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Tach is too fast on LS1 NB swap

Old 03-22-2016, 11:48 AM
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Default Tach is too fast on LS1 NB swap

One of the little things that I have still yet to work out on my car, is the tach speed. I went the FM route and wired in a hot wire with a 2.2k resistor to jump up the signal, but the gauge shows high all the time. I have seen where some people did not wire in the extra hot, any tips?
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Old 03-22-2016, 10:10 PM
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How far off is it? Maybe try taking the resistor out?
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Old 03-23-2016, 06:15 AM
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I had a Glowshift gauge and used this....Tachometer Filter
Attached Thumbnails Tach is too fast on LS1 NB swap-image_zpsgpsmpeel.jpeg  
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Old 03-23-2016, 06:44 AM
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I would say its around a 1000-1500 too high, I have not actually put a monitor on it to see. If I remove the resistor, won't that make it go up? More voltage=Higher signal? I was thinking that on nathans site, he said just hook the wire up, wondering if it is not actually needed.
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Old 03-23-2016, 07:08 AM
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I just started up my 2000 base model project and I'm having the same problem. I'll let you know if I come up with a fix. It should be back from the dyno shop tomorrow so maybe this weekend I'll have some time to look at it.

Good Luck
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Old 03-23-2016, 07:58 AM
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Cool bud, thanks
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Old 03-23-2016, 09:15 AM
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I'm running a 2002 LS1 using the ECM feed to a NB dash from a 99 with with no resistor and the tach seems right to me. It matches both my HP Tuners and Ultragauge anyway.

LOL though I have had some of our Miata brethren say it can't possible be right it's way too low!
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Old 03-23-2016, 09:26 AM
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Thats what I am wondering about, it seems to me like by me adding voltage to it has made it too high.

Last edited by stng_96; 03-23-2016 at 09:50 AM.
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Old 03-23-2016, 09:44 AM
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Yeah but wouldn't the resistor cut the voltage?
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Old 03-23-2016, 09:52 AM
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It would cut the voltage coming in from the 12v source, but it won't affect the signal that is coming from the PCM. The idea is to add an additional constant amount of voltage to the tach signal via the resisted 12v wire. The way they are saying it is below:

Tach Signal=PCM Signal(Voltage Carried)+constant voltage from other source
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Old 03-23-2016, 10:42 AM
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Ahh I see! So if the reference voltage is lower the tach signal is based on the wrong bench mark so it's off. Well it's a simple test to find out!
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Old 12-31-2017, 11:33 AM
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Was this ever resolved? Did the FM route and wired in a hot wire with a 2.2k resistor work? I have a 2002 miata and an LS1 from a 2000 Trans Am. Thanks

Last edited by 02-VIVID; 12-31-2017 at 02:30 PM.
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Old 12-31-2017, 07:15 PM
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I bought the resistor from Flyin Miata and installed it per their instructions.
It appears to work accurately.
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Old 03-14-2018, 03:36 AM
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Hi,

Don't forget that the Dakota Digital universal tacho interface will do any engine to tacho combination. It works a treat
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Old 04-16-2018, 10:26 PM
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I used HP Tuners to correct my tach info. I am using an E38 ECM though. The earlier computers may not have the same issues or settings.

https://forum.hptuners.com/showthrea...ch-(autometer)
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Old 01-18-2019, 02:18 PM
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So I wanted to update this thread for others benefit in the future. Figured out several of the things that are being discussed in this thread. First of all, 2000 and older PCM's are different than 01/02 from an F-body. As jim mentioned, he runs a 99 PCM without issue. My car has a 2002 PCM and as Grip mentioned above, I was able to go in and modify the tach output signal with the HP tuners software I bought. I tried a bunch of other combinations and resistors and such. This was the only way I was able to figure out to make it work. There is a thread on the HP tuners forum that explains some linked here. Same idea as what Grip linked above, except with the older PCM. With it I was able to successfully line my tach out.

https://forum.hptuners.com/showthrea...der-tachometer
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