Weird Tach Issue

sharper · 10-10-2022, 01:34 PM

I changed out my gauge light bulbs for LEDs recently and my tach quit working. When trying to figure it out I noticed some weird behavior. Whenever the case to the tach touched another gauge or something metal the lights in it got brighter and the tach started working. Assuming it is a ground issue so I checked all the wires and connections, even replaced some, and everything looks fine. Triple checked that the ground is connected to the right terminal on the tach. The gauges wired downstream in the series have no problems. Is it possible something internal has broken or come loose that is keeping the ground getting to the whole gauge? It is a 3.2l in a 914 with the signal coming from an MSD box if that matters. Been running for years without any problems.

sharper · 10-12-2022, 05:22 AM

Hoping an electrical guru will chime in. Another weird thing is when I turn the ignition on the guage lights go out on the tach; until it's grounded.

jbell959 · 10-12-2022, 06:12 AM

There is an internal ground connection but that is just to ground the case. If that connection was bad, the lights that are in metal bulb holders wouldn't work but it would not effect the tach itself. The tach driver board gets its ground from the ground wire connection.

The only thing that would make sense is that the ground wire isn't good or not connected to the correct terminal. Then if you touched the case to ground it would be providing ground to the internal circuit through the case and internal ground wire.

sharper · 10-14-2022, 02:24 PM

Quote:

Originally Posted by jbell959

There is an internal ground connection but that is just to ground the case. If that connection was bad, the lights that are in metal bulb holders wouldn't work but it would not effect the tach itself. The tach driver board gets its ground from the ground wire connection.

The only thing that would make sense is that the ground wire isn't good or not connected to the correct terminal. Then if you touched the case to ground it would be providing ground to the internal circuit through the case and internal ground wire.

I agree with you but I've checked all wires and connections. I even ran a ground straight from the terminal (its corrected to the right one) and still have the same issues. Its got to be something loose or wrong with the internals I suppose.

jbell959 · 10-14-2022, 02:40 PM

The only other thing that could explain that behavior is if the ground terminal is broken. If you have a multimeter you can check the continuity between the ground connector on the tach and the case

sharper · 10-15-2022, 06:15 AM

Quote:

Originally Posted by jbell959

The only other thing that could explain that behavior is if the ground terminal is broken. If you have a multimeter you can check the continuity between the ground connector on the tach and the case

Good call. I checked and there wasn't any continuity. Pulled the tack apart and this is what I found. The bottom right slot is where the ground pin comes through. Brushed away the burnt stuff and the connection was severed. I'll be testing my soldering skills now.

jbell959 · 10-15-2022, 06:27 AM

That's a known failure point. I didn't realize you had a 3.2 tach otherwise I would have pointed you there first.