My 75 tachometer has been acting a little odd lately. I seems like it was sticking at about 2800 for a bout a min after start up. I thought is was cold and sticking a little but now it stay there any time the key is on or running. Key off reads zero. So its not a sticking needle. Has any one opened one up for repair? I repaired my clock last year by replacing 2 blown caps for the power supply. I may just look in the classifieds for a replacement. Let me know if anyone has experienced this…

-Best

Robert

Check your fuses and tach connection at the coil

Also, I noticed while driving in this morning when it stopped responding ( stuck at 2800) that a press of the brake pedal made the tach jump around. May be a voltage supply problem?

Grounds.... Check your grounds.

And have I mentioned you need to check your grounds?

Also fuses.

--DD

Fuse #9

I check my fuses and all are good. I am thinking it may be ground related at this point. I pulled the fuse for the instrument and console and the Tachometer immediately stated to work and respond. This was the 4th fuse from the right Any one know of hand where the dash/IC gets it ground? Will take a closer look over the week end. I may just pull the IC and bench test the Tach, taking all the harness out of the diagnostic picture…. but for now I think the Tachometer is still functional.


-Robert

There is a brown wire connected to the back of the tachometer that provides the ground. Use a DVOM and measure the resistance between that brown wire and a clean spot on the body. If it is greater than 0 ohms, you have a ground problem.

The ground point is above the fuse panel on the left side. Drop the fuse panel down and you will see it. Undo the M6 nut (10mm wrench) and clean the connections. Ohm check between the tach brown wire and the ground loop. Wiggle the wires while testing to see if it is broken internally.

Thanks for the tips I will be sure to look into this in the morning and post back with results

-Robert

...problem seems to come and go. ground checked fine as did power supply and my bench test was about the same.

Key on the tach jumps up and sometime returns to (0). I did some testing with the battery charger On, even though the engine will crank and start fine, and the tach works/ starts working again. Does the VDO tack need extra voltage to get it to turn on or something? My charging system has been working hard with the new CiBe 85watt headlights lately with the stock alternator. Related?

Still not sure if tach is bad or going bad...


-Robert

In the interest of covering all of our bases, is your idle really high? Could it be running when you think you turned it off?

Seriously, I wouldn't think it is internal. I suspect a ground, connection at coil, or a fuse.

My thinking too, that is why I bench tested new ground, power and new coil connection. Only engine ground to test next....Ill keep digging///thanks

-Robert

BTW, I am still running the original Point/condenser configuration and coil

....For any one following along on this thread, I replaced the tach with a 911 tach and tested and no failure(reads low as expected) and later a replacement 914 tach was installed and the problem was inside the failing tachometer. I have the unit apart and nothing is screaming Im broken! When I get a little free time I will try to diagnose the circuit board in the unit. Im betting on a dry electrolytic capacitor. Very common failure item for parts in the day...


Best

-Robert