BMW tachometer

[antonch]

BMW tachometer expects a square wave signal. MS folks use the following wiring scheme as shown below. Is that how I want to wire up mine?

[jrussell]

No need for all of that complexity. You can just use a tach output from the ECU. You will only need to fit an external pull-up to 12V (the 1K-10K in that pic).

[BenFenner]

This man speaks the truth.

[rob@vems.co.uk]

Connection will be like this:

And the tacho config like this:

[antonch]

Awesome, that will make things easier. I am not sure why MS has such complexity.
How do you determine what the resistor value should be?
How about Divider?

I have 3 trigger events per crank cycle.

[rob@vems.co.uk]

The bigger the resistor the smaller the pulse that the tacho receives.  Start with 10K and lower the value accordingly (two 10K resistors in parallel will give you 5K)

[GintsK]

Did simple input  signal for IGBT, MOSFET or output signal from stepper chip work without pullup?

[BenFenner]

I can ask DanMartin what value he used for his BMW tach. He looked it up somewhere...

[antonch]

Rob, your values worked out of the box, didn't have to modify/tune anything. Very odd how you knew them

[rob@vems.co.uk]

Did simple input  signal for IGBT, MOSFET or output signal from stepper chip work without pullup?

No because the IGBT, MOSFETs etc channels only switch to ground, so you need a power feed, and you need the resistor in there or when the thing switches to ground the current will be massive.

The way it works (sorry if people already understand this) is that when the channel is closed the tacho sees ~12v on its trigger line, then when the channel opens the voltage drops to zero, then back up to ~12V when the channel closes again.

[rob@vems.co.uk]

Rob, your values worked out of the box, didn't have to modify/tune anything. Very odd how you knew them

They're the ones I use on the Nissan tachos, I didnt know anything about the BMW at all.
What size resistor did you use?

[antonch]

Rob, your values worked out of the box, didn't have to modify/tune anything. Very odd how you knew them

They're the ones I use on the Nissan tachos, I didnt know anything about the BMW at all.
What size resistor did you use?

It's a cheapo RadioShack pot, supposed to be at 10K, but when I checked it was set to 9.2kOhm. Its funny, I turned on the car it took me a while to realize that the tacho worked right off the bat.

[gunni]

I have always used a 1-2k ohm resistor and never had problems,

I think I´ll be installing the resistor on the inside between 12v and the appropriate channel pin.

[jrussell]

If you use P259 Ch. 4 which seems to be pretty standard these days there are pads on the PCB for a pullup resistor.

Just please be careful. ;-) Its rather easy to damage things in there if you're not very skilled. The space is tight in that area.

In fact I really discourage the average owner to open their case, especially if it was a fully assembled unit you bought. Most areas of the World have support people that will gladly make modifications for you.

[GintsK]

Did simple input  signal for IGBT, MOSFET or output signal from stepper chip work without pullup?

No because the IGBT, MOSFETs etc channels only switch to ground, so you need a power feed, and you need the resistor in there or when the thing switches to ground the current will be massive.

....

I mean FET or IGBT input. There must be 12V square impulse already and no need for pullup.