Tach problem
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Le Mans Master
Joined: Jul 2004
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From: Clifton Park, NY ............Clearwater, FL ... 85 Original Owner
Tach problem
Since coming out of hibernation a few weeks ago the tack in my 85 works only intermittently. Always lit but reading zero most of the time. Where do I start?
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Melting Slicks
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, 04:52 PM #2
seabright
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This link covers the tach filter..... may help (?):
https://www.corvetteforum.com/forums/...-problems.html
seabright
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This link covers the tach filter..... may help (?):
https://www.corvetteforum.com/forums/...-problems.html
Melting Slicks
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Doh.
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Le Mans Master
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From: Clifton Park, NY ............Clearwater, FL ... 85 Original Owner
, 04:52 PM #2
seabright
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Default
This link covers the tach filter..... may help (?):
https://www.corvetteforum.com/forums/...-problems.html
seabright
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Member Since: Aug 2002
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Default
This link covers the tach filter..... may help (?):
https://www.corvetteforum.com/forums/...-problems.html
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If the problem was the tach filter or an issue OTHER THAN the cluster then the scanner and the cluster would fail at the same time I'd think. If the cluster display is random and the ALDL scanner consistent then I'd think the problem is more likely internal to the cluster.
I didn't check a wiring schematic for the tach but I would think this diagnostic is certainly the easiest and certainly "non destructive".
Last edited by WVZR-1; May 25, 2015 at 09:19 AM.
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How intermittent is it? I'd consider connecting an appropriate scanner at the ALDL and drive with it to see if it's consistent in readout OR maybe intermittent like the cluster.
If the problem was the tach filter or an issue OTHER THAN the cluster then the scanner and the cluster would fail at the same time I'd think. If the cluster display is random and the ALDL scanner consistent then I'd think the problem is more likely internal to the cluster.
If the problem was the tach filter or an issue OTHER THAN the cluster then the scanner and the cluster would fail at the same time I'd think. If the cluster display is random and the ALDL scanner consistent then I'd think the problem is more likely internal to the cluster.
The ECM measures the time between DRPs (Distributor Reference Pulses) and calculates the RPM from that.
The dash uses the TACH output from the ignition module inside the distributor, filters it and displays the result. Problems are most likely caused by the tach filter (a well-known source of problems).
Since I haven't seen a picture yet in this thread... On my car the tach filter is bolted to the top bell housing bolt on the driver's side. It's below the connector on the large oil pressure sender in this picture (the silver part is the mounting bracket):
Last edited by Cliff Harris; May 26, 2015 at 12:17 AM.
Burning Brakes
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From: lincoln ri
pay it forward
mine was an intermittent problem; tach would surge like crazy then go back to normal, all the while the engine was steady. I recently had some engine work, and while doing some checking, twisted some wiring out of the way and voila, the problem left (for now)....However, I had purchased a filter through Ecklers; still sitting in my tool box in the package I'm sure. If you think you need it, just let me know, it's yours!!! No charge of course!!
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The method for determining RPM is completely different in the dash vs. the ECM.
The ECM measures the time between DRPs (Distributor Reference Pulses) and calculates the RPM from that.
The dash uses the TACH output from the ignition module inside the distributor, filters it and displays the result. Problems are most likely caused by the tach filter (a well-known source of problems).
Since I haven't seen a picture yet in this thread... On my car the tach filter is bolted to the top bell housing bolt on the driver's side. It's below the connector on the large oil pressure sender in this picture (the silver part is the mounting bracket):
The ECM measures the time between DRPs (Distributor Reference Pulses) and calculates the RPM from that.
The dash uses the TACH output from the ignition module inside the distributor, filters it and displays the result. Problems are most likely caused by the tach filter (a well-known source of problems).
Since I haven't seen a picture yet in this thread... On my car the tach filter is bolted to the top bell housing bolt on the driver's side. It's below the connector on the large oil pressure sender in this picture (the silver part is the mounting bracket):
For diagnostics the filter could be just by-passed and eliminate the expense. The use of an older hand held tach/RPM device I'd think could also be used to confirm the "intermittent" issue requiring no expense or the by-pass.
mine was an intermittent problem; tach would surge like crazy then go back to normal, all the while the engine was steady. I recently had some engine work, and while doing some checking, twisted some wiring out of the way and voila, the problem left (for now)....However, I had purchased a filter through Ecklers; still sitting in my tool box in the package I'm sure. If you think you need it, just let me know, it's yours!!! No charge of course!!
Last edited by WVZR-1; May 26, 2015 at 11:17 AM.
Thread Starter
Le Mans Master
Joined: Jul 2004
Posts: 6,228
Likes: 698
From: Clifton Park, NY ............Clearwater, FL ... 85 Original Owner
mine was an intermittent problem; tach would surge like crazy then go back to normal, all the while the engine was steady. I recently had some engine work, and while doing some checking, twisted some wiring out of the way and voila, the problem left (for now)....However, I had purchased a filter through Ecklers; still sitting in my tool box in the package I'm sure. If you think you need it, just let me know, it's yours!!! No charge of course!!
Thread Starter
Le Mans Master
Joined: Jul 2004
Posts: 6,228
Likes: 698
From: Clifton Park, NY ............Clearwater, FL ... 85 Original Owner
Tach Filter Update
Thanks to vetteboy88 I received and installed new tach filter and unfortunately it didn't resolve my problem. Except for the first couple seconds when car is started and reads 2700 it stays on zero. I really don't feel like pulling the dash for this