermittently.
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The
THIRD_PARTY_SYSTEM_INSTALLEDerror displays on the Tachograph Health page in the Samsara dashboard.
If tachograph downloads fail frequently, confirm whether a third-party device is causing interference on the tachograph CAN-C line.
The method for testing cabling depends on your setup. Use the workflow that matches your configuration to test the cabling and temporarily isolate the Samsara Vehicle Gateway (VG).
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Disconnect the CAN-C (red) lead from the vehicle side to stop interference from any third-party device.
Leave CAN-C unplugged from the truck side so that only the Samsara VG remains connected to CAN-C.
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Attach the resistor cap to the open CAN-C connector on the Y-end of the Samsara cable, as shown in the following image.
Vehicle-side CAN-C disconnected
Samsara VG CAN-C with resistor cap attached
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Turn on the vehicle and drive for approximately five minutes to confirm whether tachograph downloads resume normally.
When the Samsara VG is connected through the vehicle’s Fleet Management System (FMS) interface, third-party devices on the same CAN line can interfere with tachograph downloads. Use this workflow if the third-party device can’t be fully disconnected:
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If possible, temporarily unplug the third-party device to allow the first tachograph download to complete.
The first download may take longer and is more likely to be interrupted by interference.
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After the first successful download, reconnect the third-party device and check whether subsequent downloads continue normally.
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If the issue continues and the third-party device cannot be disconnected or powered down, connect the VG directly to the back of the tachograph using a CBL-VG-CHGV or CBL-VG-DHGV cable instead of the FMS interface.
Note
When the VG connects directly to the tachograph instead of through the FMS interface, some vehicle diagnostic data—such as fuel consumption and eco-driving metrics—may not be available. If interference is resolved, you can return to the FMS connection to restore full data access.
If tachograph downloads succeed only when a third-party device is disconnected, that device is likely introducing CAN-line interference.
To maintain reliable tachograph downloads, ensure the following:
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Keep the Samsara VG as the only device connected to the tachograph CAN-C line.
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If the third-party system must remain connected, work with the device provider to:
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Move the device to a different CAN channel, or
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Adjust its configuration to disable functions that conflict with tachograph communication.
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Use FMS cabling when possible to preserve access to additional vehicle diagnostics, provided it does not cause interference. If tachograph functionality is not required and interference occurs, contact Samsara Support to disable tachograph communication.
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