Hi There!

I tried to make a DIY circuit to convert the modern ECU 5v pulse of RPM for my VW GOLF-2 which upgraded to GOLF-4 engine to the original 12v analog tachometer with the most known circuit of a relay coil, resistor and couple of diodes but it failed after 2000 RPM, it drops to zero when reach 2000.

I asked the new genius GPT and he gives me a pretty complicated circuit in code which is very hard to read, so I draw it on a paper as as I can understand it and need to ask pros. before applying it on my car.

 

I asked the new genius GPT and he gives me a pretty complicated circuit in code which is very hard to read, so I draw it on a paper as as I can understand it and need to ask pros. before applying it on my car.

Really surprised using a relay it worked to 20K! Did you mean maybe 2K rpm?
That circuit won't work as drawn using a N channel mosfet.
Here's one of several ways to level shift the 5 volt pulse to 12 volts.

 

 

Version #2 including the TVS diode:

 

Version #2 including the TVS diode:
View attachment 357177

WooW, Thanks a lot my friend, I'll try it and back to you.

The GPT suggest adding a Decoupling capacitor: 100 nF at +12 V, Can I add it before R3 and R2? or there's a certain position?

 

I would connect the capacitor at the R2-R3 junction.

 

Has the TS verified that the tach will not work with the 5 volt signal??

 

Has the TS verified that the tach will not work with the 5 volt signal??

Not sure. I'm still trying to figure out how in the world he was getting 20K rpm out of that engine.
I would also like to see that relay circuit that fails at 20K rpm mentioned in post #1.

 

Not sure. I'm still trying to figure out how in the world he was getting 20K rpm out of that engine.
I would also like to see that relay circuit that fails at 20K rpm mentioned in post #1.

The old SUN tachometers did use a quite unusual relay to deliver a charge from a cap to move the pointer. It worked quite well until things wore out or the battery got weak.
AND, triggering off of the ignition pulses instead of one plug would be 4x the speed, maybe. OR maybe contact bounce multiplied the pulses per firing.
But an amplifier with a gain of 2.5 is not that hard. On transistor should do it, but the load resistor selection will be a challenge.

 

Not sure. I'm still trying to figure out how in the world he was getting 20K rpm out of that engine.
I would also like to see that relay circuit that fails at 20K rpm mentioned in post #1.

Yes, The VW Golf-2 analog tach works with 12v only, and I mean 2K rpm, sorry for that. The tachometer drops when reach 2000 rpm, and it's not stable when accelerate but it's stable without accelerating on 9-10K rpm.

I didn't use a relay, I used the relay's coil only, I think it's equivalent to a resistor of 10K, I'm not sure about the value.

 

2000RPM equates to 333.333 Hz, Within the capability of a tach relay.

 

@sghioto @ronsimpson
Here's a video for that issue, the RPM didn't go over 2K

 

@sghioto @ronsimpson
Hey guys,

Here's the circuit but unfortunately it doesn't work,




Here's a video for the first test, I have tested the input and output with a tester pen works with 5 volts, the input of ECU out (5v) make the tester flickering while the output is always 12v with no filker and it makes the tester always ON, so the RPM always zero.

I noticed that after the 100K resistor, there's no volts on the multimeter and the tester pen didn't work, I think the output voltage after the resistor is very small.

anyway, the circuit is working without any short or blown fuse but the output is always 12v.

I draw the circuit in the Crocodile Technology and it works perfectly.

So What's wrong?

Thanks for your help

 

So What's wrong?

Appears to be several bad connections or Q1 is shorted
Check the connections circled in blue.

 

Appears to be several bad connections or Q1 is shorted
Check the connections circled in blue.
View attachment 357502

I checked it for a hundred times, nothing wrong, it's exactly like the graph.

These are my readings:

After R1 the voltage became 0.67 v
After R2 the voltage became 0.85 mV
After R3 the voltage drops from 13.8 to 10.3
The input to the Q2 base is 0 V

 

Then the 5 volt ECU pulse does not go to zero volts between pulses.
Do you have a scope to check the pulse.

 

Then the 5 volt ECU pulse does not go to zero volts between pulses.
Do you have a scope to check the pulse.

Unfortunately No, Could we change the design if it's really didn't goes down to zero?

 

Should be able to.
Using the voltmeter what is the reading when measuring the pulse directly from the ECU?

 

@sghioto
Actually the signal is very fast and doesn't read exactly 5 on multimeter, It reads about 6-8 volts and can't catch it when goes down.

 

If the pulse is 5 volts it should never read above 5 volts and that will depend on the duty cycle.
If the duty cycle is 50% the voltage should read appx 2.5 volts
The meter is measuring the average voltage as the signal goes from low to high.
The 6 to 8 volts is telling me the signal is not a 5 volt pulse.
Really need to see the pulse on a scope.
Is the ground connection on the circuit board the same as the ECU ground ?