Custom Tach for GM

[The Dickens]

I figured I'd just not retained it in one of the classes. I now have 4 books from them now, and just didn't catch it while flipping through them. Thanks guys!

[Danny Cabral]

Tom probably went over that in the classes and I didn't think I'd ever use, so put no effort into retaining.

There's some instruction on it, in the Holley EFI Tier 2 Training Manual (page 17 - "Inputs & Outputs" section). See last paragraph below.
https://forums.holley.com/showthread...4640#post74640 (Related Forum Thread)

Holley EFI Tier 2 Training Manual: Output Types PWM+ ‐ Like the +12V output type is used when the other side of the load is grounded. When the output is active the ECU “pushes” 12V to the load. This is often called an active high output or high side output. Unlike the +12V type it is not simply on or off. You can set it up to smoothly control the duty cycle as a function of multiple variables. The letter code for “+12V” in the Pin Map is “H.” Think of “H” as active High. After enabling an output and assigning it this type it will show up in the Pin Map as an unassigned output with an “H” immediately to the left. This signifies it can be clicked and dragged to any available pin that has an “H” beside it. PWM− ‐ Like the Ground output type is used when the other side of the load is powered. When the output is active the ECU “pulls down” the low side of the load. This is often called an active low output or low side output. Unlike the Ground type it is not simply on or off. You can set it up to smoothly control the duty cycle as a function of multiple variables. The letter code for “+12V” in the Pin Map is “H.” Think of “H” as active High. After enabling an output and assigning it this type it will show up in the Pin Map as an unassigned output with an “H” immediately to the left. This signifies it can be clicked and dragged to any available pin that has an “H” beside it. PWM Setup Parameters - Because actuators and solenoids vary greatly, you need some adjustments instead of having a one size fits all solution. One thing you need to specify is the frequency at which you drive the actuator. You have two options in the software‐ setting a fixed frequency of your choosing or using a frequency that is a function of engine speed. For most actuators a fixed frequency is appropriate, so you select "Fixed" frequency for the Type and enter in the desired frequency in the field below. This frequency is in Hz, or the number of cycles per second. If you want vary the frequency with engine speed, you set the Type to "RPM Derived" and enter either/or a Multiplier or a Divider. this deserves some explanation. If both the Multiplier and Divider parameters are both zero, the output frequency in Hz is equal to the engine speed in RPM. If the engine speed is 1000 RPM, then the output frequency would be 1000 Hz. A 1:1 ratio usually results in a frequency that is too high for most applications. For a one pulse per fire frequency for a V8, you would need a divider of 15. This would result in a 4 pulse/rev. A divider setting of 30 would result in 2 pulses/rev, which happens to be what many GM tachometers use.

[The Dickens]

Awesome, that's the info I needed & couldn't find. I had no clue what signal was coming out for that, and didn't want to hack my way to an answer. I really think a changing equation for the current output settings would be a great tool and save a lot of confusion, questions, & tech calls. Thanks for the help. Tom probably went over that in the classes and I didn't think I'd ever use, so put no effort into retaining.

On a different not what is the most reliable fuel pressure regulator for you. Wanting to ditch the in tank one for an inline with dual inlet and single return. Aeromotive has not worked well for our shop ever. Right now, I'm leaning toward a Weldon and would have one on the way, if I could find one of our venders that sold them. But doesn't seem to be a line any of ours carry. Thanks again.

[S2H]

Holley EFI Tier 2 Training Manual: If you want vary the frequency with engine speed, you set the Type to "RPM Derived" and enter either/or a Multiplier or a Divider. This deserves some explanation. If both the Multiplier & Divider parameters are both zero, the output frequency in Hz is equal to the engine speed in RPM. If the engine speed is 1000 RPM, then the output frequency would be 1000 Hz. A 1:1 ratio usually results in a frequency that is too high for most applications. For a one pulse per fire frequency for a V8, you would need a divider of 15. This would result in a 4 pulse/rev. A divider setting of 30 would result in 2 pulses/rev, which happens to be what many GM tachometers use.

So set your Multiplier as 1 and your divider as 30 for the GM tach which is actually looking for a 4 cylinder signal.
(And why GM chose to be stupid like that, we will never know.)

[The Dickens]

Alright Danny, I need a hand. This is my second time setting up a custom tach output, but the first one was to drive a diesel tach for a low RPM engine, that took something like a 168 triggers per revolution.

I'm trying to get my Camaro tach to work and I was getting nothing. So I hooked up the A28 pint to and got double reading like it should. So I went back to my wire and made some changes. It moves it now, but it's all wrong as the GM Tach pegs at startup and only came down a little at 850 rpm. I have an LED installed to test out different output settings, and when it was setup like the tach, RPM Derived and set to come on with time it would flash like crazy with the motor not running.

I would love to know what signal you get from RPM derived gives, I would think 1 pulse per revolution or 4, but I want to know so I can make intelligent adjustments and not a guess. I wish they would have put in my idea to have the actual math the output shown and change with the settings, so when you are doing something for the first time you could figure it out without guessing and testing.

After messing around a little before deciding I might as well just ask someone who has done this before. I as you see it would read about 1400 when idling at 950.