Mk3 Ls400 Rev counter going haywire

[Malc]

Just got back from a 4 day 900 mile trip from Kent, to and around Wales and almost immediately we left  the Rev Counter dipped to zero, only revving would bring it back up to any sort of useful ( or not ) reading but never more than about 1200 revs.  So it's definitely not stuck.

No amount of cajoling would get it back to it's usual impeccable performing self.

Any ideas on getting it back to normal performance ?

Thanks for any thoughts everyone.

Malc

[Malc]

.....  ok, so today it's starting to hit 3 to 4000 revs when I know it should be about there, and more at 1200 when I know it shouldn't BUT very often at about zero when it should of course be higher as it's driving at about 30 - 40 mph.

No ideas coming from you guys doesn't bode well 

I've asked ( texted )  my usual mechanic of about 9 years standing and he hasn't commented ( yet )

What do I do next I wonder  ?

Malc

[steve2006]

The earlier models had a tendency to stick at zero but a thump on the dash normally sets them off again but not the series 3.

It could be the cluster itself, a loose connector plug at the rear or the tacho signal is missing.

I think the signal is supplied from one of the coil packs but don't have a wiring diagram, if anyone has I would be happy to trace the tacho signal.

[steve2006]

Tacho signal originates from one of the ignitors, these are stacked together under a black plastic cover mounted on the driver's side inner wing.

[Malc]

Thank you so much for this Steve. I'll pop this info across to my indy to help him on his way !

I'll let you know when he's sorted the issue ( or not ).

And if anyone has more great info as indicated then that will be very much appreciated

Thank you

Malc

[steve2006]

More info, one of the ignitors will have an extra wire on the plug connector, either a plain black wire or a black wire with a white stripe on the later models, this is your tacho pulse feed wire.

[Malc]

Thank you

Malc

[malcolmdc]

Quote

 

This may be a clue to my own, similar, problem (see previous posts) on my low-mileage Mk I. I will investigate for said wire - if its intact, what is the next step?

[Malc]

Ok guys, sadly I have received these words of wisdom from my indy ............  not sure what's next !!! 

Maybe it's best just to live with it not functioning properly  .......  BUT it has today begun a better behavioural pattern actually   so who knows, it might settle down.

Lexus issue. This will be the dash cluster as the rev counter motor would have given up and unable to correctly pick up the signal. It is very rare for the coil to stop sending the switching pulse as this normally causes a engine misfire and power would be down. I would safely say a new dash or repair of dash both of which I can't do.

Malc

[Malc]

well well well ...... who'd have thought a good by-product of this extremely hot weather ( I guess )  .............  today driving for about 20 miles in stop start traffic and a little dual carriageway,  amazingly the rev counter is behaving impeccably, back to normal ..........  just flabbergasted 

But HAPPY  ...............................  heaven knows why the fault has vanished, but it has, and some  10 months later 

Malc

[Spacewagon52]

It is a not well known but the LS 400 was designed with the capability to repair itself. There are numerous examples posted on this forum to confirm this.