Analog tachograph

Analogue tachographs record speed, distance travelled and driver activities on paper record charts. In older MAN and Mercedes trucks, buses and vans, faults in the power supply, signal transmission or recording mechanism can cause incomplete records. However, an incorrect speed reading does not automatically mean that the tachograph is faulty: the gearbox drive, pulse sender, tyre size and calibration also affect the measurement.

Always identify parts using the vehicle's chassis number, together with the data plate, approval number and existing tachograph configuration. Visual comparison or selection based solely on the model name is never sufficient. Analogue systems can be found in older MAN F2000, Mercedes SK, Atego and Sprinter models, among others, depending on the year of manufacture and specification. For cab-mounted installations, the dashboard cut-out, mounting points and cable clearance must also match.

Distinguish between mechanically driven tachographs with a drive cable and electronically controlled analogue versions with a pulse sender. There are also round instruments and rectangular slot-in models, with different arrangements for one or two drivers. Check the nominal supply voltage of 12 or 24 volts, connector pin assignments, speed range, chart approval and signal outputs. The tachograph constant k, expressed in revolutions or pulses per kilometre, must be matched to the vehicle characteristic coefficient w in accordance with the specified adjustment procedure. The effective tyre circumference is also a determining factor. A housing that fits therefore does not guarantee accurate recording.

The round tachograph, also known as the speedometer type, is the classic and best-known design. This unit served as both a speedometer and a clock on the dashboard. The front could be hinged open, allowing the round paper tachograph chart to be placed behind the dial.

The rectangular analogue tachograph in radio or DIN format appeared in more modern dashboards in the 1990s and early 2000s, before the introduction of the digital tachograph in 2006. Manufacturers wanted to separate the speedometer from the tachograph. This version had the shape and dimensions of a conventional car radio and was often fitted above the windscreen or in the centre console. Instead of a hinged cover, the unit used an automatic slide-out tray, similar to that of a CD player, into which the round paper chart was placed. The speed was transmitted digitally to a separate speedometer in the dashboard.

Even with a rectangular housing, the paper tachograph charts were always round. The chart rotated like a clock, completing one revolution every 24 hours. Mechanical recording styli moved outwards from the centre, scratching speed, distance travelled, and driving and rest times onto the chart.

A jumping speedometer needle may indicate a binding drive cable, a worn drive connection or an interrupted pulse train. Missing recording traces can be caused by damaged recording styli, incorrectly positioned record charts or a faulty chart drive mechanism, among other things. If the clock has stopped, check the permanent power supply before condemning the clock mechanism. Oxidation in connectors and insufficient contact pressure often cause vibration-related intermittent failures.

1. Document the fault: compare the speed reading, distance counter, clock operation and recording traces. Check that the correct, undamaged record chart is properly positioned and that the closure locks fully.
2. Measure the power supply: use a multimeter to check the permanent supply, switched supply where fitted, and earth directly at the connected tachograph. Measure the voltage drop across the supply and earth cables under load and compare it with the manufacturer's specifications.
3. Check the signal path: inspect wiring harnesses for chafing and connectors for corrosion. On electronic versions, use an oscilloscope to measure the pulse shape, amplitude and frequency at a known rotational speed, following the correct wiring diagram.
4. Inspect mechanical drives: check cable routing and connections wherever accessible without breaking seals. Have the inner cable, square drive ends and drive gearing assessed for wear and play.
5. Confirm the measurement error: have an approved tachograph workshop check speed, distance and time recording using suitable test equipment. Assess tolerances against the applicable inspection requirements, rather than relying solely on a comparison during a road test.

Repair power supply problems, damaged wiring or a faulty pulse sender first; otherwise, a replacement tachograph may exhibit the same symptoms. Avoid tensile loads on connectors and excessively tight bends in mechanical drive cables. Do not use unspecified lubricants in the recording mechanism: contamination can damage charts and recording styli.

Have work on sealed components, calibration and resealing carried out by an authorised tachograph workshop. Check beforehand whether retaining an analogue version is legally permitted for the vehicle and its intended operation; retrofit requirements may apply to international transport. After replacement, have the recording function and any linked speed functions checked before returning the vehicle to service.