Tachometer is giving false high reading?

[wilezcoyote]

Well I got a new toy today, an Auto X-ray EZ-Scan 5000. Found out I need a new O2 sensor to make that pesky intermitent S.E.S. light to go away.

Then I used the real time monitoring & logging mode. I thought I had a high idle problem because the tach has been reading around 1300-1600 rpm at idle lately. Well according to the Auto X-ray it's idling perfectly at 800rpm +/-5rpm. Anybody got any ideas about what's going on and how to fix this?

I took the scanner and hooked it up to 3 other vehicles and checked their tach readings against the scanner. They were all dead on.

 

[Hrtbeat1]

Well I got a new toy today, an Auto X-ray EZ-Scan 5000. Found out I need a new O2 sensor to make that pesky intermitent S.E.S. light to go away.

Then I used the real time monitoring & logging mode. I thought I had a high idle problem because the tach has been reading around 1300-1600 rpm at idle lately. Well according to the Auto X-ray it's idling perfectly at 800rpm +/-5rpm. Anybody got any ideas about what's going on and how to fix this?

I took the scanner and hooked it up to 3 other vehicles and checked their tach readings against the scanner. They were all dead on.

I found the same thing but with EASE. There is a filter (that is discontinued) that is in the harness that comes off the distributor. The filter itself is attached to one of the tranny mounting bolts (high on drivers side) it has one wire going in and one coming out. Im guessing it is like an EMI filter, but mine was shorting out a bit. Some of the strands of wire were showing on one side. My tach worked alright unless I "got on it" then it would jump to 6K, you could tell the engine was winding up but it wasn't at 6K and climbing. I have hooked up a remote tach directly to the wire coming off the distributor (bypassed the filter) and it works correctly, dead on with my scanning program. With the electronics in the display I haven't tried bypassing the filter for fear of toasting something. I still haven't figured out how to replace or fix the problem.