Tach Bounces while Shifting

[Duffman]

I have had an ongoing problem with my tach bouncing when I shift. It goes up about 1500 to 2000 rpms when I shift. Doesn't seem to bounce while in gear, or at idle. I searched for this topic but didn't see anything. Any thoughts?

[240260280z]

The tach signal goes through the distributor. My guess is a loose electrical connection (behind tach, or ignition key or at distributor , and or vibration in the drivetrain/oil spindle.

Edited September 8, 2011 by Blue

[EScanlon]

Are you letting your foot come up on the gas while you shift?

Sorry, had to ask.

E

[Duffman]

Ok, Enrique, that was cold!!

[EScanlon]

I DID say Sorry!

But... are the RPMS ACTUALLY going up or is it just the Tach jumping? That may help solve the problem.

E

Edited September 9, 2011 by EScanlon

[Duffman]

Tach jumping, not rpms.

[EScanlon]

Wow, sheats the bit out of me!

It sounds almost like you've developed some form of electric "back-pressure" in the circuit to the coil which gets released when you're no longer accelerating. That would be an interesting one for the EE's in the crowd to mull over.

Is it only on acceleration or also while decelerating? Either way it's just plain unusual.

Wish I could help more.

E

[240260280z]

I think Enrique may be on to something: I'll try to take a peek inside a tach and check out the circuitry. There has to be some kind of pulse to current converter integration circuitry that drives the tach needle.

[240260280z]

Web search found this:

Source Reference : http://www.dinoplex.org/tachoconversion/index.html

Current driven tachos are wired in line with the primary coil wiring, such tachos can be easily identified by the cable loop at the back of the tacho. Smith tachos of this type are labeled "RVI". The tacho is not actually wired to any of the ignition components, the wire connected to the coil (+) terminal just induces a magnetic field for each ignition impulse via a small wire loop on the back of the tacho, which is then picked up by a small coil within the tacho. This type of tacho was installed until the late sixties to early seventies, when the automotive industry moved to ignition kickback driven tachos.

Kickback controlled tachos have a single wire connected to the negative primary terminal on the coil, and use the inductive kickback of each ignition, which creates a short impulse with several hundred volts at the primary terminals to display the engines RPM. Smith tachos of this type are labeled "RVC".

In the early eighties, voltage controlled tachos, also called "electronic tachos", were installed when the industry switched to electronic controlled ignition systems. A voltage controlled tacho receives a 12 volt square wave signal to display the engines RPM. Nowadays, most if not all aftermarket and oem tachos units are voltage controlled tachos.

The tacho types listed above are not interchangeable. Connecting a modern voltage controlled tacho directly to the coil in an older kickback setup might damage the tacho. Also, connecting a kickback controlled tacho to a modern capacitive ignition system can also damage the tacho due to the higher primary voltage.

For many setups tacho adapters are available from a range of companies. As example connecting an electronic tacho to a kickback setup can be easily handled by a standard tacho adapter. There is also a another tacho adapter type available to connect older kickback tachos to newer electronic ignitions by means of a tiny transformer to create the kickback impulse. As current driven tachos are difficult to integrate with an external adapter, the scope of this document is to offer an easy way to do a conversion instead.

How does the conversion work

The Datsun 240Z current driven tacho is based on a two transistor circuit, which amplifies the induction impulse from the coil wire loop. The conversion circuit listens to a 12 volt square wave input from a modern electronic ignition and then creates a 500mV 1ms induction impulse signal similar to the original one, which is then directly fed to the current driven tachos input stage.

This is realized by a relatively simple circuit with just one transistor and a voltage regulator, the signal processing is handled by an AVR ATtiny13A microcontroller. You need an AVR compatible programmer such as the low cost AVRISP mkII, or someone with a programmer to flash the microcontroller with the firmware. The firmware is provided for downloading below. Cost for all parts is 6-8€, The installation is simple as the circuit is soldered directly to one input pin of the tachos pickup coil like a daughterboard. The tacho itself is not modified, so the conversion is easily reversed if required.

Building and installing the conversion circuit

Here is a shopping list of the required parts:

1 AVR ATtiny13A Microcontroller DIP 8 IC1

3 10K Ohm Resistor 0204/5 R1, R2, R5

1 270K Ohm Resistor 0204/5 R3

1 6.8K Ohm Resistor 0204/5 R4

1 0,1uF Capacitor C1

1 10uF Electrolytic Capacitor C4

1 100 uF Electrolytic Capacitor C3

1 1N4001 Diode D1

1 MC78L05A Voltage Regulator TO-92 IC2

1 BC547B Transistor TO-92 Q1

1. Flash the ATtiny13A microcontroller with the firmware (download). Solder all components to a small prototyping board. A board with 9 x 10 holes fits nicely. Solder R1 and C1 below the DIP socket to save some space (see image below on the right side). The output wire of R4 (below the red wire in the picture) ends up in a solder pad for installing the board later on, see blue area in the middle picture below.

Here are the Eagle files with the schematic and the PCB layout as a download: 240Z Tacho Conversion.zip

2. Remove the 240Z tacho from the dashboard by unscrewing the two winged nuts on the back and pulling the tacho forward. Make sure its the correct type with the white wire in a loop on the back. Remove the screws holding the black plastic bezel, then remove the electric terminals, the white coil loop and the two screws on the back to get to the PCB.

3. Install a third terminal using a screw and nut in the unused hole for the 6V terminal (see picture below on the left). Solder GND (black wire) to the ground post in the middle, +12V (red wire) to the +12V post on the left and TACH_IN (orange wire) to the new contact on the right side.

4. Identify the small coil below the current loop terminal (the coil is on the right picture below in the lower left corner). The black wire of the coil connects to a terminal on the tachos PCB, on the right picture below marked with a green circle.

5. Solder the conversion boards TACH_OUT solder pad (blue marking in the PCB picture above) to the terminal with the black wire coming from the coil (green circle on the picture above right). Take care that the board does not touch the right terminal/red wire.

6. You are now ready to test the tacho conversion. The firmware will move the tach needle to ~800 RPM for 100 ms after the tacho is powered on, so if you apply power to the +12V and GND external terminal, you should be able see the needle moving if the converter has been installed correclty.

Should the tacho not work, make sure that you have installed and wired all components as documented, and that the ATtiny13A chip is flashed correctly. With a scope you should be able to measure the incoming tacho square wave signal on pin 6 of the ATtiny13 chip and the output PWM wave signal on pin 5 (on power up you should see a negative square wave for 100 ms).

7. If you have a frequency generator capable of supplying a low frequency square wave signal, connect the ground output of the generator to the external GND terminal, and the signal output to the TACH_IN third terminal. With the frequency generator set to 50 Hz, the tacho should now display 1000 RPM (for a 6 cylinder engine it's 100 Hz=2000 RPM, 150 Hz=3000 RPM, 200 Hz=4000 RPM etc.)

If the needle moves but indicates a slightly different RPM value, adjust the tacho by turning the calibration potentiometer, which can be accessed through a hole between the two direction light sockets (Warning: use a plastic screwdriver or put tape on the screwdriver shaft so you won't create an electrical contact between the trim pot and the outer enclosure! This could kill the tach transistor which is located next to the terminal marked with a green circle. If this should happen to you, use a BC547C as replacement).

My '72 tacho turned out to be quite precise after calibration, with a maximum RPM display error of less than 3% at 4000 and 6000 RPM.

8. You are now ready for the reinstallation of the tacho. Create a short wire for the TACH_IN output for easier connection after the tacho has been reinstalled (see image with blue wire below), then reinstall the tacho and connect TACH_IN to your electronic ignition.

240Z Tacho Conversion.zip

240z_itacho2square_v3.hex.zip

[240260280z]

From Opel GT Forum. Seems similar to Z:

Tach Circuit

The tach uses a relatively simple analog circuit with two transistors.

- The first transistor detects the pulse from the points being opened. The pulse from the points is actually an oscillation that lasts a couple of milliseconds. It starts out with an AC component of about +/- 50V that damps down to a DC voltage (about 24V) in a couple of milliseconds (ms). Once the spark is done the voltage drops to about 12V DC and stays there until the points close again.

- This is not an easy signal to deal with, so the first transistor is used to convert it to a simple square wave pulse with about a 2ms duration. The output of this stage is then a set of pulses of 2ms duration on and the rest off, with one pulse for each time the points open.

- The second transistor acts as a current amplifier so this pulse train can be used to drive the meter. The higher the duty cycle of the pulses, the more the needle moves.

- The meter will go to full scale with about 10ma of current.

- Interestingly, the tach actually works on 6V internally. Part of the circuit drops the 12V input to 6V to operate the tach.

[240260280z]

Datsun 1200 tach

Datsun 1200 tach

Datsun 240z

[Duffman]

Blue, I appreciate your diligence in finding the above information, have always thought I had tach issues that could be fixed by some kind of adjustment. Will try to digest the info and see if there is anything I can do. Not my area of strength by any means!