Tach to Pcm

[LeesTruk]

You're gonna need what's called a "Pull-UP" resistor. Just put a 1K ohm resistor from your tach power wire to the tach signal wire. Also, if using an aftermarket tach, make sure it's set for a 4 cyl. engine. The PCM tach output is designed with that sort of signal pulse

[1970 CST Short Wide]

Originally Posted by LeesTruk

You're gonna need what's called a "Pull-UP" resistor. Just put a 1K ohm resistor from your tach power wire to the tach signal wire. Also, if using an aftermarket tach, make sure it's set for a 4 cyl. engine. The PCM tach output is designed with that sort of signal pulse

I have a in dash Factory style tach but not factory original How do I change it to 4 cyl or can I

[Mr.Chevy4x4]

If the tach is reading approximately half of what it should be, then you don't need the resistor. If the tach is barely or not moving at all then you need to put a 1K resistor from a switched power source into the tach feed wire. This will boost the signal.

Now, back to reading half of what it should. The signal from the PCM is similar to what a 4 cylinder would produce. This is why you can switch aftermarket tachs to 4 cylinder mode and they read correctly. If the tach you have isn't switchable, you can have a tuner change the tach output to make the tach read correctly.

Mike

[1970 CST Short Wide]

Originally Posted by Mr.Chevy4x4

If the tach is reading approximately half of what it should be, then you don't need the resistor. If the tach is barely or not moving at all then you need to put a 1K resistor from a switched power source into the tach feed wire. This will boost the signal.

Now, back to reading half of what it should. The signal from the PCM is similar to what a 4 cylinder would produce. This is why you can switch aftermarket tachs to 4 cylinder mode and they read correctly. If the tach you have isn't switchable, you can have a tuner change the tach output to make the tach read correctly.

Mike

Thank you, that helps a lot
Steve

[68c10airstream]

I have the 8,000 rpm aftermarket stock cluster kit installed and the pcm put out the wrong signal and after talking to avatar "critter" from "lone star dyno" i sent my pcm to him and he did the required mods and tested it on 2 different tachs before he sent it back. He said the signal needed to be changed in 2 ways, i believe from 4 cyl to 8 cyl and had to rescale it. He said this worked better than the pullup resistor, but i can't verify it. He was fair and took the time to double check it before sending it out. His name is Terry.

I did the ls1nova cluster changes and between eric (ls1nova) and terry it came out great.

[1970 CST Short Wide]

Originally Posted by 1970 CST Short Wide

Thank you, that helps a lot
Steve

Tried the 1K resister and it made my tack go backwards

[BBPanel]

Originally Posted by 1970 CST Short Wide

Tried the 1K resister and it made my tack go backwards

The purpose of the pull-up resistor is to change the +5v pcm tach signal to +12v - it worked for me on my Autometer tach. Sure you have the resistor in the correct direction - have no idea what it will do if not.

Here's a reference: http://aeromotiveinc.com/wp-content/...leShooting.pdf

[1970 CST Short Wide]

Originally Posted by BBPanel

The purpose of the pull-up resistor is to change the +5v pcm tach signal to +12v - it worked for me on my Autometer tach. Sure you have the resistor in the correct direction - have no idea what it will do if not.

Tried both power and lead to pcm. Lead did nothing power went backwards