05 Hummer/55 Chev conversion..

[Chuck L.]

Using the stock pcm, and trying to get a tach signal to a new SUN Retro tach..
I called their tech desk..He said the tach needs a 5v sq wave signal, and to check the wiring for correct position.
White wire/pin 10/green connector is good to go.[In correct pos., and has continuity].
I pulled up the tun file, and see that the tach parameters are checked as having a y/no choice for a 12v pullup signal. It is now active.

**I checked the signal for a 5v sq wave, but was using my Fluke VOM.. Got 0. Probably not fast enuf to trap that signal???
**Is that something I need a scope to look at?
**Would having the 12v pullup active, be interfering w/ the signal??


THANKS!:grin:

[Taz]

Hello Chuck,

Some Tachs require an external powered Tach pull up circuit to function. See the link below for a basic diagram...

http://forum.efilive.com/showthread.php?16025-Tacho-Pulses&p=144246&viewfull=1#post144246

Does the Sun Retro Tach feature a 4-6-8 cylinder switch ? Once you get a signal adjust G1204 and G1205 until the Tach reads accurately.


Regards,
Taz

[Chuck L.]

Hello Chuck,

Some Tachs require an external powered Tach pull up circuit to function. See the link below for a basic diagram...

http://forum.efilive.com/showthread.php?16025-Tacho-Pulses&p=144246&viewfull=1#post144246

Does the Sun Retro Tach feature a 4-6-8 cylinder switch ? Once you get a signal adjust G1204 and G1205 until the Tach reads accurately.


Regards,
Taz

TAZ, thanks for the quik hit!
The pcm I have, does put the 12 v pullup signal on the tach wire, [as I understand it, anyway].
Are you saying I need an external pullup signal??

The tach has several cyl choices, up to 10. I made sure it was on the 8 cyl setting.

I'm going to try turning the 12v pullup off, and see what that does.

More later!

THANKS!

[Taz]

Yes sir ... some Tachs won't function without a hard wired pull up circuit (despite how the tune is configured). These are usually a 12 volt circuit. I have never used a Sun Retro Tach - you mentioned it needs a 5 volt signal. Check with the manufacturer on the proper voltage of a pull up circuit.

If you need a 5 volt source - I would recommend the Fuel Tank Pressure Sensor circuit (5 volts) - it's not an engine management sensor (wouldn't tap into one of those) - and you probably weren't going to use it in the swap anyway. On 2005 Gen III trucks - often C1 (Blue) pin 46 (gray wire) - not sure on your 2005 Hummer.

Also, the LS1-B PCM outputs a "4 cylinder" Tach signal. Setting the Sun Retro Tach to 4 cylinder mode may preclude the need to modify the G1204 and G1205 values.


Regards,
Taz

[Chuck L.]

Yes sir ... some Tachs won't function without a hard wired pull up circuit (despite how the tune is configured). These are usually a 12 volt circuit. I have never used a Sun Retro Tach - you mentioned it needs a 5 volt signal. Check with the manufacturer on the proper voltage of a pull up circuit.

If you need a 5 volt source - I would recommend the Fuel Tank Pressure Sensor circuit (5 volts) - it's not an engine management sensor (wouldn't tap into one of those) - and you probably weren't going to use it in the swap anyway. On 2005 Gen III trucks - often C1 (Blue) pin 46 (gray wire) - not sure on your 2005 Hummer.

Also, the LS1-B PCM outputs a "4 cylinder" Tach signal. Setting the Sun Retro Tach to 4 cylinder mode may preclude the need to modify the G1204 and G1205 values.


Regards,
Tazthe setting for cyls to 4, nothing.
Loaded the .tun with the 12v pullup switched to off, nothing.
w/ the VOM on a/cv, no signal even close to 5v. Have a scope coming to see if the pcm is even outputting on the #10 pin....

The mfgr tech said the tach works on a 5v signal. Made no mention of a pullup requirement. Will call him back.
A couple posters on LS1tech have made mention of a resistor, such as the one you posted about the GTO thread.
I may get a pack of resistors and see if I can make the tach AND the pcm scram!!:doh2:
A good old fashioned "smoke test" is coming up!:grin:

Thanks for the help!