View Full Version : New Tach 89 PS 190

05-16-2008, 01:08 PM
I bought a new VDO tach for my 89 PS 190 351 and it does not register RPM. I made sure the settings on the back are set correctly but still no reading. The reason for the change is I did not trust my old one. It had sporatic readings. What tool can I buy to read RPMs to measure rpms outside of the tach and any ideas on why the new one does not even work?

Thanks in advance,


05-17-2008, 01:18 AM
All tachs have these things in common.

They are hooked up to the positive and negative side of the battery. So they need a good power supply and a good ground.

The signal is connected to the negative (minus) terminal on the ignition coil.

When diagnosing, I like to use my Tach/Dwell meter because I know it reads correctly. I hook it up as per the instructions and compare it to the tach in the dash. If the readings are different I would then try hooking my tach dwell meter in place of the dash mounted one. You want to verify that there is a good B+ and B- available to the dash tach. As well as a good signal from the negative side of the coil. The leads on my Tach/Dwell meter are long enough that I can move them directly to the battery if I wanted to try and isolate each circuit. The Tach/Dwell meter should work the same hooked up in place of the dash gauge as it would when connected to the engine/battery. If it dosen't then there is a problem with one of wires in the harness. Use a process of elimination to find the problem i.e. move the Tach/Dwell meter ground lead to the battery negative and see if it works correctly. Do the same for the positive lead and then the signal lead, one at a time.

Of course, if the Tach/Dwell meter works correctly when substituted for the dash tach, then the harness is OK and you just have a defective tachometer.
Hoping this helps you out some...

05-18-2008, 04:27 PM
Thank you!