I am working on an 88 Volvo 760 Turbo 2.3L. When the vehicle is not warmed up, the tachometer needle twitches upward about 300 RPM and then quickly back to the correct reading. The engine has steady RPM. I checked the CKP sensor wire at the Dist. and found a nice, clean, even square wave pattern that changed proportionately with engine RPM. The clean waveform is also seen on the input signal wire to the power stage. I checked both the signal wire from the power stage and signal wire to the inst. cluster which showed a waveform much similar to that of an inj. pulse, but with a 12 volt steady section in between the on/off time voltage spikes. This steady voltage section is the same as the clean waveform signal steady voltage of 5 volts as seen straight from the CKP and into the power stage. I am not sure if this stepped up wave form seen coming from the power stage and also into the inst. cluster is a good pattern, but it seems to stay the same even when the vehicle warms up and the twitching tach. stops twitching, so I am assuming that it is not the problem. The tachometer display/head has been replaced in the past for unrelated issues, but the twitching tach. problem was there before and after the replacement of it, so it is fairly safe to rule that out as well. I have replaced the coil and it seems to have dampened the twitching some, but the problem is still there. Any ideas on what I should check next, or what might be causing this problem? Thanks for your time.

 

I would try an Aftermarket tach.