Tach to Pcm

[dayj1]

Originally Posted by aknovaman

Hate to rain on someones parade but your theory on the PCM tach output is incorrect. First the older PCM red/blue actually puts out a positive pulse from ground to about 8 volts. This transition is sufficient to trigger most aftermarket tachometers. The newer ecu don't work the same way. They actually ground the tach output wire for each tach event. The purpose of the pull up resistor is to do that, pull up the voltage to 12 volts between each tach event. Then with each tach event, the pcm output is grounded, thus transitioning the voltage across the external tach threshold. This can be verified using a simple oscilloscope to see the waveform. If you don't have the pull-up resistor connects to 12 volts, the newer PCM output will float anywhere from 1-8 volts erratically. There needs to be a solid/consistent voltage transition across the external tach in order for it to function reliably.

Respectfully, Aknovaman

Based on what you wrote, I believe you and I are in agreement on the operation of the newer ECMs. At least I think we're saying the same thing. I haven't noted the same behavior as you on the older PCMs, but I certainly haven't used/tested every hardware revision. From your description it sounds like some may have an internal pull-up resistor.

[Liltroy]

Originally Posted by aknovaman

Hate to rain on someones parade but your theory on the PCM tach output is incorrect. First the older PCM red/blue actually puts out a positive pulse from ground to about 8 volts. This transition is sufficient to trigger most aftermarket tachometers. The newer ecu don't work the same way. They actually ground the tach output wire for each tach event. The purpose of the pull up resistor is to do that, pull up the voltage to 12 volts between each tach event. Then with each tach event, the pcm output is grounded, thus transitioning the voltage across the external tach threshold. This can be verified using a simple oscilloscope to see the waveform. If you don't have the pull-up resistor connects to 12 volts, the newer PCM output will float anywhere from 1-8 volts erratically. There needs to be a solid/consistent voltage transition across the external tach in order for it to function reliably.

Respectfully, Aknovaman

I was wondering when you were gonna come back on here! Welcome back!!

[aknovaman]

I have not visited this site mainly due to one member who appears to be a know it all jerk with a few years’ experience with LSx platforms but not mature enough to stay out of other people’s business. He has repeatedly insulted me and my 45+ years of experience in electronics and auto mechanics.

That being said, if someone wants to contact me about wiring, tuning or parts compatibility issues with their projects, via PM I will assist. I have neither the time nor patience to concern myself with other peoples self-righteous attitudes.

[Willie Makeit]

Originally Posted by aknovaman

I have not visited this site mainly due to one member who appears to be a know it all jerk with a few years’ experience with LSx platforms but not mature enough to stay out of other people’s business. He has repeatedly insulted me and my 45+ years of experience in electronics and auto mechanics.

That being said, if someone wants to contact me about wiring, tuning or parts compatibility issues with their projects, via PM I will assist. I have neither the time nor patience to concern myself with other peoples self-righteous attitudes.

i understand how you feel. seems this site has an over-abundance of such types. it's tiring.

[aknovaman]

I have thick skin but value my time greatly.

[Willie Makeit]

Originally Posted by aknovaman

I have thick skin but value my time greatly.

hear. hear.

[Liltroy]

Originally Posted by aknovaman

I have not visited this site mainly due to one member who appears to be a know it all jerk with a few years’ experience with LSx platforms but not mature enough to stay out of other people’s business. He has repeatedly insulted me and my 45+ years of experience in electronics and auto mechanics.

That being said, if someone wants to contact me about wiring, tuning or parts compatibility issues with their projects, via PM I will assist. I have neither the time nor patience to concern myself with other peoples self-righteous attitudes.

Roger that! I haven't been on here much lately either. And Just for the record, I appreciate EVERYTHING you've taught me and done for me!

[solidaxel]

Thank you both for your time and explanations and for answering the problems with my Tach
I am keeping this thread for future reference

Now on to the speedometer!