RPM meter with mind of its own

[Hendrous]

Got into our 98 discovery tdi on the weekend to drive down the south coast and pretty soon noticed the rpm meter was wobbling around and only loosely following the engine sound. I asked my wife who normally drives it to work how long it has been doing this and she said ...oh about a month' I asked why she didn't think to mention it and she said 'well the horn doesn't work either so I figured it was normal. "

Hmmmm.

There also appears to be a problem with the auto where it seems to be slipping or losing power just in top gear.
(Again if the horn doesn't work- why would you mention this before packing the car to go on a 600k drive?)

I'm wondering if anyone can suggest if these two problems auto related. The engine sounds and drives fine until you get into top gear and yet the rpm meter is wobbling up and down when in neutral.

With 360 000 k.s on the one auto I'm expecting I need to get a replacement transmission, but any advice on the tacko issue and whether this might be hinting at bigger problems would be greatly appreciated.

I have to rego it in about four weeks so I'm trying to weigh up whether all the $$$ I'm going to be up for will be more than the cost of replacing it with a newer version.

Chrs

Hendrous

[one_iota]

With regards to the tacho there are two causes that I know of:


There is a wire connection on the back of the alternator that picks up the signal for the tacho. From memory it has a 90 degree plug that connects to a tab on the alternator. This can work loose and result in the symptoms you describe. I crimped mine with pliars to give a tighter connection.
The other is that the alternator is on the way out: worn brushes for instance. This could be tested with a voltmeter. However if it has been like this for a month your battery would probably be flat by now.
Hopefully it is the former and not the latter or the Auto box.

[p38arover]

The tachos are driven by a tap off one phase of the alternator. This is a quite common technique.

So another cause (on some vehicles) is a loose fan belt. That may not apply to vehicles with serpentine belts and automatic belt tensioners.