Is this a VDO tach? If so, it needs to be adjusted for the particular engine. Is there a small hole over a button on the back? If so, this is an adjustment button. Instructions for calibrating it can be found on the web.
It is a VDO and there is no screw or hole to insert a small screw driver to adjust.
A Volvo dealer who sells parts told me that the tach has to be paired to a specific rev counter sender. I replaced the OEM with a new volvo part and there was no change.
What is baffles me is:
The RPMs read from a laser tach does not agree with the original tach nor the used supposedly tested replacement tach.
So at idle..ie with the lowest fuel supply the throttle can deliver
the original tack now reads ~650
the laser reads ~800
the replacement reads ~ 1400
I formerly was able to get the revs to over 2600 (full throttle) with old gauge which now seems to be ~2100. So it seems to be reading ~15% low. Didn't test w/ laser. And didn't full throttle w/ replacement.
I think I will keep the old and simply recompute in my mind. I am thinking it's a wire issue... but what?
there are only 4 connections and the OEM harness is labeled
-, +12, W, G (W & G are connected to the sender) I also ran new wires from send to see if results changed... they didn't so I conclude that the OEM wires are OK... connectors are new.
did not measure the voltage of the - and the +12 wires.
Could this just be a stiff sticky tach?
What's with the replacement?