LM2-tach signal goes flat

[Dr J]

I have an LM2 with a working (sort-of) tach signal. The problem is that it starts to cut out and becomes flat. The tach signal comes from an HPX which sends a 5V square wave. That signal goes to my DWUR and the LM2. It is connected to the car's tach through a transistor that isolates the signal and provides enough current to drive the car's tach. The problem has gotten progressively worse. If it were a ground problem, I would expect the other signals to also cut out. Anyone experienced this before?

Here is a picture.

[Dr J]

bump

[DRACO A5OG]

Have you tried their Forum?

[Dr J]

I had browsed their forum prior to posting here. This forum usually has a lot more responses. Plus there are a few here with LM 2s so I thought I would try it here first.

[dicklague]

I don't know the LM2 very well. I do hear the tack signal can be finicky. Wish I could be of more help

[Dr J]

Problem solved! It was not the LM-2 at all. It was the signal generated by the Electromotive HPX. It turns out the ground wire was coming loose. Tightened it up and it got rid of the flat lines.

[DRACO A5OG]

Cool, I still need to set up my LM2 to read the tach.

[Dr J]

Updating this post for posterity.

Seems the ground wasn't the problem, since I continued to get flat readings. I decided to try increasing the voltage from the HPX tach output (5V square wave) to 12V. I did a test today and everything registered well. Signal has much less noise. Also the car tach and the LM-2 register correctly.

The voltage was increased using a NPN Tip 31 transistor. The tach input (the HPX signal) went to the base through a 500 ohm resistor, the emitter went to ground and the collector went to 12V, through a 50 ohm resistor. The tach out is from the collector pin. This inverts the signal, but increases the high voltage to 12V.

[304065]

Yep, the RPM sensor looks for a +12V square wave or nearly so