on my '97 V90 3.0 I now find the Rev Counter can be erratic. Sometimes when I'm with my foot on the Brake and in Drive but stationary the revs will show less than 500 on the clock when I know its actually at around 800. Other times when on the motorway at say 2000/2500 revs it will suddenly drop to 1500 with no warning and no difference to the cars performance. It is clearly something possibly electrical affecting the rev counter but I dont know where to start to look.

The ECU sends the signal to the drive instrument panel. The problem is likely in the panel's circuit board or one of the connectors.
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'96 965, 16' wheels, Michelin Pilot Sports, rear 18mm bar + Koni, 204HP cams, 127K. Put 200K on '85 745 TD.

Or..... It could be the RPM sensor. We had a similar problem on our 95 and changed out the sensor and all was well.

DEWFPO
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1998 S90 071,245 and 1995 964 154,100

Question....

Was the tach the only symptom of the bad RPM sensor? Since the ECU is "between" the sensor and the tach I would have though there would be other symptoms? Check Engine light? Hard starting?
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'96 965, 16" wheels, Michelin Pilot Sports, rear 18mm bar + Koni, 204HP cams, 127K. Put 200K on '85 745 TD.

On our 95 960, the only noticeable condition was an occassionally, erratic tach, the rpm sensor replacement cured that.

DEWFPO
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1998 S90 071,245 and 1995 964 154,100

Where is the RPM sensor, how much is it and how easy to replace?

The rpm sensor is located at the rear of the engine just above the trans bellhousing. One bolt holds it on and it tkaes small hands to reposition it. My wife repositioned the new sensor while I screwed the bold down with a long extension.

I bought the rpm sensor either thru FCP Groton or Borton Volvo, I don't remember which. It cost about $45 IIRC.

DEWFPO
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1998 S90 071,245 and 1995 964 154,100

Wouldnt I have other instrument problems as well if there was a connection problem?