Tachometer showing wrong RPM

[Nez'sRanger]

My 89 2.9 has this strange problem with the tachometer. When I first start it, usually when cold, the tach will give me a reading nearly 2,000 higher than it really is. It usually normalizes later on, but it's pretty annoying. Any idea what's up?

 

[Briansshop]

I'm having EXACTLY the same issue. Not finding any info...

 

[RonD]

Tach signal comes from a splice in the Coil "-" wire, between TFI and coil, a tan/yellow wire(or dark green/yellow)
If the engine is running OK then that signal is OK

The splice runs to the Computer(pin 4) and to the cluster, both tan/yellow wires(or dark green/yellow)
Don't know where the splice is, but the tan/yellow(or dark green/yellow) wire from TFI to Coil is not that long so it may be at the TFI connector or coil "-" connector
The Computers tach signal has an inline resistor

Computer would set a 2 digit code if Tach signal was erratic, like:
18 (R) Check base timing & advance function – Timing Tests
.........(M) Ignition TACH signal erratic – Ignition Systems


Tach also needs a good ground at the cluster, loose or intermittent ground would cause the high low RPM on Tach
Its a separate ground for Tach only on the cluster

If there has been no codes or engine running issues related to spark timing then problem is most likely at or in the cluster


Watch yard/garage sales for timing lights and hand held Tach/RPM meters, good to test signal at the cluster, and of course in engine bay