unexplained jump in tach readings

That is strange. If the engine is running well, I would say the issue is with the tach itself. You can check the signal between the tach and the inductive source, which I assume on those is a negative inductive signal.

 

I had the opposite problem, i.e. the tach was registering an incorrectly low rpm reading. I know others have also had their tach out of wack, but only reading too low, not too high. The fault in my case was the tach itself. The spring and gears in the gauge needed to be cleaned, and that has (so far) fixed the fault.

 

I had tach issues on my 87 328 as well. Mine was reading too low. It is an electronic tach for the 328 and there are no gears etc.

There is a calibration potentiometer on the tach. It gets corroded over time and the calibration resistance goes off. Mine would would work fine and then drop to half for no real reason. Known and not uncommon issue for these.

You can pull the tach and pop the case off the back. If you turn the calibration pot back and forth a few times it will usually clean up and work fine. Pay attention to where it is set so that you can put it back where it was. My timing light has a tach function so I just used that with the engine running and the timing light in my lap and the tach hooked up to reset the calibration. Ran the engine to 4000 on the timing light and then set the tach to 4K with the pot. You could probably replace the pot with a new one if you are an electronics handy fellow. Not me, I solder like crap.

Cleaning the pot worked for a while for me.. but eventually I bit the bullet and bought new tach. They run about $800 dollars from Ricambi... or PM me and I can give you a source in the UK. It was about $350 after currency exchange and customs fees etc. for a new yellow boxed one (date stamp on new one was 2003). The orange of the lettering doesnt match the other gauges so that is a minor issue. My other gauges are all faded slightly but evenly to a visible color difference. Yellow-orange instead of the bright orange of the new one.

My old tach is in the garage so let me know if you want a photo of the potentiometer location.

 

@ NW328GTS

I wouldnt mind that photo as my take won't go over 2500 rpm at all and figure it needs a cleaning.....might just buy a used one some where if the cleaning doesnt fix the problem..

 

My 1979 308 GTB shows (occassionally) 4K when at about 2K. Working through lots of other 308 issues. Not there yet.

Tach reads negative coil lead, doesn't it? Could faulty ignition (points/or condenser/or coil) cause faulty signal for tach?

 

My tach is kinda spot-on, but I've noticed lately that while cruising, the SPeedo & the fuel gauge will bounce around. The MPH might float by 20mph. But if I'm accelerating/decel, then they settle down (I'm guessing more juice in the circuit) Must be a bad ground.

 

CCCVIII will check engine frame ground first.

 

CCCVIII will check engine-frame ground first.

 

Just had the same problem on my 86' 328 and the problem was a short in the console dimming switch. Replaced it and the tach went back to normal. Hope this helps

 

bringing back this old thread...ahh never mind ..better to start new thread altogether

 

Did you ever get your Tack working properly?

 

never did Roc but then again I didnt really try as I found a mint one (mine was kinda faded/worn) for a great price so I'm just gonna install the new one

...when I take the old one out I'll see if I can get it adjusted as a spare