Tach ouput dying at ~6000

LT401Vette · Post by **LT401Vette** » Mon May 29, 2006 7:13 am

Yesterday I punched my car and it didn't shift..... Tried it again looking at the tach and saw that the tach needle went up to about 6000 RPM then started going down instead of up... The transmission TCU is using the MS tach feed also, so it is never seeing shift RPM and the motor just runs to rev limiter.

I have recently taken my wheel off and yesterday when it went back on, it landed in the wrong spot. So now I can run a trigger angle >49 degrees. I figure I have to fix this just to be sure I am getting full spark advance, but could this cause the tach signal to loose it at the higher RPM's?

The thing is, yesterday morning it was shifting with the same 49degree trigger angle, nothing changed by latter when it started doing this. That was the first time this motor cracked 7400 RPM....

LT401Vette · Post by **LT401Vette** » Wed May 31, 2006 6:22 pm

Ok, after a little further look into this, it appears that it is the hard rev limiter that is making the tach output take a dive. I have it set for cut fuel and spark, so once it hit the rev limiter the lower number of sparks makes the tach start dropping...

The problem is, there seems to be a lag or disagreement between MS and my TCU. What I think brought this on is that I raised my shift points to 6800 RPM from 6500, but the MS makes it to the 7300 hard cut before the transmission computer sees 6800, the sparks cut, rpms appear to fall and it never shifts...

The funny thing is, I was struggling with the shifts happening too soon before. When it was set at 6500, it kept shifting at 5900...

I dunno, I'll have to capture some more logs from the transmission point of view. And shut off the spark cut.

What would it take to have the tach output not impacted by the spark cuts?