Revs reading high

[TJ]

Hi,
during a recent ride my FJ again started to bug me again.
All of a sudden the tachometer displayed rev readings twice as high as usual.
What confuses me is that it is not a faulty reading in the sense of bouncing or being not reliable,
it shows consistently the doubled value... (My FJ has a slight vibration at around 4k, now I feel this vibration at a reading of ~9.1k)

A  couple days later during another ride the revs dropped to normal for about half an hour
but then raised to double again and stayed faulty since.

Has anyone ever had similar issues? What could it be? The instrument cluster? Wiring harness?
Tach and wiring is still stock 

[roverfj1200]

Most of the time I have seen tachos operating erratically it has been a battery on the way out. But in your case it sounds like the pickup wire is shorting across 2 sender wires thus doubling the reading. That is by no way a solid guess. I have never looked at the routing of the tacho wiring on a diagram before. When I have fitted aftermarket tachos to cars and so on there was a switch to select 4 ,6 or 8 cylinders and if the wrong one was selected it would show incorrect reading. not sure if there is such a switch on a FJ tacho or if it would have been needed if the tacho was used on 2 and 4 cylinder bikes. Well that is all I can think of.

[Flynt]

All of a sudden the tachometer displayed rev readings twice as high as usual.

A  couple days later during another ride the revs dropped to normal for about half an hour
but then raised to double again and stayed faulty since.

Has anyone ever had similar issues?

I had this symptom on my '92.  The fix was to improve the grounds for the coils and make sure the tach ground is exactly the same (zero ohms resistance between the two).  I added a ground wire to the coil ground which is normally accomplished via the mounting bolt, but I think just cleaning up the bolt's connection to the frame would have fixed it.  If the tach "ground" and the coil "ground" are not the same, you'll get what you're seeing on the tach.

Frank

[FJ_Hooligan]

Also check as many connectors as you can.  Slight corrosion on a connector pin can cause irregular signals especially with vibration.

Check main battery ground cable on the rear of the engine case

[TJ]

Added "rebuild wiring" to the winter todos 

For the moment i'll check the ground probably tomorrow.
And will keep you updated.

Thanks for the quick suggestions 

[PaulG]

Also check as many connectors as you can.  Slight corrosion on a connector pin can cause irregular signals especially with vibration.

Check main battery ground cable on the rear of the engine case

Basically the same issue and cure for me. I narrowed it down to corrosion on the coils and the ignitor box under the fairing. Very fine emory paper for the connections, and used a jewelers file set to try and push the emory inside the ignitor box connectors.  Sprayed it out with some contact cleaner, then lubed it all up with dialectric grease before reassembly.

Had an intermittent problem with it a few yrs ago, esp after sitting for a while under it's cover.  Haven't had an issue since.

[TJ]

Finally found the cause of the wrong readings!
(No worries I just had two quick weekend sessions, wasn't searching the cause for over a week)

Last weekend I checked the ignition coils wiring, especially ground.
Removed some corrosion and... nothing changed, still high rev readings.

Today I checked the resistance of the wires between ignition coils and tach, as well as battery ground and tach.
Identified the big connector (under right inner fairing panel) as a source of high resistance.
Measured ~17 ohms from female to male side of the connector
Applied some contact cleaner and contact spray, measured again: ~1.3 ohms
Not great but at least better. Same goes for the rev readings.
Still 300-500 higher than it should, but at least not twice as high and kinda useful again...

Thanks alot for the suggestions!
Saved quite some time

Also I'm definitely going to rebuild/replace the wiring harness at some point.