Tach reads 1/2 of what it should

[1970 CST Short Wide]

Factory Tach reads 1/2 of what it should coming from ecm. I put a 1K ohm, 1/2 watt resistor in line as suggested with no results. Any body have any other ideas

[homemade87]

I had the same problem . Tuner changed the setting in the pcm . Cant remember if he had to set it on 4 or 8 cylinder . But it fixed mine from reading 1/2 .

[ls1nova71]

Needs to be 4 cylinder

[homemade87]

I just looked at my setting . There is an engine section . From there go to tach output . It has resolution low & high . Tuner set mine at 3 & 3 . Not sure how it is calculated but it works for mine . If you have HP tuners its an easy fix .

[1970 CST Short Wide]

Originally Posted by homemade87

I just looked at my setting . There is an engine section . From there go to tach output . It has resolution low & high . Tuner set mine at 3 & 3 . Not sure how it is calculated but it works for mine . If you have HP tuners its an easy fix .

What is HP tuners?? Something I can buy??

[87chevy.com]

Google hp tuners.

It's about 650 ish

[homemade87]

Originally Posted by 1970 CST Short Wide

What is HP tuners?? Something I can buy??

Unfortunately it is expensive . Did you have someone do any tuning on your pcm . If so they can do it for you . If you have someone local to you you could pay them to do it for you as well . I do not know of another way to fix it .

[Liltroy]

Watch this video. I just made it yesterday.
Common LS swap tach issues and how to fix them
https://www.youtube.com/watch?v=FS7aRfYkFAQ

[davepl]

$80 adapter box can convert 4-cyl to 8-cyl and back and forth. I use this in my LS3 Camaro to drive the factory tach and it works great:

https://smile.amazon.com/gp/product/...?ie=UTF8&psc=1

[Liltroy]

converter box is unnecessary unless you just have money to blow.

[65blackfleetside]

I realize this is an older thread but some of you may be having similar Tach Issues with your aftermarket tach's on your LS powered projects.

seems as though that installing the resistor helps some but I found it did not work.

Here is what I have: 2004 5.3 LS3 Truck motor
2-1/4" Autometer tach mounted on dash of my '51.

Connected:
red to 12V power
Black to ground
Green to White Signal wire out of ECM

Autometer tach was set on 4 cylinder position

Nothing.

I then bridged the signal side of the tach with 12 volt power side like in the video in the previous post.

I tried this bridge different size ohm resistors starting with a 1000 ohm 1/2 watt resistor
Tach worked but was registering double RPM than what it should have been

I then bumped up to a 5000 Ohm 1/2 watt resister- with same type over sensitive / i.e. tach registering 6k when should only be 3k.

I am also getting code:P0654 "Engine Revolutions Per Minute Output Circuit Malfunction"

I then called Autometer tech line.
Tech guy told me that my signal was corrupt out of ECM. I need to buy the $100.00 autometer 9117 tach adapter box.

This gets wired in from the coils directly abandoning the white ECM tach signal wire.

So I am thinking that BP Harness set my tach signal within my ECU that is somehow corrupting it/ not allowing it to

[dayj1]

Originally Posted by 65blackfleetside

So I am thinking that BP Harness set my tach signal within my ECU that is somehow corrupting it/ not allowing it to

You have the opposite problem of the original poster (you're reading double the RPM instead of 1/2). It actually sounds like BP set your ECM for 8 cylinder output and you set the tach to 4 cylinder. Have you tried setting the tach to 8 cylinder with the 1K ohm pull up resistor? It sounds like that would get you up and working.

[Liltroy]

Originally Posted by 65blackfleetside

I realize this is an older thread but some of you may be having similar Tach Issues with your aftermarket tach's on your LS powered projects.

seems as though that installing the resistor helps some but I found it did not work.

Here is what I have: 2004 5.3 LS3 Truck motor
2-1/4" Autometer tach mounted on dash of my '51.

Connected:
red to 12V power
Black to ground
Green to White Signal wire out of ECM

Autometer tach was set on 4 cylinder position

Nothing.

I then bridged the signal side of the tach with 12 volt power side like in the video in the previous post.

I tried this bridge different size ohm resistors starting with a 1000 ohm 1/2 watt resistor
Tach worked but was registering double RPM than what it should have been

I then bumped up to a 5000 Ohm 1/2 watt resister- with same type over sensitive / i.e. tach registering 6k when should only be 3k.

I am also getting code:P0654 "Engine Revolutions Per Minute Output Circuit Malfunction"

I then called Autometer tech line.
Tech guy told me that my signal was corrupt out of ECM. I need to buy the $100.00 autometer 9117 tach adapter box.

This gets wired in from the coils directly abandoning the white ECM tach signal wire.

So I am thinking that BP Harness set my tach signal within my ECU that is somehow corrupting it/ not allowing it to

Yes. They have set your pcm to output an 8cyl signal as shown in the video in the comments above. All you need to do at this point is change your tach back to reading an 8cyl signal and go back to the resistor shown in the video on the power jumper. You do not need any converter box for a tach signal. The only time I would recommend one is if you are using a gen4 ecm (late 07-up) as it it can be difficult to get the tach dialed in with the tach output in the ecm. But even then you can usually get it within 100-200rpm error without a converter box.
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[65blackfleetside]

Thanks for the Heads up guys. I will try flipping the dip switch on the tach from 4 cylinder to 8 and go back to the 1000 ohm resister.

Question about the factory ECM tach settings:
What does the factory setting on the Gen III ECM 4 cylinder or or 8 cylinder?

Why does every LS forum that I read including the Autometer website say to set the tach to 4 cylinder?

[Liltroy]

Originally Posted by 65blackfleetside

Thanks for the Heads up guys. I will try flipping the dip switch on the tach from 4 cylinder to 8 and go back to the 1000 ohm resister.

Question about the factory ECM tach settings:
What does the factory setting on the Gen III ECM 4 cylinder or or 8 cylinder?

Why does every LS forum that I read including the Autometer website say to set the tach to 4 cylinder?

Because from the factory all gen3 pcms output a 4cyl tach signal. Again, the answers to all of your questions are in the video I posted above
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[Liltroy]

Here is the video link again
https://youtu.be/FS7aRfYkFAQ
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