This is my tacho reading at idle speed, just over 250rpm. When I connected my scanner the idle speed is reading 724rpm so it looks like the tacho is under reading, it carries on when driving too when I'm at 70mph, it's showing just over 2000rpm which I know is incorrect. Any ideas?

 

toganga, when you turn off the car, does the tach drop to say -500 RPM (ie, below zero?)? When you start the car and the gauges do their full sweep, does it go all the way to the top of the RPM scale or does it fall short (by say about 500 RPM)? When at 70 mph, is the tach still off by the 500 RPM? If you can answer yes to all of these, you simply need to access the needle and reposition it. Somehow the needle has slipped and is off. When setting the zero, you will want to put the instrument cluster into the diagnostic mode and then hit the TRIP button twice. This will put the gauges into their full sweep mode and when they come to rest, all gauges are at the minimum reading. That would be the time you put the tach on at zero. From there, the tach should be accurate again.

Now, if the tach is not off linearly (ie, not off by 500 RPM across the whole scale), then you have a bad tach and unfortunately, the only fix is to get a new instrument cluster. The crankshaft position sensor is doing its job as the computer is seeing the actual engine RPM, it is only the indication that you see that is wrong.

If you need me to answer anything, let me know.

 

Thanks for your reply Thermo, it does read -500 with the ignition on and is only reading just over 2000rpm at 70mph which I know is incorrect, so looks like the needle needs repositioning.

As for the full sweep at engine start up, mine has never done this and I've owned the car for 14 years, even when I went into test mode, the needles twitched a little but didn't do a full sweep, is that incorrect?

 

topanga, been a few years since I owned my x-type, so, I may be mixing up vehicles. If you want to full sweep the gauges, simply depress the TRIP button and hold it while you turn the car to the RUN position. After a few seconds (normally 3), the message center will say "Engineering Test". At that point release the TRIP button. PUsh it 1 or 2 more times and this will cause all the gauges to do a full sweep one time. At the end of this, all the gauges go to their "zero" position (ie, low point). This is when you would want to reposition the needle back to 0.

 

Mine don't do a full sweep either, even in engineering mode. Never have and everything works fine. It does make a strange sound (did that same sound at ignition on when the battery died). Sounds something like a toy wind up plastic gear slipping.

 

Thanks again Thermo, but like Dell says above, there is no full sweep at all. Never mind though, hopefully repositioning the needle will do the trick.

 

Just an update to this problem. I took the plastic 'glass' cover off the instrument cluster after firstly prising off the walnut trim. A few screws later and the cluster just pulls out. I just moved the tacho needle with my hand, put the ignition on and reset it to zero. So far so good, it's been accurate so far but not sure why it went off in the first place.

 

Most instrument cluster needles are just a compression fit onto the servo shaft.
If not pressed sufficiently on, then they can be a little loose and drift on shaft, but if pressed to heavily then they can set the needle too low and rub on the dial face.

I cut a piece of thin cardboard from a cereal packet or similar as a spacer, cutting out a 3-5mm slot in it so I can slip the cardboard under the needle's hub shoulder, then press the needle down.
Remove the cardboard and there should still be decent clearance for the needle to operate throughout its range of motion.