MakoShark77

I've been trying to read up on this but it's just adding to the confusion.

I did an LS swap in my 77 and I have kept all the stock gauges. To get the Tach working do I have to buy an adapter like the SGI-8 from Dakota Digital or will installing a pull up resistor do the trick?

'75

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Shark Racer

I ran through this a while ago. With LS swaps, it depends on the ECM. The GTO ECMs are fairly problematic, and require a converter. With early LS motors, I believe it's as simple as matching the tach type to the expected tach type of the Corvette tach. With later LS motors, there's a pulse per rev value in one of the tables.

If you tell me what ECM you have, I can pull a tune and tell you which field needs looking at.

Patro46

You change the pulse multiplier . For whatever reasons, the LSX engines are on what us old schoolers call a 4 cylinder tach output. We used HP Tuners with my LS swap on my 74. Think you got issues? Try dealing with a mechanical tach AND speedometer on an LS swap! At least with HP Tuners and an electronic tach it's just a matter of adding an X2 mutiplier into the PCM's equation without having to purchase a piece of 3rd party hardware.

MakoShark77

I have the 09354896 PCM. Blue and red connectors. #10 pin on the red connector (white wire) is connected to the stock Tach (brown wire).

So, if I understand you guys correctly. I just have to make some table adjustments to the tune and don't need any additional hardware?

Shark Racer

Yup - no need for additional hardware. With the PCM you're using, if you're using HP Tuners there's a tach output setting. "Resolution - High" = 6, "Resolution - Low" = 6. These are the defaults.

24/6 = 4.

For an 8 cylinder tach you'd replace this setting with 3 (24 / 3 = [8]).

MakoShark77

Thanks a bunch Shark Racer! I will give this a try asap. I don't own HP Tuners but that is what the local Tuning shop I'm using has. At this point I just have a bench tune and I guess he didn't mess with these settings because he didn't have my details on this. Sounds easy enough.

Illegal Vette

This is correct, I did it to mine.

MakoShark77

OK, I'm going to re-vive my own old thread here.

Due to life's happenings, I have just got back to this. Found a very reputable tuner in my area and scheduled an appointment. I ended up a with a great tune that will do for now, but I still don't have a functional Tach. According to what HP Tuners is telling him the PCM doesn't even see a Tach and the resolution outputs were already set at 3, as described by Shark Racer. So, obviously I'm going to have to verify I'm on the correct PCM pin and verify continuity to the Tach. Is there a way to bench-test the Tach itself? It worked before I did the LS swap, so I'm assuming it's still good. But, you know what they say when you assume...

lionelhutz

The output pin on the PCM needs a pull-up resistor. Wire a 1000 ohm, 1/4 watt resistor between the tach wire and an ignition switched source.