WOT Tach Problems
07-19-2005, 07:51 PM
#1
WOT Tach Problems Ok, I have posted this on another board but have had no luck. Here is a summary of my posts.
I have a 94 Z. The problem normally shows up in first gear. Basicly when the car is WOT in first gear the tach will rise up to about 5500rpms and then just smack over to 7000. I know for sure the motor isnt going to 7000, just the tach. Currently the car has a TH350 trans, but with the previous 6 speed it had the same problem. On motor it is normally fine, but on spray it happens probably 7 times out of 10.
I hooked a Auto Meter tach up to the tack output on my MSD 6a. The Auto Meter tach read purfectly.
So, I decided to hook up the factory tach to the tach output on my MSD 6a. This did not solve the problem. My factory tach and the autometer tach were getting the same exact signal and the factory one was still off. So I figured it had to be something wrong with the factory cluster. I just got a different gauge cluster and put it in to make sure my old one was bad and the new one does the same thing. Lastly I cut the gauge cluster ground wires and re grounded them to a different known good ground to see if that would help. Still no change.
Does anyone have any guesses????????????
I have a 94 Z. The problem normally shows up in first gear. Basicly when the car is WOT in first gear the tach will rise up to about 5500rpms and then just smack over to 7000. I know for sure the motor isnt going to 7000, just the tach. Currently the car has a TH350 trans, but with the previous 6 speed it had the same problem. On motor it is normally fine, but on spray it happens probably 7 times out of 10.
I hooked a Auto Meter tach up to the tack output on my MSD 6a. The Auto Meter tach read purfectly.
So, I decided to hook up the factory tach to the tach output on my MSD 6a. This did not solve the problem. My factory tach and the autometer tach were getting the same exact signal and the factory one was still off. So I figured it had to be something wrong with the factory cluster. I just got a different gauge cluster and put it in to make sure my old one was bad and the new one does the same thing. Lastly I cut the gauge cluster ground wires and re grounded them to a different known good ground to see if that would help. Still no change.
Does anyone have any guesses????????????
07-19-2005, 10:26 PM
#3
Yeah this is driving me crazy. I dont want to have to drive around with the AutoMeter tach in the car anymore. I thought about just getting a shift light, but this just really bugs me. A little more info. The odd thing is on spray the problem is worse.?????? On spray most of the time as soon as it comes in the tach just goes straight to 7000 and stays there untill i get off the gas. On motor the tach just rises smooth untill about 5500 then jumps to 7000 and in 2nd and 3rd is noticably jumpy. If im crusing in 3rd and then pull it down to 2nd and hammer it the tach will jump up to about 6000 and slowly rise to my 6400 shift point. Then I can hit the recall on the AutoMeter tach and it will say that the car only went to 5800 or so the entire time. I just dont get it. Since the tach signal is 12v all the time and only the frequency changes does it make a huge difference if the wires have been spliced or not? It has been doing this well before I spliced into any of them but I was just wondering.
07-19-2005, 11:40 PM
#4
It's possible the high res signal from the opti could be breaking up but this is usually accompanied by a misfire. Just the fact the autometer is working ok may nullify this. However, the autometer may not be as sensitive to input signal destortion as the factory tach. It may be hearing noise on the tack line from the pcm is my best guess. You might try a tach filter on the lead heading to the tach from the pcm.
