Mack&Cheese Posted May 13, 2017 Share Posted May 13, 2017 On my '91 Mack RD688S the tachometer doesn't move much at all and usually stays in the 1400 RPM range regardless of engine speed. Where should I look first? Thanks! Quote Link to comment Share on other sites More sharing options...
j hancock Posted May 14, 2017 Share Posted May 14, 2017 Magnetic sensor may be dead. Quote JimIt doesn't cost anything to pay attention. Link to comment Share on other sites More sharing options...
Mack&Cheese Posted May 19, 2017 Author Share Posted May 19, 2017 Thanks Jim, I will start there first. Appreciate the feedback! Quote Link to comment Share on other sites More sharing options...
Mack&Cheese Posted June 13, 2017 Author Share Posted June 13, 2017 As it turns out the tachometer input is mechanical. Changed out cable with new obtained from Mack and tachometer exhibits the same fault (when running the tachometer won't go below 5-600 rpm and no higher than 1400 rpm. Very sluggish. In my troubleshooting I also found that the incandescent light bulb that is suppose to be plugged into the back of the tachometer was not plugged in, so I suspect that the insides of the tachometer are full of dust and contributing to the sluggish operation, and apparent sticking of movement. Anyone know how to take these apart?? Quote Link to comment Share on other sites More sharing options...
j hancock Posted June 14, 2017 Share Posted June 14, 2017 The bezel is crimped on to the case. Gauge shops have a tool to open the crimp, repair and then re-crimp the bezel. If you take the gauge out, try some gentle compressed air to blow the dirt out. It might work? Tachs are not too expensive so if it doesn't clean up put a new one in. Quote JimIt doesn't cost anything to pay attention. Link to comment Share on other sites More sharing options...
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