Simulating an Alternator Stator Signal for Tacho Driving

[stirlsilver]

Hello everyone,
I pretty much have everything sorted out on my MegaSquirt installation however I have a problem in trying to get the the stock Tacho to work.

The car is a 1993 Land Rover Discovery and the factory wiring has the Tacho connected to the W terminal of the alternator. Currently I have this tacho signal connected to IAC2 and I have confirmed that I am getting a frequency signal to the gauge. However the needle isn't moving.

Now, before you tell me not to bother using MegaSquirt and simply connect the Tacho back to the alternator - my problem is that the engine has a new alternator which doesn't have this signal (I know if can be modified by soldering onto one of the phases inside) and the pulley is a different diameter. So, even if I did go this way, it would be out of calibration.

Anyway, how can I drive this Tacho from megasquirt? Do I simply need to use a coil to get a higher voltage? I'm a bit hesitant on this idea because the voltage levels in alternators aren't high. I'm starting to wonder if the tacho is looking for a +12V to - 12V sine wave to make it work? If that is the case, how do I do this??

Any tips or suggestions very much welcomed!

Thanks,
Stirling

[Broke4speed]

Not sure if this is any help, but it could be useful. Looks like you'd need some sort of Hz multiplier.
http://vwdiesel.net/forum/index.php/topic,13499.0.html

[DaveEFI]

I'm thinking the number of pulses per engine revolution from an alternator are likely to be different than from ignition or MS.

Is it possible to get a rev counter from a petrol engined version? With the same number of cylinders as your new engine?

[stirlsilver]

Thanks for the feedback guys. The vwdiesel link is handy, it seems to show it as a 0-15V signal, just at a much higher frequency, so it is possible that MS is simply not providing a signal at a high enough frequency. I think the new version of the firmware (1.4.0) allows even greater adjustment of the frequency output for a tacho. A way for me to test would be to simply rev the engine and see if the needle shows any life.

I wish it was as easy as simply grabbing a petrol alternator. Unfortunately Petrol Land Rover Discoveries take the tacho signal from the alternator...

Any other tips or suggestions very much welcomed!

Stirling

[jsmcortina]

Could you swap out the tacho from a petrol version?

James

[DaveEFI]

Jaycar do a speedo corrector. That is designed to correct an electronic speedo when you change to an non standard diff ratio or wheels, etc. But not sure if it would have the range to work in your application, where the principle is the same. It basically looks at the original pulses, and multiplies or divides then on the output.

I had problems with the rev counter in my Rover SD1 when I went wasted spark. Non of the various adaptor circuits was 100% perfect. It is an open frame unit when removed from the nacelle. I got a cheap used one, and removed the electronics and fitted a modern circuit - basically one chip designed for the job and a few components. So all that remains of the original is the meter movement.

[stirlsilver]

Could you swap out the tacho from a petrol version?

Hi James, sorry for the looong delay in responding. The tacho I have is from a petrol car, which is why it is all a bit bizzare. It seems like the diesel an petrol models have the same triggering method.

Jaycar do a speedo corrector. That is designed to correct an electronic speedo when you change to an non standard diff ratio or wheels, etc. But not sure if it would have the range to work in your application, where the principle is the same. It basically looks at the original pulses, and multiplies or divides then on the output.

I had problems with the rev counter in my Rover SD1 when I went wasted spark. Non of the various adaptor circuits was 100% perfect. It is an open frame unit when removed from the nacelle. I got a cheap used one, and removed the electronics and fitted a modern circuit - basically one chip designed for the job and a few components. So all that remains of the original is the meter movement.

Thanks for the suggestion Dave, I actually ordered a frequency generator which I will use to test to see if I can work out what frequencies work (if I can get it to respond). Do you remember what chip you used to drive the meter movement? I haven't quite figured out what signal is actually sent to the movement, but it is possible that it is a voltage or perhaps a PWM signal. Any further information you have would be very helpful! Below is what I am dealing with:

[DaveEFI]

Chip I used is an LM2917. But there may well be something better now. The actual meter on mine is a simple volt meter, driven by variable DC.

[stirlsilver]

Thanks Dave, just the sort of information I needed! I have a frequency generator on the way, and I'll use it to determine how it is that this Tacho behaves, and also use that to determine what is used to control the moment. Voltage sounds reasonable to me, but then again it could be current as well? Anyway, we shall see.

Thanks,
Stirling

[stirlsilver]

Hi all, just to close out this topic, the latest update 1.4.0 for ms3 sorted this out. With this version supporting a scalable tacho output, all I needed to do was scale the output by 750% and hey presto! It worked!