Erratic Tachometer (Page 1/1)
lurtz SEP 12, 10:39 PM
Good Evening.

I am experiencing odd behavior with the tachometer in my 84 4spd MY8 Notch. The tachometer is on the fritz and is bouncy or reads incredibly low when I start the vehicle after sitting. After I have run about 10 miles of my 50-mile commute, the tach will resolve its issues and function generally satisfactorily with smooth sweeping motions and accurate rpm displayed.

The bouncing or low readings will sometimes be throttle-dependent. The rpm's will register close to correct but 1000 rpm short when on the throttle, but if I release the pedal and close the throttle, the rpm's will drop to zero sometimes.

During this tachometer break down, the car does not seem to be affected. No ignition issues or engine misfires.

From what I recall, the tach receives it's signal from the icm. Is it safe to assume that if the icm was suspect I would experience running issues?

Is there a tach filter on an 84 duke? If so, is there a way to test it?

Has anyone else experienced this before?

Does anyone have a suggestion. As to how I can test this issue?

Thank you all for your help.

------------------
1984 Base Coupe Red Stock MY8

2002 Grand Prix GTP 40th Anniv. Stock (Now Donor Material)

[This message has been edited by lurtz (edited 09-12-2022).]

theogre SEP 12, 11:48 PM
Yes is a Tach Filter and likely bad. Tach itself may also be bad.
Small can thing attach to engine, around the thermostat, and 2 pin plug to wiring.
See my Cave, HE Ignition

Isn't a test for it.
Just pull the plug and leave it out because iffy filter can make engine "dead" to fry I-coil, ICM, etc. w/o more warning.

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

lurtz SEP 13, 12:05 AM
Are there replacement tach filters?
theogre SEP 13, 11:40 AM
Some aftermarket, not from GM.
rodney dickman has them but some say doesn't work on some HEI engines... use search.
Maybe some ICM hate these... Even if you get ICM from GM/ACDelco now, isn't same as OE or some others and ICM may not play well w/ 3rd party T-filters.

Read all cave page. If ICM isn't grounded right w/ crap screws etc then will have tach and other problems.