How to get your Tachometer working with your LS Swap.
Something we see posted countless times on facebook groups is the age-old question, how do I get my tach to work with my LS Swap? Well, today we’re here to dissect that question and hopefully, provide a solution.
Let’s start with the basics. How does a tach actually work?
A tachometer, rev counter, tach/tacho uses electrical pulses with a frequency proportional to the speed of the engine which is used to move the needle on your dash. Before any fancy EFI or ECU’s this pulse was directly connected to the negative side of the coil. As tech advanced, this pulse then was generated by a separate circuit in the ECU, which is how it is produced in the GM LS series of vehicles.
So, if the tach output from the computer is connected, why isn’t my tach working?
Not all tach signals are equal, nor are all tachometers. The LS ECM/PCM is actually programmed to output the same signal calibrated for a 4 cylinder tach (More on this later). This signal can also be weakened when the operating system is changed (for mafless tunes and conversions). The variance continues for some factory and aftermarket tachs. Some factory tach’s will be looking for this data over CAN (A digital computer language), some may derive the engine speed from the crank or cam sensor data directly and some may require a 12v or 5v pulse. If you’re feeling like getting this tach to work is a lost cause, don’t worry, here are some things you can try;
Now the easiest, cheapest, and most common way to resolve the tach issues, is to fit a pull up resistor. A pull up resistor is essentially a “Booster” for the signal, allowing the potentially weakened signal to drive the tach. This is simply a resistor that bridges across the already existing signal wire and a 12v+ power wire. When doing this you will leave the tachometer still connected as it normally would be. Connect one leg of the resistor to the signal wire on the tachometer, and the other leg of the resistor to the power wire of the tachometer. We recommend starting with a 10K-1/2 watt resistor, though you may go as low as 5K- ½ watt. The connected setup will look as below;
These resistors are commonly available from your local Jaycar or electronics store. The Jaycar Part number for the 10k Resistor is RR0596.
So you tried the pull up resistor and you have a signal, great, but the calibration is wrong and reading off. This is most likely due to the PCM output being what most would consider a “4 Cylinder” tach output or 4 pulses per revolution. Luckily, this can be changed within most reputable tuning software’s. The tach output pulse can be modified to suit almost any typical tachometer, be it a 4-, 6- or 8-cylinder tach. As we use HP tuners, we are most familiar with this and will be explaining how this works for this particular software. The tach output can be found in the Engine - General – Section of the VCM editor suite.
The Resolution High and Low relate to how many periods of the crank pulse, in this case, Gen 3 LS1 which is a 24x Crank pulse, stay low or high, creating the pulse. The math is simple and the results are below;
The Gen 4 engines (LS3, L98, L77 etc) present a similar issue with the 4 cyl tach output, however, the settings to change this are slightly different.
The resolution is 15 as the trigger wheel for the Gen 4’s has 58 teeth (as opposed to 24 on gen 3)
If you need to change these values and don’t have access to HP tuners, your local tuner can change these values while on the dyno. If this isn’t a possibility for you, you can send us your PCM and we can change this on the bench.
If none of the above have resolved the tach issue, the next solution is a Tach adapter module like the SGI-100BT.
This module allows you to modify the tach output, regardless of the input, to suit the tach you are driving. It can also calibrate the speed output to suit. It connects via Bluetooth and can be calibrated form your smartphone. It also has the following features;
- Speed calibration from 25% - 400%
- Multiple speed outputs available for multiple devices.
- Electronic pull-up built in; high or low voltage input.
- Tachometer settings from 1-16 cylinders and 5V or 12V signal type.
- Diesel applications: outputs for 4, 6 or 8 cylinder tachs.
- Can read off flywheel sensor (available separately), timing gear sensor or alternator W-terminal (stator tap).
- Bluetooth technology syncs unit to available mobile app for programming and setup.
- Built-in programming buttons and LED display for setup in lieu of app.
These can be purchased from our shop HERE;
We hope this clears up some confusing surrounding the notorious LS Tach outputs.
LS The World!