Cruisinfanatic

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?

antfarmer2

Tach filter.

MavsAK

Where does the tach filter mount, and connect to?

I know one end is off the dizzy, but the wire kind of disappears where you can't see it.
Does it have a second connector that eventually leads to the dash?
I suspect mine is shot too.

antfarmer2

Google your year and tach filter but belive it is a condencer looking thing by your distributer.

antfarmer2

, 04:52 PM #2
seabright
CF Senior Member


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Member Since: Aug 2002
Location: santa cruz ca

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This link covers the tach filter..... may help (?):

https://www.corvetteforum.com/forums/...-problems.html

MavsAK

Yeah..it's buried beneath two wire looms in my 89, where no human hand can actually reach it. The engine, wiper motor, fuel line, and wiring harness is all in the way. The hell was GM thinking.

Doh.

MavsAK

Any recommendations on which filter brand I should get? Or avoid?
Ecklers has one for 30 bucks on fleabay?

Cruisinfanatic

no link there

WVZR-1

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.

I didn't check a wiring schematic for the tach but I would think this diagnostic is certainly the easiest and certainly "non destructive".

antfarmer2

There was look it up by his name and find original puter stupid here.

Cliff Harris

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):

vetteboy88

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!!

WVZR-1

I don't disagree that it's "delivered" differently to the dash (CKT 121) the first check for a NON OPERATIONAL Tach is to confirm the reference signals at the distributor module. CKT 121 is the second/next to confirm. I don't doubt that it's a CKT 121 (filter most likely) but it's entirely possible that it's an internal cluster issue) also.

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.

A quite nice gesture for sure. "PAY IT FORWARD" seems to happen here frequently

Cruisinfanatic

PM'd

Cruisinfanatic

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