Diagnosing a tachometer (Page 1/1)
tjoudrey MAY 30, 09:07 PM
I am working on a 85 Fiero GT which has had its tachometer go bad. I have managed find the signal wire coming from the Ignition coil and through the tach filter producing 'AC' voltage bottoming out a 7v while idling, increasing with the engine speed. I have tried to repair the original tach and connecting a tach from a different fiero with no luck. Can anyone think of anything I am missing?

Thanks.
Gall757 MAY 30, 11:10 PM
Send a PM to forum member J Gunsett.
Mickey_Moose MAY 31, 02:20 PM
Been a long time since I work on them, but the tach is frequency dependent not voltage (it counts the pulses).

Now the part I don't remember is the min voltage level needed to operate the tach, but this 7 volts you are reading now is measured how? Are you using a regular meter or one that measure true RMS? A regular meter will read the level to be lower than it really is. Using an oscilloscope is the best way to measure the signal. Also where are you measuring this signal (at the tach or at the motor)?

If you have an 12v AC wall wort you can use this to test the tach (you feed the 12 VAC signal into the tach) - again I forget what reading you should get, but if the needle rises to some value and stays constant you know the tach is at least "working".

The 2nd tach, do you know for sure it is working? I would suggest try swapping out the tach filter if you know the tach is working - or try the tach another car. Another thing to check is that the connector on the back of the pod is good as it is possible for the "tabs" that make the connection could be missing or moved out of place (assuming the pins on the connector are also good).


edit: ok, I believe the tach should read 1200 RPM if using a 12 VAC wall wort (60 hz signal) - assuming a V6 tach

[This message has been edited by Mickey_Moose (edited 05-31-2019).]

tjoudrey MAY 31, 06:11 PM
I think I may have sorted it out, I pulled my tach out of the cluster and connected the 12v and grnd leads to a 12v battery, I then rubbed a second positive lead against the coil lead on the board and voila it moves, So does that mean that the filter in the car is faulty?
Gall757 MAY 31, 08:57 PM

quote
Originally posted by tjoudrey:

I think I may have sorted it out, I pulled my tach out of the cluster and connected the 12v and grnd leads to a 12v battery, I then rubbed a second positive lead against the coil lead on the board and voila it moves, So does that mean that the filter in the car is faulty?



you can bypass the filter to find out.
tjoudrey JUN 01, 02:17 AM
This is slightly embarrassing but I feel a need to post this for anyone with a similar issue in the future. It turns out I was neglecting to plug in the second connector when testing the new tach.