Tachometer filter on a 1990
Thread Starter
Heel & Toe
Joined: May 2006
Posts: 20
Likes: 0
From: Oxelösund
Tachometer filter on a 1990
My tach presently shows twice, or about twice, the real rpm. The tach filter was not mounted to any ground spot but hung loose behind the engine, below the distributor. When I hold the filter against a sure ground, the tachometer shows zero. It is active, but no signals from the distributor seem to go through when I ground the filter/capacitor.
Does this mean that I should replace my filter and that I can hope for correct readings after?
Does this mean that I should replace my filter and that I can hope for correct readings after?
Le Mans Master
Joined: Feb 2005
Posts: 5,146
Likes: 3
From: The Motor City
If when you ground it, nothing happens. Ungrounded, it works but innaccurate would seem that there is something is wrong with it. It should be mounted/grounded.
As far as "correct" readings, these tachs. are not the best, but it might be reasonable, and if calibrated perhaps close.
As far as "correct" readings, these tachs. are not the best, but it might be reasonable, and if calibrated perhaps close.
Le Mans Master
Joined: Oct 2003
Posts: 6,130
Likes: 2
From: seattle WA
it's supposed to be grounded. Funny, but my tach on my 87 is extremely close to scanner readouts-within 25 RPMs. Another thing you might check is the bulkhead connection, their not sealed and water from rain, and sometimes the cowl drip tube dumps on the connection, the tach pin is in on the lower row-if you open it look for a greenish/whiteish look-that's evidence water has gotten in, blow it out, reconnect and seal top and sides of connection. Learned this from Gordon Killebrews book.
Racer
Joined: Oct 2005
Posts: 405
Likes: 0
From: 1984 crossfire, Alhaurin de la Torre Malaga, Spain
Originally Posted by ghd
My tach presently shows twice, or about twice, the real rpm. The tach filter was not mounted to any ground spot but hung loose behind the engine, below the distributor. When I hold the filter against a sure ground, the tachometer shows zero. It is active, but no signals from the distributor seem to go through when I ground the filter/capacitor.
Does this mean that I should replace my filter and that I can hope for correct readings after?
Does this mean that I should replace my filter and that I can hope for correct readings after?
In your case, the capacitor may be faulty and may be shorted to ground. If you measure the filter DISCONNECTED at BOTH sides, it should read more than 1 MegaOhm from input or output to ground. and the input to output should be approx 10 kOhms. This value may vary quite a lot.
I heard the the filters are still available, but if you can´t get it, an electronic hobyist can make you one, tell him you need a T filter, low-pass, resistors 5.6 kOhm input and output and 0.1 uF to ground.
Resistors of 0.25 Watt are quite sufficient.
Good luck
Racer
Joined: Aug 2005
Posts: 336
Likes: 3
From: State of Confusion
But if that doesn't work and you are using an aftermarket ignition coil, try putting the factory unit back in. It caused tach issues in my car, not the same as your situation, but worth a try if anything else,Best of luck,
Adam.
Le Mans Master
Joined: Aug 2000
Posts: 8,838
Likes: 31
From: Australia
Originally Posted by trulytex
I have a 1990 and my tach is off by 300%. It shows 2100 RPM's at idle. Where is the tach filter located on the 1990? I would like to check mine.
'90 Vette tachos are well documented for many and misc problems which were fixed for 91.
These appear to be across the board problems totally unrelated to the tach filter.
Racer
Joined: Oct 2005
Posts: 405
Likes: 0
From: 1984 crossfire, Alhaurin de la Torre Malaga, Spain
Originally Posted by rodj
Join the club.
'90 Vette tachos are well documented for many and misc problems which were fixed for 91.
These appear to be across the board problems totally unrelated to the tach filter.
'90 Vette tachos are well documented for many and misc problems which were fixed for 91.
These appear to be across the board problems totally unrelated to the tach filter.
So when someone has the problem, PLEASE test the following. Put a 10 kOhm resistor in series with the output and one in series with the input.
When that does not help, ground the filter at the ground of the tach as close as possible with a fat short wire. So we can close in at the problem
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Thread Starter
Heel & Toe
Joined: May 2006
Posts: 20
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From: Oxelösund
My filter measures about 15 kOhm from input to output, so the resistors seem accurate. I didn't try the MOhm scale when I checked for shortage to ground, I only used the 200 k scale, with zero as result. So there may be a too low resistance to ground, as the grounding of the filter leaves the tach with to weak a signal.
The filter is no longer produced as an original part, but there are replacements from several sources. I have ordered one from rodneydickman.com, who sells a cheaper and more slim design one than the original.
I will tell what happens when I get it and puts it in place. If the tach isn't working then, I will try Nicos idea of putting more resistance in line with the filter, I could use the old filter as resistor.
The filter is no longer produced as an original part, but there are replacements from several sources. I have ordered one from rodneydickman.com, who sells a cheaper and more slim design one than the original.
I will tell what happens when I get it and puts it in place. If the tach isn't working then, I will try Nicos idea of putting more resistance in line with the filter, I could use the old filter as resistor.
Melting Slicks
Joined: May 2004
Posts: 2,524
Likes: 10
From: Lufkin Texas
St. Jude Donor '05-'06
My 90 had the same condition with it's tach when I got it. Even down to the filter being hanging loose behind the distributor and not grounded. At first suggestions from a GM dealer tech friend I went to ask, I replaced the filter to no avail. I ended up getting a GM refurbished 90 instrument cluster and the tech said the filter wasn't needed.I was concerned that the instrument cluster change would effect my odometer but I learned that mileage is stored in the CCM. I dont know if the refurbs are different or have a built in filter but my tach has worked great for over 2 years without the filter.
Hopefully a new filter will fix yours.
Hopefully a new filter will fix yours.
Le Mans Master
Joined: Aug 2000
Posts: 8,838
Likes: 31
From: Australia
Originally Posted by skateparkdave
My 90 had the same condition with it's tach when I got it. Even down to the filter being hanging loose behind the distributor and not grounded. At first suggestions from a GM dealer tech friend I went to ask, I replaced the filter to no avail. I ended up getting a GM refurbished 90 instrument cluster and the tech said the filter wasn't needed.I was concerned that the instrument cluster change would effect my odometer but I learned that mileage is stored in the CCM. I dont know if the refurbs are different or have a built in filter but my tach has worked great for over 2 years without the filter.
Hopefully a new filter will fix yours.
Hopefully a new filter will fix yours.
When running mine lags badly and if key left on , engine stopped will slowly climb to redline!
Pro
Joined: Jul 2002
Posts: 703
Likes: 0
From: Milwaukee Wi
my trouble started day i upgraded to a "stock replacement" high energy HEI coil and module. Accel advertises much more voltage and i believe it. it killed tach readout at about 4500 rpms. my cam was rated to 5000 then. i removed GM filter and all was well. swapped cam for one that spins to 5800-6000. it then disturbed readings again once rpms climbed. now Autometer and Innovate suggests using an aftermarket tach filter or tach adapter with Autometer tachs used on late model cars(HEI). my tach will be installed W/O the adapter and if i see reoccurance I will order Autometer adapter. i wonder if that adapter could be used with stock tach?
