Function generator to test a car tacho meter?

[cittra49]

Im looking for a function generator to bech test a couple to tachometers i have. They have shiftlights and i want to make sure that they are calibrated so i dont blow my engine =) , the meters have +12V , earth and signal at the back. When i look at the datasheets for the function generator is says
Amplitud Sinus max. @ 50 Ohm    10 Vpp
Amplitud Sinus max. @ tomgång    20 Vpp

The function generator must ba able to giva a +-12VDC signal?

Here is a lonk to the function generator https://www.elfa.se/sv/funktionsgenerator-1x5-mhz-uni-utg9005c/p/30064113?channel=b2c&price_gs=1788.75&wt_mc=se.cse.gshop.sv.-&source=googleps&ext_cid=shgooaqsesv-na&gclid=Cj0KCQiAnOzSBRDGARIsAL-mUB0S5CsuyVsm1G-DfxWP-9cHlg83BJZiAUNpKKPhcLvoDlfIAnDZ_LkaAutVEALw_wcB

If that generator works for my needs , then ill hook up my powersupply with 12v to +12 the earth to earth, and the signal generator to signal and the earth from the signal generator to the earth aswell?
 

[danadak]

Tach model # and manufacturer name ?


Regards, Dana.

[cittra49]

One is for my volvo 740 -1986 model, and the other one is bought from summit racing some years ago so the signal is not TTL , the tachos are connected to the - side on the ignition coil.

Im looking to get a setup like this


My concern about the function generator that i linked is if it can produce a 12V square wave.

[rstofer]

It would seem pretty easy to use a transistor between the signal generator and the tach.  Add a 1k base resistor and stuff the square wave (offset to 0V, signal is 0V..1V more or less) into the resistor.  Tie the collector to +12V through a 1k resistor and take the output from the collector.  It will vary between +12V and ground.

Tie the emitter to common ground.

Almost any signal generator can produce a 1V square wave.

[cittra49]

Ok, i guess that does the trick  , im pretty new to all the electronics stuff, my experience in this field is "change the stuff thats visible broken" , but i managed to fix some car stereos, tvs and monitors so far  , any chance you can make me a quick schematics how to connect everything and a part number or something for the transistor?

Im really greatful for the help 

[Rerouter]

Hello, Its a crossing zero input required from memory, atleast 8V p-p with 0V offset, 10V p-p is best,

The input to the tacho is usually about 1K to 5K so it should be close to your function gens maximum rating, however exceeding this will not blow it up,

V8 should be 200Hz @ 3000RPM
V6 should be 150Hz @ 3000RPM
V4 should be 100Hz @ 3000RPM

if you have a different engine this calibration may differ on your output signal, however these are the usual calibrations.

If its reading wrong, the main chip on the board will be a frequency to voltage converter working in current mode, there will be a reference RC pair, and there will be a low side resistor that scales the meter, either can be tweaked to calibrate it.

Throw me a PCB image and i can likely dig up better info on the thing, I see so many different makes and models that they have blurred a little. from memory the pinout is marked on the top side of the PCB.

[rstofer]

Ok, i guess that does the trick  , im pretty new to all the electronics stuff, my experience in this field is "change the stuff thats visible broken" , but i managed to fix some car stereos, tvs and monitors so far  , any chance you can make me a quick schematics how to connect everything and a part number or something for the transistor?

Im really greatful for the help 

See attached:

I might start with somewhat higher value resistors to limit the current that could flow into the tach.  I get real nervous when I'm not the one doing the work.  As it is, only about 12 mA can flow through the collector resistor, I might change both to 2.2k to cut the current in half.  The thing is, I don't know what the tach circuit demands.