Strange behaviour of tachometer

[larsmhh]

Hi!
I took my DeLorean for a ride yesterday about 130 miles and on the last 30 miles I recognized that - looking on the tachometer - the motor appeared to run with about 4000 - 5000 RPM, driving 60 mph on the 5th gear...
Standing on traffic lights the tachometer still was showing high values with about 2000 RPM, but the engine sounded absolutely normal and not running high, so I think there is an issue with the tachometer.
Tipping the accelerator showed normal behaviour of the tachometer needle (going up and down), but simply 1000 to 1500 RMP too high. Also when driving 60 mph the 4000 to 5000 RPM were shown "normally", no jumping or failing of the needle.
Maybe somebody knows about this issue and has an idea, where to search for a fault or what to do...
Thanks in advance,

Lars

[Bitsyncmaster]

That is a strange failure. My guess would be you have noise on the ignition spark. Make sure the capacitor on the ignition coil is still connected and good. Seems like your getting double the RPM reading.

[Jason]

What does the volt meter read when this happens? I'm not sure why, but low voltage at the instrument cluster will make the RPM meter read high. My car will do this if the headlight and AC are on and for some reason RPM drops below 700 or so. Instrument cluster (and headlights) dim, volt meter reads low, and the RPM meter goes high. My alternator just can't quite keep up at low rpms.

[David T]

Usually the tach either bounces around, reads low or is dead. They usually don't read high. First get another tach and verify the correct RPM's. If the tach is really not accurate it is often due to a bad connection somewhere.

[Ron]

What does the volt meter read when this happens? I'm not sure why, but low voltage at the instrument cluster will make the RPM meter read high. My car will do this if the headlight and AC are on and for some reason RPM drops below 700 or so. Instrument cluster (and headlights) dim, volt meter reads low, and the RPM meter goes high. My alternator just can't quite keep up at low rpms.

Put a smaller pulley on it...

[Jason]

Put a smaller pulley on it...

I just increased the idle speed a bit. Which also helps with AC while sitting in traffic.

[Helirich]

I recently had an issue where the "dash services" fuse blew. Somehow power was able to backfeed to the guages. So they didn't completely quit. But they were reading erratic. Installed a new fuse and all is well.

[larsmhh]

I recently had an issue where the "dash services" fuse blew. Somehow power was able to backfeed to the guages. So they didn't completely quit. But they were reading erratic. Installed a new fuse and all is well.

Hi!
Thank you for all your answers!!!
I will check the capacitor, the fuse (should be fuse No. 5) and the voltage (shown on the instrument cluster-voltmeter and measured withe a voltmeter) this weekend and write about my findings.
As I can remember, the voltage was normal, showing between 12V and 13V on the gauge, but , when I think it over, it was the first time this year that I drove with the AC switched on...
Unfortunately I did not check the tachometer while switching the AC on and off...

Best wishes and thanks again,
Lars.