![]() ![]() The math is simple and the results are below 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 tach output can be found in the Engine - General – Section of the VCM editor suite. 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 pulse can be modified to suit almost any typical tachometer, be it a 4-, 6- or 8-cylinder tach. Luckily, this can be changed within most reputable tuning software’s. This is most likely due to the PCM output being what most would consider a “4 Cylinder” tach output or 4 pulses per revolution. So you tried the pull up resistor and you have a signal, great, but the calibration is wrong and reading off. The Jaycar Part number for the 10k Resistor is RR0596. These resistors are commonly available from your local Jaycar or electronics store. We recommend starting with a 10K-1/2 watt resistor, though you may go as low as 5K- ½ watt. 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. When doing this you will leave the tachometer still connected as it normally would be. This is simply a resistor that bridges across the already existing signal wire and a 12v+ power wire. A pull up resistor is essentially a “Booster” for the signal, allowing the potentially weakened signal to drive the tach. Now the easiest, cheapest, and most common way to resolve the tach issues, is to fit a pull up resistor. If you’re feeling like getting this tach to work is a lost cause, don’t worry, here are some things you can try ![]() 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. The variance continues for some factory and aftermarket tachs. This signal can also be weakened when the operating system is changed (for mafless tunes and conversions). The LS ECM/PCM is actually programmed to output the same signal calibrated for a 4 cylinder tach (More on this later). Not all tach signals are equal, nor are all tachometers. So, if the tach output from the computer is connected, why isn’t my tach working? 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. Before any fancy EFI or ECU’s this pulse was directly connected to the negative side of the coil. How does a tach actually work?Ī 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. 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. ![]()
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