New OEM tach issue can't find info
04-04-2008, 11:27 PM
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New OEM tach issue can't find info I can't find anything on this searching or with stickies here or on HPTuners forum.
1st gen Fbody w/original tach. I started with a 680 ohm jumper and it would only work on idle undershoots. A 560 made it work up to 1800 RPM, and a 460 ohm worked up to 2600 RPM. The tach is within 50 RPM, but once it hits the speeds above, it dives to zero and won't come back up until the engine is 3-400 RPM below the cut out point. It looks like the lower I go on the resistor, the more span I get.
The original tach worked when I took the SBC out and put the LS2 in. My multimeter says the tach output from the PCM gives a good linear frequency up to at least 5k RPM and the math works out for the frequency to be what it should output at that speed. I changed the tach drive period in my tune (HPTuners) from 3 to 6 and 9 and the PCM will drive correctly and the tach is where the problem is.
For now, I am leaving the tach drive at 6 and just doubling the speed in my head so I can read up to 5200 RPM while breaking the engine in.
Has anyone run into this before?
1st gen Fbody w/original tach. I started with a 680 ohm jumper and it would only work on idle undershoots. A 560 made it work up to 1800 RPM, and a 460 ohm worked up to 2600 RPM. The tach is within 50 RPM, but once it hits the speeds above, it dives to zero and won't come back up until the engine is 3-400 RPM below the cut out point. It looks like the lower I go on the resistor, the more span I get.
The original tach worked when I took the SBC out and put the LS2 in. My multimeter says the tach output from the PCM gives a good linear frequency up to at least 5k RPM and the math works out for the frequency to be what it should output at that speed. I changed the tach drive period in my tune (HPTuners) from 3 to 6 and 9 and the PCM will drive correctly and the tach is where the problem is.
For now, I am leaving the tach drive at 6 and just doubling the speed in my head so I can read up to 5200 RPM while breaking the engine in.
Has anyone run into this before?
