Innacurate Autometer Tach
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Joined: Apr 2008
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From: Dumfries, VA
Car: 1985 Z28
Engine: 334 Stroker Superram 222/230
Transmission: Full Manual 700R4 / 3k Street Edge
Axle/Gears: 3.90 Eaton, Moser, Richmond & More
Innacurate Autometer Tach
Wanted some opinions on what to do here.
A few weeks ago I found an Autometer Phantom 5" tach in a Honda at the junkyard that appeared to be in good shape, so I took it. Redid the calibration loop that had been cut to restore 8cyl operation and installed in the car.
It appears to be reading low by a small amount that gets slightly more exaggerated as RPM increases. At a 900rpm idle verified with both my intellitronix digital cluster and laptop reading the EBL flash, the tach appeared to read 50-100rpm low. Slowly increased RPM and observed the reading become more off with RPM. At 3000rpm, it is reading low by 200-300. That's the highest I tested at and haven't driven the car yet to further investigate.
At first I figured it could be poor power/ground. I verified the ground was good but I had the power tapped into the fuse block where I measured a difference of 0.8V vs battery with engine off.
Redid wiring so that the tach was now receiving battery power directly via a switched relay along with my digital cluster. Still the same result. Tach reads dead on zero with engine off, so I know the needle itself hasn't been skewed.
Right now I'm considering one of three possibilities. 1, try a pull-up resistor. 2, try a current limiting resistor inline with the signal wire (Which for whatever reason I had to do with the digital cluster or it went ape above 3k). 3, Send tach to autometer for repair/calibration.
Anyone had an issue like this?
A few weeks ago I found an Autometer Phantom 5" tach in a Honda at the junkyard that appeared to be in good shape, so I took it. Redid the calibration loop that had been cut to restore 8cyl operation and installed in the car.
It appears to be reading low by a small amount that gets slightly more exaggerated as RPM increases. At a 900rpm idle verified with both my intellitronix digital cluster and laptop reading the EBL flash, the tach appeared to read 50-100rpm low. Slowly increased RPM and observed the reading become more off with RPM. At 3000rpm, it is reading low by 200-300. That's the highest I tested at and haven't driven the car yet to further investigate.
At first I figured it could be poor power/ground. I verified the ground was good but I had the power tapped into the fuse block where I measured a difference of 0.8V vs battery with engine off.
Redid wiring so that the tach was now receiving battery power directly via a switched relay along with my digital cluster. Still the same result. Tach reads dead on zero with engine off, so I know the needle itself hasn't been skewed.
Right now I'm considering one of three possibilities. 1, try a pull-up resistor. 2, try a current limiting resistor inline with the signal wire (Which for whatever reason I had to do with the digital cluster or it went ape above 3k). 3, Send tach to autometer for repair/calibration.
Anyone had an issue like this?
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someone972
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Jun 12, 2016 03:24 PM
