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Tachometer troubleshooting

Discussion in ''How To' & 'Handy Hints'' started by Doob, Aug 22, 2021.

  1. Doob

    Doob Member

    Messages:
    52
    Location:
    Perth
    I thought I might share my experience in trouble shooting issues with a VDO tachometer, when hooked up to my stock standard VW/Bosch coil & distributor igniting system.

    After spending a lot of time, effort and expense in rebuilding my 1600cc motor, I thought it prudent to add some instrumentation to monitor it, so I fitted VDO oil temperature, oil pressure and tachometer gauges. The oil temperature and oil pressure sender units and gauges performed as expected. However, the tachometer was erratic and reading high. After a call to the supplier of the tachometer, I spent some time checking all electrical connections and tachometer switch settings, replaced the ignition points, checked dwell angle etc. but to no avail. After a ‘Google’ search I tried adding various resistors into the circuit from the tachometer to the coil. This perhaps helped reduce the erratic response a little, but the reading remained inaccurate. Eventually, I returned the tachometer to the supplier to check calibration. The unit checked out OK on the bench, but the supplier did discover a service bulletin that suggested adding a diode to the circuit (see attached).

    After adding a 1N4004 diode, the tachometer settled down nicely. [Cathode toward the tachometer & Anode to the -ve terminal of the ignition coil].

    This worked for me, and from thesamba forum posts listed below, it seems to have worked for others (but maybe not all).

    I do wonder though, if there is an underlying issue with the ignition system causing the corrupted/noisy signal to the tachometer – and that this diode fix is masking a problem that should be further investigated?

    From thesamba.com ;

    https://www.thesamba.com/vw/forum/viewtopic.php?t=248223

    https://www.thesamba.com/vw/forum/viewtopic.php?t=505340

    Attached - AISAT Service Bulletin from 1998
     

    Attached Files:

    grumble likes this.
  2. Barry

    Barry Well-Known Member

    Messages:
    6,181
    Location:
    Abbotsford NSW
  3. Wiz

    Wiz Well-Known Member

    Messages:
    564
    Location:
    Near Ballarat
    Good to know. How does it measure temp?
     
  4. Barry

    Barry Well-Known Member

    Messages:
    6,181
    Location:
    Abbotsford NSW
    Gathering it measures the temp of the dizzy - not the oil temperature
    That said - it is the "change from normal" that is the base line for me

    better than nothing

    Tacho works a treat
     
    Mordred and Wiz like this.
  5. syncro

    syncro Well-Known Member

    Messages:
    10,371
    Location:
    Southern Highlands
    Sounds logical. A different voltage at the gauge to the distributor could do this. It could be a Volkswagen thing with such a long cable run to the engine.
     

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