Reduce pulse frequency for rev counter

Generally with that kind of thing I see people use the computer that came with the donor engine to solve that problem. If nothing else you can find a car with the same motor, as many cars should use the same style motor from differing models from the same manufacturer.

Paul

You can use a phase locked loop to convert the frequency.

If you can give a little more information then I may be able to give you a definitive solution to the tach giving an incorrect reading. Most tachs for 4 cylinder engines read every other ignition pulse as one for a 6 cylinder will read every 3rd pulse. If you can tell the make and model of the car as well as the year it will let me make a much more informed solution to the problem.

Jim

Thanks for the reply,

The car is a '95 Nissan 200sx S14 and used to have a SR20DET 4 cylinder engine that dose pulse with each firing. It has since had a 94' (Series 1) R33 RB25DET 6 cylinder engine installed.

The Skyline workshop manual as useful as a chocholate tea pot compared to the 200sx one and have not been able to find out if the rpm signal is generated off all 6 cylinders. But from comparing the boost controller (which has been configured correctly for this engine) to the rev count display its easy to say it is picking up 6 pulses. e.g 2000rpm = 3000rpm on the rev counter. This holds true up to just over 5k engine speed and the needle goes past 8k and off the scale xD

I hope this helps, and thanks in advance

James

You really have two easy solutions that I see. 1. Have the gauge recalibrated are I believe that the tach out of a 300z is the same size and plugs directly in to the dash and will resolve your rpm reading so that you will see the correct reading. Hope this helps you.

Jim

You need to build a small digital circuit that divides by 3 and multiplies by 2....

The solution is even easier than creating another interfacing/interfering circuit. Most Automotive Instrument repairers can recalibrate the Tacho by replacing a couple of components in the instrument. Its usually an hours labour and sundry parts.

Just remove the Tacho from the cluster, and drop it in to the instrument guy. Makes life way easier in the long run.

I'm assuming it is a analogue instrument, but digital ones can be "adjusted" in the same way.

If your really keen then you can hunt for a data sheet on the IC that drives the Tacho movement/display and reverse engineer your own recalibration.

Tacho's are essentially only a frequency to voltage converter and have a reference "tuned" circuit which determines the output voltage to the meter movement/display against the input frequency of the ignition pulses.

The change usually is to the tuned circuit which consists of a capacitor/resistor network. Note most Automotive Tachos are not exactly precision instruments but are usually accurate to +/-200RPM.

Cheers

Pez

Hello, if the car has a model that has a six cylinder as a model/year option then the tach may have jumpers internal of the gauge to convert it to the correct range.

They could be a jumper plug or could be a soldered link.

I had installed a factory tach in a Nissan truck I owned the only thing I had to do was add the signal wire from the distributer and install a power and ground in the instrument cluster.

Having the factory manual was a big help.

Also most all after market tachs use the jumper method to adjust from 4-8 cylinders.

metalSmith's