Here's what I know:

1. The ECU drives the tach by outputting a square wave that goes from battery voltage to ground, at a frequency corresponding to how often a spark plug fires. (like any tach does)

2. There is a "test tach" connector in the engine bay for mechanics to connect to.

3. If you turn the ignition on and do not start the car, there is battery voltage on the tach wire. Since it's not pulsing to ground, the tach does not display anything.

4. Without the engine running, if you pulse the tach wire to ground through a 1k ohm resistor, the tach will indicate something. I've not left it on long enough to see just what.

Here's what I assume:

1. Honda would not put a test tach connector in the engine bay for mechanics to use if it was easy to break the ECU by connecting your test tach up wrong or maybe even shorting it to ground.

2. Assumption 1 means it should be safe to pulse the tach wire, without disconnecting the ECU, for the purpose of displaying rpm information on the tach.

Here's my question:

Does anyone have any evidence to support my assumptions?
Has anyone ever used the "test tachometer" connection, or I should say, mis-used it?

Here's why I ask:

I'm building a programmable shift indicator, and it would be beneficial to the end user to be able to VIEW the program settings on the tach. Obviously this "forced" tach display would only happen during program mode, and only if the engine was not running. It would also be a quick verification of the tach's accuracy. I have found my tach to read 100 rpm high at redline.

I'm usually the guy who answers questions like this one, but since i'm dealing with OTHER OWNER'S ECU's, I thought I'd look for some confirmation.

 

If the system is designed correctly (which I believe Honda would have done), I would think the outputs are safe from being permanently grounded...no evidence, just years of design experience. Many alarms and remote starters will use that tach testpoint to determine if your foot is off the gas.

What's the duty cycle on the wave, 50% I would assume? If the engine is running and you ground the connector, does the instrument cluster still show 0 rpm?

 

[QUOTE]Originally posted by MacGyver
[B]What's the duty cycle on the wave, 50% I would assume?