Mychron 4 rpm jumping

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Mychron 4 rpm jumping

Postby derick9480 » Mon Apr 18, 2016 8:28 pm

Took the car out for the first time this year and noticed tach is getting funky readings on rpms. It will stay at 4400 or where ever it's set for few seconds and then jump to 1200 or so then back to what it set at. It does this every few seconds or so and just keeps jumping back and forth. We changed from the motor from a zr3 to a zr4 this year but haven't changed anything with tach.We didn't have any problems last year with it. Cht and egts are good. I know it got to be something simple. Any ideas ???
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Re: Mychron 4 rpm jumping

Postby WILD BILL » Mon Apr 18, 2016 9:11 pm

Is it the same coil you used on the ZR 3?
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Re: Mychron 4 rpm jumping

Postby slowgoat » Mon Apr 18, 2016 9:28 pm

I have had the same issue. The rpm line looks like a heartbeat. Some runs are worse than others. I have no clue what the issue is. If anyone can figure it out please fill us in.
I thought the rpm wire was getting interference so I covered it, wrapped it, etc. No change. Thought a ground issue but that didn't change anything either
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Re: Mychron 4 rpm jumping

Postby derick9480 » Mon Apr 18, 2016 9:41 pm

it is not the same coil we ran with zr3 but it is a new one I bought from craws with the zr4.
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Re: Mychron 4 rpm jumping

Postby Leml » Fri Apr 22, 2016 9:53 pm

I had same problem last year. Try a different coil. I called Mychron they told me its was interference so replaced both switches and rewired car with ground to engine block to frame as Mychron suggested and same problem. attached coil kill wire straight to Mychron wire still had rpm jumping around. At wits end swapped coil from my old raptor and rpm reading normal. Call Fred he should remember this.
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Re: Mychron 4 rpm jumping

Postby derick9480 » Sat May 07, 2016 8:55 pm

So after putting new switches,checking grounds. Tach was still jumping . I took the tach wire from the mychron out of terminal block cut it and warped it around the plug wire a couple times taped it up good and it reads perfect !!!! Way better than it ever has. Just thought I would let you guys know that maybe having same problem.
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Re: Mychron 4 rpm jumping

Postby Heppner146 » Wed May 18, 2016 10:51 pm

I had the same issue with a new ZR4. Take the wire that goes to the MyChron and wrap it around the spark plug wire. I had the same issue pop up after I changed the coil. Wrapped the wire, problem solved.
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