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Old 03-05-2010, 08:50 PM   #1
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Default Tachometer Jump/Half RPM

The tach begins life showing half of the true RPM, ie. if the engine is turning 3k RPM, the tach shows 1500RPM. Then when it feels appropriate, it begins to jump around without any real logic that I can see. I've read all the threads on this, checked all the ground I could find, pulled the cluster out and checked wiring, pins, etc. Checked the wiring at the ECU and it seems to be in working order. The stink of it is that my Link ECU readout tells the true RPM, so how they show two different reading baffles me. Please send help before I set the car on fire.

Thanks,
Michael
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Old 03-05-2010, 08:57 PM   #2
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Sunpro monster tach, seriously, i have another tach, if you want me to send it to you, that could be it, if everything else is in working order.
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Old 03-14-2010, 11:18 PM   #3
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No need to replace the tach, it appears to be doing its job. The main problem I'm having trouble fixing is the half rpm on the tach. I spliced the bl/wh wire from each coil together so I know for a fact that both signals should be getting to the tach. Unfortunately it is still reading half of what it should be. I'm beginning to think that one coils isn't sending out the signal. Has anyone ever heard of that?

Michael
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Old 03-14-2010, 11:35 PM   #4
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Yup, mine did that exact same thing. Turned out to be an issue with my COPs harness. Switch coils, wires, plugs, etc, see if anything changes. You're on the right track suspecting your coil. Is it running ok otherwise?
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Old 03-15-2010, 08:50 PM   #5
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Well I spliced both the signal wires together last night, to ensure that both signals were connected and that should've fixed it. Well as you might expect it didn't. So I first clipped the wire on the passenger side coil. Started car, idled just the same as before (half rpm). I then cut the other signal wire. Started car and no tach at all, which is what I would've expected. So now I have to source another coil, even though the coil still works, it isn't sending a signal out for the tach. I wish there was a way around that, as replacing a coil that is functioning except for the tach signal just seems silly.

Comments/Suggestions Welcome.

Thanks

Michael
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Old 03-15-2010, 10:31 PM   #6
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Quote:
Originally Posted by TheBandit View Post
I wish there was a way around that, as replacing a coil that is functioning except for the tach signal just seems silly.
With a couple of transistors, you could build a circuit to simulate the tach pulse off of the coil trigger wires.

Basically this:


Last edited by Joe Perez; 03-16-2010 at 01:39 PM. Reason: Added series 1k to Q1 / Q2 base
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Old 03-15-2010, 11:26 PM   #7
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Joe,

What kind/size of transistors would you use for this application? Also, what is the significance of the 1K resistor?

Thanks,

Michael
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Old 03-15-2010, 11:30 PM   #8
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Quote:
Originally Posted by TheBandit View Post
What kind/size of transistors would you use for this application?
Any small-signal NPN would be fine. RadioShack® NPN Transistors (15-Pack) - RadioShack.com


Quote:
Also, what is the significance of the 1K resistor?
It provides the voltage pullup that the tach needs, yet prevents the transistors from exploding the instant they turn on.
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Old 03-16-2010, 12:23 AM   #9
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should also suffice...

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Old 03-16-2010, 01:39 AM   #10
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Braineack, is this you?
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Old 03-16-2010, 01:50 AM   #11
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Yeah, I probably shoulda drawn a 1k resistor in series with the base of each transistor. (Between the base and the brown or brn/yel wire).

That's what I get for drawing circuits while intoxicated...

Oh, Brain, you mis-spelled Emmmmmmmitttttttter.
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Old 03-16-2010, 09:29 AM   #12
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I didn't make that circuit, Patmx5 did.
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Old 03-16-2010, 01:39 PM   #13
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My schematic above is revised to include resistors in series with the base of the two transistors.
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