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Tachometer fault

I have a Siemens VDO tachometer with engine hours counter. It's a KI "W" Inductive, N02 012 165 (168). The unit stopped working some time ago. The rev counter is firmly fixed at zero, although the pointer does twitch a little when the ignition switch is turned. The hours display is there but is flashing & has been stuck on 1213.2 for about 40 hours. I have checked the "W" connection at the alternator & at the tacho; both readings are about 7.5 volts. Also, the 12volt supply to the unit seems to be ok. The unit has what appears to be a "reset" button. Pressing it had the same effect as turning off the ignition &, after a few seconds, things returned to how they were. Everything else on the instrument panel is functioning normally. The engine is an Isuzu. Any ideas please?

Asked by: chrissy  | 8.52pm, Wednesday 23 March

WW says:

The supply and signal from the alternator seem to be OK, although since the W terminal signal is AC with the frequency varying with engine speed it is difficult to check with a multimeter. The alternator might not be supplying an AC signal.
Another possibility is that the tachometer has lost the programming. One of the terminals is a constant 12V supply and if this was isolated the memory might have been wiped. The button at the back is not a reset; it is used to set up the instrument.
For more details refer to the manual which can be found at http://www.dnvdo.com/files/8207/20120601/125302-698902c73010d374ca087168ab30a80a.pdf

Rupert Smedley  | 10.34AM, Thursday 24 March

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