td42 tacho intermittently not working

[nibbsy]

Hey, i have a 96 GQ with a TD42 and tacho has started to jump around and drop to zero rpm and go all over the place. Just after some info on how to figure out if its the dash or the tacho sensor itself. I have had the sensor out of the the block and looked at it, it looks ok. I have checked the sensor with my multimeter on AC frequency and it appears that its sensing ok, its about 40Hz at idle and steadily climbs and the reading doesnt drop out or jump around. Is this the right way to test the sensor????
Is there anyway to test the dash itself??
It would also appear that i am having a bit of a voltage issue with my altenator too, i think the regulator might be playing up as it takes a long time after start up to get the voltage up to 13 odd volts and i have the batt light on the dash as well as the water in fuel light up. these eventually go away after the car has been running for a while but it seems like it is taking longer now than it used to. I am wondering if anyone knows whether the voltage problem could be affecting the tacho as the tacho problem has only started recently and the voltage issue has been going on a bit longer??
Any advice would be appreciated.
Cheers
nibbsy

[Mrowka]

My 1994 TD42 did the same thing. Turned out to the be crank position sensor (which runs the tach) had gone south.

How a magnet with no moving parts can stuff itself up is beyond me, but it did, I replaced it, and my tach has been ducky ever since.

[garth]

got same prob in one of my GQ;s with an manual trans. I played around for ages trying to get it to work in the end gave up and had to send the whole cluster in to get repaired - cost a fortune to fix but was a common problem I was told.
My GQ ute with an auto trans also has a non working tacho, I don't thrash it like my manual truck so haven't bothered to get it repaired- does the tacho effect the trans computer therefore trans operation at all?

cheers garth

Low charging voltage sounds like a diode let go(sub 13v?), are the batts going flat? and yes complications can occur from an unrectified current spike going through the electronics but is unusual.