Tacho on-off time problem

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MegaHelp
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Tacho on-off time problem

Post by MegaHelp »

The Jeager Tacho from the Renault 5 GTTurbo does not seem to accept the 50/50% on/off tacho signal. It likes to see a very short signal. It does work, but over 3000RPM the tacho drops back to zero. Using the output port settings I can get it up to work using a logic coil output. If I set the dwell very low it goes up to 7000RPM.
Would like to see a dwell setting for the tacho output.
ol boy
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Re: Tacho on-off time problem

Post by ol boy »

You can try an integrator circuit. Output from MS to a 1uf cap then a 1k resistor to ground. On the raising edge you'll get a single short spike as the cap charges up. Try it out in test mode first, an oscope would be nice to have.

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306 SBFord, Torquer II EFI intake, 60 lbs injectors, 8 LS2 coils, VS Racing 7668 turbo, 4R70W, MS3x fw1.4 w/built in trans controller.
MegaHelp
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Re: Tacho on-off time problem

Post by MegaHelp »

Thanks, going to try that.
ol boy
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Re: Tacho on-off time problem

Post by ol boy »

Sorry, I got my widgets crossed. It's a differentiator circuit. Aka.. first order high pass filter.

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306 SBFord, Torquer II EFI intake, 60 lbs injectors, 8 LS2 coils, VS Racing 7668 turbo, 4R70W, MS3x fw1.4 w/built in trans controller.
4DPerformance
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Re: Tacho on-off time problem

Post by 4DPerformance »

Have you tried a high volt spike using a relay coil and transistor ?
MegaHelp
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Re: Tacho on-off time problem

Post by MegaHelp »

Yes.
Works on 99% of the cars that have tach problems.
Not on this specific one.
I have tried 2 different cars with the same instrument cluster, both have the same problem.
I do need to try that filter....
4DPerformance
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Re: Tacho on-off time problem

Post by 4DPerformance »

Funny as mine works fine with that coil method

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MegaHelp
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Re: Tacho on-off time problem

Post by MegaHelp »

Hmm... That surprises me.
Can you tell me more about the coil that is used ?
I am using a relay coil. That does not do the trick for me.

By the way.. Your Turbo is not working!
You might have forgotten a couple of bolts. They are still on the dash.
4DPerformance
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Re: Tacho on-off time problem

Post by 4DPerformance »

Is just a standard 5 pin 40 amp relay

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Six_Shooter
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Re: Tacho on-off time problem

Post by Six_Shooter »

I had this same problem with my Datsun tach, not with MS though, since I was running a Delco ECM at the time (I have since tested a spare Datsun tach with an MS and had the same issue).

The Datsun tach also does not like a long pulse/mid to high DC over about 4000 RPM, or it too will drop to 0 RPM, or spike to full sweep before dropping back down.

What I ended up doing was building a circuit based on a 555 timer, that will only provide a short output pulse regardless of the length of the input pulse. It detects the first edge of the pulse, pulls to ground very briefly, IIRC I think I set it to about 3 ms, then does nothing until the next rising edge is detected. I don't have the schematics for the circuit, but it should not be hard to find with a Google search for 555 timer projects.

My tach will now read to full scale (8000 RPM) without any issue, it would read higher if the tach went higher, since testing I went much higher than the tach was capable of reading without it dropping back to zero.
Tha Toy: 1973 Datsun 240Z Turbocharged, and loads of fun, now MS'd
Tha Otha Toy: 1923 T-bucket Hot Rod, Currently Sniper'd
Tha Daily: 2005 Chevy Blazer
Tha Summer Daily: 1987 Buick Skyhawk hatchback
Tha Long Term Project: 1985 GMC S-10 Jimmy, hasn't been fun for a while
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