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Author Topic: Faulty tach signal
Sean Allen Verified Driver Made Donation to Website
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Car #: 54!
Year : 90'
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I seem to have a faulty tach signal and I'm not sure why. The tach works up until about 4.5-5k rpm's and then it goes crazy. Jumps up and down, shift lights flash on and off early and then around 6.5krpm's the stock tach starts to work again. Any idea's?

Keith in WA Verified Driver Made Donation to Website
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Yep, I had that and I've seen a couple different reasons with other people having the same issue.

Some people had an instrument cluster issue. Some people had a loose ground. (Unfortunately I don't remember which ground.) My problem was a wire going bad to the cluster where the big white connector is under the steering column. If I zip tie the wire bundle to the right place, the wire makes a good connection and the tach works perfectly fine. If the wire bundle isn't bent the right way, the needle bounces like mad at the same RPMs yours does.

--------------------
Keith Novak
(Will work for tires)

Sean Allen Verified Driver Made Donation to Website
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The thing is, the stock tach does it but so does my "magnatrometer" which gets its tach signal from the diagnostic box near the drivers front shock tower. I will look around under the dash for a loose ground wire, but I don't remember seeing one.

This is the magnatrometer: http://www.rumblestripracing.com/DLT3.html

Keith in WA Verified Driver Made Donation to Website
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I tried to search it but couldn't find the links where people were struggling with the same thing. I think there was a ground issue in the engine bay that people had, not under the dash. Mine was just a wiring issue going straight to the tach.

Since your magnatrometer is doing the same thing, it sounds like something upstream from where both it and your stock tach is getting the signal.

--------------------
Keith Novak
(Will work for tires)

Sean Allen Verified Driver Made Donation to Website
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Could it be an ECU issue? (This isnt a spec miata)

Muda Verified Driver Made Donation to Website
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Keith -

I have the DTL3 and it works fine. I remember reading the schematic on my 1.6 and the tach wire is the same in the instrument panel, diag. box or the ecu. Might want to try connecting it from the ecu and if you get the same problem then it is probably a ground.

--------------------
Muda Motorsports
"We're all here 'cause we're not all there."

Greg Bush Verified Driver
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Year : 90
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The ground wire on the engine harness by the CAS can cause this. It is a ring terminal that goes under a nut on one of the brackets the holds the connectors.

Its easy to forget to tighten it and for some reason this messes with the tach. Saw the same thing on an AiM system after a fast head swap.

Sean Allen Verified Driver Made Donation to Website
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Could this be it?

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Sean Allen Verified Driver Made Donation to Website
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It's not really near the CAS, and it seems to come from the fuel rail, but it's the only ground wire that I could find anywhere near where you are describing. And it WAS disconnected.

Greg Bush Verified Driver
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CAS was the closest thing that I know everyone has heard of.

That looks like it.

   

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