Strongsperformance

I have 03 silverado and if I hook the tach output up to my white wire after ecm and/or at gauge cluster it reads 400 off at idle and gets greater as rpm increases.
I have dropped the output to 50% just to get that reading.
If I remove my cluster and install it in my buddies truck it works correctly using his microsquirt the exact same way I use it in mine.
If I hook up the green output from Microsquirt (set to 50% ) to white wire output from ecm going to cluster it reads inaccurate. But works correctly on a autometer tach.
What am I doing differant on truck. Thank you for any help

Strongsperformance

UPDATE:
today I installed a bcm out of a 03 suburban and if fixed the tach issue. Mine was a v6 truck so bcm was altering the reference and now its fine BUT
I have a flashing security for remembering the pass lock now. But since I have ms3 if does not affect my starting because my pump is wired separate.
Is there a simple fix to either change my factory tach output from bcm or make truck recognize new bcm.
Just curious if it's as simple as leave key on for 10 min 3 times in a row.
Thanks again

Hulign

I think the easiest thing would be to get vats turned off. Anything else seems like a pain. Any pics of your install? You didn't happen to retain AC did you?