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Tach reading low?


danielp3344

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So I finally got my speeduino working pretty well (hopefully I'll get a writeup on here soon), but I'm having a weird issue. I replaced the stock coil with an LS1 type coil with an internal igniter. I connected the dark green/yellow wire that used to go to the coil power to the igniter signal on the LS1 coil (so basically the same setup as factory). The tach responds very well, but reads low. it hovers below 0 at idle, 0 seems to be about 1000 rpm and 1000 on the tach seems to be right about 2500 from the engine. Any ideas? This ECM has a dedicated tach output but it doesn't seem like I should need it here. For what it's worth I found an old video I took from before the ECM swap and the tach worked normally then.
 


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RonD

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danielp3344

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Mine does have a resistor(I think), large brown plastic thing inline with the wire over by where the coil used to be. I cut the connector that goes to the TFI module and have the coil wire and SPOUT connected together so the ECM drives the coil with what used to be SPOUT. I hope that makes sense. The stock PCM is gone, so nothing to unplug there.
 

RonD

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No, not understanding

What ecm are you using

Then TFI module/distributor and coil are self contained for startup and RPM spark advance(distributor has the weighs and springs inside)

The ecm is/was just used for "load advance", i.e. the vacuum advance on older system, thats what SPOUT wire was for, load advance, which is why you disconnect it to set base spark timing, just like you would disconnect vacuum advance on older system to set base spark timing

But the coil "-" is the coil "-" so it has the correct tach pulse as far as I know, on a V6 single coil it pulses 3 times for each RPM, 6 for 4-stroke full cycle 2 RPMs
 

danielp3344

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The ECM is a speeduino as stated above. I removed the PCM, TFI module and the stock coil. Now the distributor only supplies a timing signal to the ECM, which fully controls the coil. I have connected the coils igniter signal (which should IIRC behave just like the negative side of a regular 'dumb' coil) to the wire that used to go to the negative side of the coil (the dark green/yellow wire), that connection is right at the TFI connector if it makes any difference.
 

RonD

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Never heard of it just thought speeduino was your slang for speedometer, like speedirineo, lol sorry

So speed was working but not tach in the dash

Yes, I would try the tach out from the new ecm, they list in "features" which usually means required :)
So it should have the correct pulse that Ranger tach expects

Calling it an "igniter signal" may mean its not the same as ground pulse used in most systems
 

danielp3344

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Lol yeah I guess I can see that, I did say they needed a better name...

51974
Here's what an LS1 coil looks like inside, the circuit labeled 'igniter' is just a set of transistors so the ECM can pulse the coil without having to handle the full current and without noise/high voltage coming back from the coil (both bad for sensitive electronics obviously). But that should be the same signal as the negative side of the coil was getting as far as I can see... I guess I can just disconnect the coil entirely and hook the tach output from the ECM to the tach input pin on the original harness if that makes sense.
 

RonD

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Regular tachs want a 12volt square wave signal this is probably lower voltage at lower RPM and gets higher with increased RPM

Yes, hook the ECM tach OUT to the dark green/yellow wire to tach in dash that should have the full 12v square wave all the time
 


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