Tacho output - Pin 11 - dropping off at idle

Phil Waters · Post by **Phil Waters** » Sun Jun 25, 2006 9:35 am

Hi - hoping someone can maybe explain this problem - although it isn't a major issue at the moment (a winter fix probably to use one of the spare outputs)

I am running Extra code with an EDIS 4 on a V2.2 board, code is 29q2, all on a cam'ed & ported Ford Zetec, using the standard flywheel and sensor.
Whilst trying to cure a misfire (turned out to be leads

) I found I had my trigger angle set to 23 deg and the spark was not inverted. According to the current instructions for the current mods to the board (which I did at the beginning of last year) it should have been 0 deg and Inverted. I changed these setting and the car ran much better.

but

The only thing is now my rev counter doesn't register the rpm at idle. I have the idle set to around 1k rpm, and the needle drops to around 400rpm and sometimes 0 on my Tacho, but on the Megatune readout it is fine, hovering around the 1k rpm mark. As soon as I raise the engine speed the needle picks up to where it should be and matches the Megatune readout fine.

Any ideas as I am

cheers,

Post by **jsmcortina** » Sun Jun 25, 2006 10:18 am

Strange. I think that your rev counter doesn't like the signal. The tacho output from Megasquirt is likely to be similar. It creates a short pulse for each tach event. At idle this is a very small duty cycle. As rpms increase it becomes a larger duty.

James

Phil Waters · Post by **Phil Waters** » Sun Jun 25, 2006 11:50 am

Thanks James - as I said, maybe worth trying over the winter when my car isn't used (no windscreen = very cold

). If it doesn't change anything it isn't a great loss and might help me justify jazzing up my dasboard

Any idea why it would have been fine with the wrong spark settings (23 deg and non-inverted spark?
I take it I now have these right?

ion332 · Post by **ion332** » Sun Jun 25, 2006 9:10 pm

It seems there are some aftermarket tachs that only deal with "stock" type signals and their accuracy is mediocre at best for the 100+ dollars you pay for it.

Now, after talking to their "tech support engineer" (note the quotes) my basic premise was something like this:

"Ok, my tach doesn't work at low rpms, is innaccurate below about 3k. I have connected it both from the coil return AND the tach output, so exactly WHAT signal does this stupid thing want to see?!?"

His response was that it wanted to see a 5v signal of a certain pw. Which doesn't seem right offhand, because the pulse spike on the return of the coil is more than that AND the stock tach is VERY accurate, just doesn't handle my new RPM range (old tach is 8k max, car revs now to 9k+)

My response was well, is it counting triggers or what is it doing? Or is it using some primitive integration to generate a voltage to move the tach needle?

At that point he starts babbling about me having to talk to MSD (I have an MSD coil) modified ignition systems blah blah blah...

So, I feel your pain on this one.. I tried the tacho output too, and it still totally inaccurate (I ran the tach output per the diagram, with a 12v output, although I *have* considered biasing the base to run it to 5v output)

Anyway, here is the issue in a nutshell:

1) My stock tacho is PERFECTLY accurate, even when the aftermarket tach is piggybacked on the same line. This rules out a problem with the MSD coil, since the stock tach is connected to the coil return.

2) Stock tach works fine with the MS signal. This rules out the MS signal.

So, basically, there is some inherent design flaws in many of these aftermarket tachs. That is the ONLY conclusion I can come to at this time.