LS swap - tach is off
Thread Starter
Burning Brakes
Joined: Sep 2020
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From: CT
LS swap - tach is off
78 stock gauges running LS with term X. Tach output of termX is v8 signal and works just fine with the stock tach. My issue is at idle the tach is reading 100rpm higher than the termX handheld. As I give it gas say 2000-2500rpms is about 300rpm higher than handheld. At 5000 it’s like 400-500rpm high. I honestly think it’s the stock tach just being old but figured I would Ask here. Its alittle annoying as I thought I was shifting at 5500-6k but really was like 5000-5500.
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It's probably just the electric tach. You could try a new tach board. But as long as you know it's off about 500 at 5,500 it should still work for you.
Instructor
Joined: Jun 2023
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From: California
My 82/LS1 is similar, it's off anywhere from 100-400 rpm. It's usually showing higher than actual, especially during acceleration. I've always assumed it's just a 42 year old tach struggling to keep up with the workout.
Melting Slicks
Joined: Jun 2011
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78 stock gauges running LS with term X. Tach output of termX is v8 signal and works just fine with the stock tach. My issue is at idle the tach is reading 100rpm higher than the termX handheld. As I give it gas say 2000-2500rpms is about 300rpm higher than handheld. At 5000 it’s like 400-500rpm high. I honestly think it’s the stock tach just being old but figured I would Ask here. Its alittle annoying as I thought I was shifting at 5500-6k but really was like 5000-5500.
So for my LS3 / 480 GM Crate motor the ECU outputs a signal which requires a pull up resistor to properly trigger the factory tach ~ 4.7K resistor tied to 12V. The GM ECU also outputs a signal like a 4 cylinder engine, meaning my tach reads 1/2 of the actual reading.
Verified using a PC connected via ODB2 ports. PC Reads 2000 RPM , Tach reads 1000 RPM.
For a waveform to read 6000 RPMS Factory Tach, a signal of ~ 400HZ, 100HZ = 1500 RPMS etc.
I added a product "Tachmatch" to correct the ECU Output. I tried to reprogram the ECU But didn't work for me.
TachMatchTM03aInstructionManual4k.pdf (technoversions.com)
What ECU are you using ?.
Thread Starter
Burning Brakes
Joined: Sep 2020
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From: CT
Without using a oscilloscope to look at the wave form it would be hard to tell what's wrong.
So for my LS3 / 480 GM Crate motor the ECU outputs a signal which requires a pull up resistor to properly trigger the factory tach ~ 4.7K resistor tied to 12V. The GM ECU also outputs a signal like a 4 cylinder engine, meaning my tach reads 1/2 of the actual reading.
Verified using a PC connected via ODB2 ports. PC Reads 2000 RPM , Tach reads 1000 RPM.
For a waveform to read 6000 RPMS Factory Tach, a signal of ~ 400HZ, 100HZ = 1500 RPMS etc.
I added a product "Tachmatch" to correct the ECU Output. I tried to reprogram the ECU But didn't work for me.
TachMatchTM03aInstructionManual4k.pdf (technoversions.com)
What ECU are you using ?.
So for my LS3 / 480 GM Crate motor the ECU outputs a signal which requires a pull up resistor to properly trigger the factory tach ~ 4.7K resistor tied to 12V. The GM ECU also outputs a signal like a 4 cylinder engine, meaning my tach reads 1/2 of the actual reading.
Verified using a PC connected via ODB2 ports. PC Reads 2000 RPM , Tach reads 1000 RPM.
For a waveform to read 6000 RPMS Factory Tach, a signal of ~ 400HZ, 100HZ = 1500 RPMS etc.
I added a product "Tachmatch" to correct the ECU Output. I tried to reprogram the ECU But didn't work for me.
TachMatchTM03aInstructionManual4k.pdf (technoversions.com)
What ECU are you using ?.
Thread Starter
Burning Brakes
Joined: Sep 2020
Posts: 866
Likes: 187
From: CT
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Do you have a buddy with a signal generator and an O-scope?
I'd bet the issue is the tach board needs to be replaced (no guarantee that a new one will be any good, of course). It sounds like the tach output from your Terminator X is dead-on.
I'd bet the issue is the tach board needs to be replaced (no guarantee that a new one will be any good, of course). It sounds like the tach output from your Terminator X is dead-on.
Melting Slicks
Joined: Jun 2011
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I'm running the holley Terminator X which has a standard V8 tach signal. It works just fine with a speedhut tach set to 8cyl mode. I'm aware of the stock GM 4cyl tach output and I knew the holley wouldn't/shouldn't have been an issue. WHich its not but I also know the stock tach can be off even in a stock setup.
Did you / or did holley indicate the need for a pull up resistor ?
Thread Starter
Burning Brakes
Joined: Sep 2020
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From: CT
I tested the Tach gauge using my PICO oscilloscope. Also has a signal generator. So I know the stock tach can be accurate. Being off by 100-400 RPM and without a O-scope to measure the signal, I would point to the stock tach vs the Holley unit.
Did you / or did holley indicate the need for a pull up resistor ?
Did you / or did holley indicate the need for a pull up resistor ?
Sorry, been driving the car and tuning it. Now I'm swapping to 3.73s, the 2.72/2.87s are killing my 3rd gear performance.
I read that I shouldn't need the pull up resistor but some say it could be necessary. Heres what I did. I used my PWM signal generator to test the tach on the car without starting the car (rear is out and the transmission is apart). With a kit of math I tested 3 points and verified the tach is indeed off (non linear). (RPM*4/60) and then set duty at 50% worked perfectly. 100hz @50%= 1.5k rpm , 200hz@50%= 3k rpm and 400hz@50% = 6k rpm. What I got was 1750, 3300-3400, 6600-6700. With key on Tach is sitting at 100rpms. I'm going to pull the tach, zero out the needle and then put a new tach board on with a petameter so I adjust the output to get me close.
Thread Starter
Burning Brakes
Joined: Sep 2020
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From: CT
FWIW for the next guy who finds this thread. I pulled the tach yesterday and installed the new calibrated board. Getting the needle lined up was not fun or super easy.
Power up the gauge Batt/Neg and the 3rd post is for the signal. By applying the battery POS and NEG your gauge should go to ZERO. Then I used a $8 amazon/ebay PWM generator and set it to 400hz at 50% duty which should have been 6k rpms which It was, then I tested at 200hz which is 3k rpms. Seems to be dead on now at least with the generator. I had to keep resetting the needle as when I powered off the PWM generator the needle to swing down and smack the backplate and move the needle. I want to say I'm now 0-50rpms accurate at Key on engine off and I should be the same throughout the RPM range. I didn't have to adjust the pot at all which was nice and the EBay seller has a lifetime warranty on the board.
I will test it again after I get the new rear installed and everything back together with the engine running as that is what really matters.
Power up the gauge Batt/Neg and the 3rd post is for the signal. By applying the battery POS and NEG your gauge should go to ZERO. Then I used a $8 amazon/ebay PWM generator and set it to 400hz at 50% duty which should have been 6k rpms which It was, then I tested at 200hz which is 3k rpms. Seems to be dead on now at least with the generator. I had to keep resetting the needle as when I powered off the PWM generator the needle to swing down and smack the backplate and move the needle. I want to say I'm now 0-50rpms accurate at Key on engine off and I should be the same throughout the RPM range. I didn't have to adjust the pot at all which was nice and the EBay seller has a lifetime warranty on the board.
I will test it again after I get the new rear installed and everything back together with the engine running as that is what really matters.