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Tuomo, thanks for the tips. My scope doesn't have any memory but I think I can get by with it. The note about the top trim pot is very interesting! I'll see what I find there. It will be a few weeks before I get to this but your help is much appreciated.
I'm sorry to bring up this post again, but did anybody find a possible reason for my symptoms:
I run a Megasquirt system with Ford EDIS ignition system. It's a waste spark system, so there are 4 double coils. At first, by using diodes, every fire of a coil was connected to the wire for the tacho. It read double the amount of revs it was actually making.
I remove the connection of 2 coil packs, so it was fired only half of the time the engine is running. Still, it read double revs.
Now I have the tacho connected only to one coil pack (so only receives pulses for two sparkplugs), but STILL it reads twice the revs the engine is making. Sometimes, when I pull it hard, the needle suddenly drops to a 'normal' reading, stays there for a while and then jumps up again to double the revs.
You must be using Pin #2 of the EDIS-8 module to drive your tach.
This is a calculated V8 signal. Where as the later 928s use 2 coils and it reads like a 4 cylinder.
You have 2 choices,
a) (recommended) is to take a spare output (my preference is IAC 1A) and wire it to the tach, with this you can then program it to be a tachometer output, and tell it to fire at half cycle. I have used this before and it works wonderfully.
b) you can disconnect pin 2 and build a circuit like this.
I'm very interested in this thread that i just discover now. I'm from France, i own an 85 928S2 (Euro, if that makes any difference).
I recently took out the dash to do a variety of repairs : odometer had the famous broken little gearing, my voltmeter was not showing the correct value, the light bulbs were too dim, that sort of stuff.
And also, some of the A/T bulbs were not working (the "P" and the "N" and I recall the "D" also). I bought new bulbs and soldered them to the card on the tacho. All the bulbs work fine now. So far so good...
... except that now I have an issue with my tacho. Here is what happens :
- i start the engine - the needle goes up to around 850rpm, normal idle speed
- i press the throttle - the needle gets stuck at 850rpm - no movement whatsoever
- i turn off the engine - the needle also gets stuck at 850rpm - if i gently hit the dash, the needle goes down to 0.
I suspect a mechanical issue with the needle and all the systems that sits behind it, which all look like very sensitive stuff. I had to take a lot of that out so i could access the A/T bulbs, and perhaps i have not remounted it all correctly. But at this stage i wouldn't rule out an electrical problem either.
So now i've got the tach out again. My issue is I do not really know how to test it to narrow down my problem. I put a basic 12V between ports A and C in the photo above, the needle didn't move an inch which I suspect is normal. After that, i'm not sure what to do.... How should i go about narrowing down the problem? Does anyone have a detailed documentation about how the tacho is assembled and/or how to test it? (note - I don't have a square wave generator, although i could go buy one if there is really no other way to test things out).
However I have a question about the photo on the first post of this thread : Can the 5 screws in the center be the cause of my issue? I have the same 5 screws on my tacho, even if the model is not exactly the same. At some point i took them out then back, perhaps i did something wrong then?
I'm also wondering about a very thin yellow metal plate that sits somewhere in all this mechanism. The first time i unscrewed some of these 5 screws mentioned above, this plate dropped out unexpectedly (it's so thin that i hadn't even seen it was there). Because I hadn't seen it, I am not sure where exactly this plate fits, but given that all this tacho system looks very precise, I wouldn't be surprised if my problem wasn't simply that i put this plate back in the wrong place. I tried a few combinations that looked logical to me, but perhaps it's not so intuitive. Does anyone see what is this plate i'm talking about? Any help would be appreciated.
Hello people from 13 years ago. Question, so I just ordered an 30 euro oscilloscope (Fnirsi DSO153) on Aliexpress to test the tachy signal. However, it can also generate a frequency signal. Do you think it could get the tach to work with the following capabilities?
* Signal generator *
Frequency：0-10KHz
Duty cycle0-100% (rectangular and sawtooth
waves)
Amplitude0.1-3.3V
WaveformsSine wave, rectangular wave, sawtooth
wave, half wave, full wave, step wave,
anti step wave, noise wave, exponential
rise, exponential drop, DC signal,
multi tone, Sink pulse, Lorentz wave.
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