To start, this isn't an LS swap but I still think this is the best suited section for this topic.

I've searched around but haven't found anything to help solve my issue. Swapped car is a 98 Firebird using factory V6 gauges, using the Dakota Digital STA-1000 to drive the tach and speedo. Dakota Digital hasn't been able to give me any advice over the phone. The PCM I'm using has no direct tach output wire.

I have the unit set to output a 4 cylinder tach signal and both the tach and speedometer work, but shortly after the engine is started (20-30 seconds) the tach will jump 500-800 rpm higher than actual at idle (confirmed with HPTuners), and get progressively more inaccurate as rpms rise. During this the Dakota Digital unit still shows the actual rpm from the PCM on its screen. I was under the impression that the v6 cluster used a 4 cylinder tach input, but I guess that might not the case. This happens with the dakota digital box set to 4, 6, and 8 cylinder modes, and its only ever accurate on 4 cylinder immediately following engine start.

Anyone have any insight? Maybe the unit is faulty.

 

Bumping with another question that can probably point me in the right direction. I haven't been able to find out what tach resolution settings people use in their tunes when they LS swap their 99-02 v6 f-bodies?

 

I think it depends on what engine/PCM you're using. But as I understand it, the factory tach resolution on a GenIII engine is set to 6, which is 4 cylinder mode. When you run an 8 cylinder tach you need to change the tach resolution to 3. Now I believe to run a 6 cylinder tach you would set it to 4, my reasoning on this is because a GenIII engine has a 24x reluctor, so the factory setting of 6 comes from the equation of 24 (reluctor) divided by 6 (tach resolution) equals 4 (cylinder tach) or 24/6=4, when running an 8 cylinder tach it's 24/3=8, so for a 6 cylinder tach it should be 24/4=6. Or in other words, 24 (reluctor) divided by 4 (tach resolution) equals 6 (cylinder tach).

 

I appreciate you taking the time to respond. What you’re saying makes sense, I just wasn’t sure if the 6 cylinder cluster expects a 6 cylinder or 4 cylinder signal like the v8.

My hope was that somebody who has done the swap may have been able to chime in.

Anyway I did find mention on fiero forums that 3800 swaps have accurate tachs with factory v6 gauges, but not factory 4 cylinder gauges. So that lends further credance to the ICM on the 3800 outputting a standard 6 cylinder signal.

I will report back after messing with it this weekend to update for anybody searching this information later.

Thanks again!