Hi. What max voltage is in tachometer and speedometer in Civic EG on signal cables from sensors? I making timer for gauge test (like in Impreza STI) based on NE555, and I need to know what voltage is there, because I have NE555 timer set for 12V and I think it can damage gauges. Can you help me?

 

The speedometer doesn't receive a straight voltage from the VSS. If I recall it receives a 5V square wave signal.

The tachometer I think gets it's signal directly from the ECU and I am not sure what kind of signal it receives.

 

The Tachometer signal for instrumentation exits the Distributor directly (Blue wire) and then branches into 3 different directions: Tachometer (cluster), Test-Tachometer Connector (drivers shock tower), and Cruise Control.

The ECU has no connection to this signal wire. It determines RPM from the other sensors in the distributor.

 

Hmm, it looks so easy, thanks

 

Excellent info!

Do you know what kind of signal is put out on the blue wire?

The other thing that strikes me is the notion that the stock tachometer is considered inaccurate but an external one is supposed to be accurate, yet the signal is pulled from the same place....

 

I've never measured the signal output nor have I read up on it. I imagine it's a voltage pulse each time the ignition coil fires. All tachometers seem to use the same type of signal (with multipliers for various cylinder configurations) which is why you can install any aftermarket tach into any car simply by selecting the # of cylinders.

As for accuracy.....the signal is just a signal. The gauge itself is built into the cluster where it does all the processing. I'm not so sure the OEM tach has a problem of "accuracy" as much it has a problem with "latency". I can definitely tell an OEM tach seems very "damped" or "smoothed" compared to my aftermarket Stuart-Warner tach (which can move the needle incredibly fast). When you have a powerful engine, a very light flywheel, or even a very capable driver, the engine rpm changes very suddenly. The OEM setup is definitely unable to keep up. Remember the tach your economy car came with was designed for a very different market than us.

Even the Lexus LFA commercials commented they had to use a digital tach setup cause no mechanical tach could keep up with how quickly their V10 could change rpm. Of course that could well be a BS excuse on their part for being one of the 1st cars to use a cheap digital instrumentation in a high-end market.

Now the speedometer is a different story. Why on earth do EG speedometers always read 5mph low. I swear Honda did this one on purpose.

 

Probably. It's my understanding that law makers only required car manufacturers to be dead accurate on the odometer, but the speedometer could have a 10% variance and be acceptable.

It is interesting it's always 5mph low though, not a range.

 

we use a signal generator here to bench test clusters jeff our old cluster repair tech said 133hz not Khz should be 60mph on speedo
he uses the same signal generator for tach but he cant remember the the exact signal to raise the tach to 3000 but he said its exactly the same signal type as the the speedo.

 

It looks like all the Honda Timing Signals are 0-5v square wave.

The calibration then is information I think that can be found in the Factory Service Manual as to how many pulses per X output display.

escope_honda_coil_timing.jpg