Tach reads double
Thread Starter
Pro
Joined: May 2005
Posts: 592
Likes: 1
From: Shreveport Louisiana LSU
factory tach. HEI with one wire to tach. This is on an 81 with a non computer tach. I was talking about the one wire that goes to the tach plug on the dist. The computer has apparently been by-passed.
Last edited by dfolse62; May 1, 2007 at 09:52 AM.
Racer
Joined: Jul 2005
Posts: 499
Likes: 1
From: Winnipeg Manitoba
A tach reading double all the time would be a very unusual failure mode.
It would be difficult to explain by anything other than hosspower's suggestion: Its a 4 cylinder tach.
Do you have any other symptoms?
Does it read double all the time?
Does it return to 0 when you turn the ignition to RUN with the engine off?
Does it jump around?
What's your battery voltage. (The tach can be affected by low voltage)
It would be difficult to explain by anything other than hosspower's suggestion: Its a 4 cylinder tach.
Do you have any other symptoms?
Does it read double all the time?
Does it return to 0 when you turn the ignition to RUN with the engine off?
Does it jump around?
What's your battery voltage. (The tach can be affected by low voltage)
Instructor
Joined: Dec 2006
Posts: 105
Likes: 0
From: biloxi mississippi
Hope you get an answer to this problem, as i have a 78 pace with the original tack that reads exactly double the rpms - i switched out the tack with another 78 tack and the replacement reads correct, i believe it is the chip on my circuit card for the tack but can't find a number for the chip.
Racer
Joined: Jul 2005
Posts: 499
Likes: 1
From: Winnipeg Manitoba
Well, you can try putting a filter on the tach line to the instrument panel.
This might filter out any ringing that the tach is reading as two pulses.
The filter ciruit is just a couple of resistors and caps.
Its R!, R2, C1, and C2 in this diagram:
Tach Filter
You can always test the tach using a modified AC adapter as I'm showing in this diagram.
If you apply 120Hz from a FWB rectified adapter, you should read 1800Hz.
I would not attempt this unless you have some electronics knowedge.
If you don't, find someone that does.
Tach Test
Shutting the engine off:
Switching off the ignition removes the electical power required to move the needle.
If you do this quickly, the tach will freeze at its last reading.
Power returns when you turn the ignition switch back to run.
There is no signal if the engine isn't running. It should read 0.
The chip is a national semiconductor LM1819.
They are obsolete. Good luck finding one.
Its easier to buy a new board on ebay.
The Gardner boards are highly rated.
This might filter out any ringing that the tach is reading as two pulses.
The filter ciruit is just a couple of resistors and caps.
Its R!, R2, C1, and C2 in this diagram:
Tach Filter
You can always test the tach using a modified AC adapter as I'm showing in this diagram.
If you apply 120Hz from a FWB rectified adapter, you should read 1800Hz.
I would not attempt this unless you have some electronics knowedge.
If you don't, find someone that does.
Tach Test
Shutting the engine off:
Switching off the ignition removes the electical power required to move the needle.
If you do this quickly, the tach will freeze at its last reading.
Power returns when you turn the ignition switch back to run.
There is no signal if the engine isn't running. It should read 0.
The chip is a national semiconductor LM1819.
They are obsolete. Good luck finding one.
Its easier to buy a new board on ebay.
The Gardner boards are highly rated.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Racer
Joined: Jul 2005
Posts: 499
Likes: 1
From: Winnipeg Manitoba
Reads double when engine is running and in gear.
Reads correctly when in neutral.
I would think the tach circuit is working and you have either bad voltage,
or a bad signal.
I would:
Check your connection point as suggested above.
Check battery voltage when in gear.
Try a filter circuit
Le Mans Master
Joined: Sep 2002
Posts: 5,236
Likes: 898
From: Myrtle Beach SC
Just to clarify, your suggesting Gardner makes the tach printed wiring circuit boards and its easier to pick a new one up than attempt to find the semiconductor and replace it on the existing board?
Where is that board located? Behind the tach/speedo console?
Racer
Joined: Jul 2005
Posts: 499
Likes: 1
From: Winnipeg Manitoba
Kilroy:
Just to clarify, your suggesting Gardner makes the tach printed wiring circuit boards and its easier to pick a new one up than attempt to find the semiconductor and replace it on the existing board?
Where is that board located? Behind the tach/speedo console?
Just to clarify, your suggesting Gardner makes the tach printed wiring circuit boards and its easier to pick a new one up than attempt to find the semiconductor and replace it on the existing board?
Where is that board located? Behind the tach/speedo console?
Yes. The board is at the back of the tach in the dash.
You have to drop the steering wheel, take off the windshield trim,
take out the dash pad, and then the instrument panel will pull out.
Its a pain in the butt.
(This is how it comes out on my 76)
Racer
Joined: Aug 2005
Posts: 435
Likes: 2
From: Boise Idaho
I replaced a nonfunctional board with a mid america board and mine now is high as well. I don't think it is quite reading double but does read high. Maybe 50% ish high. I also put on a new filter at the same time.
I was told it is probably a bad board. I will replace it the next time I open the dash up for another issue as I'm not tearing that apart again just for that circuit board. Taking it apart is easy compared to getting it back together.
I was told it is probably a bad board. I will replace it the next time I open the dash up for another issue as I'm not tearing that apart again just for that circuit board. Taking it apart is easy compared to getting it back together.
Instructor
Joined: Jun 2006
Posts: 116
Likes: 0
From: Prairie Village Kansas
I replaced a nonfunctional board with a mid america board and mine now is high as well. I don't think it is quite reading double but does read high. Maybe 50% ish high. I also put on a new filter at the same time.
I was told it is probably a bad board. I will replace it the next time I open the dash up for another issue as I'm not tearing that apart again just for that circuit board. Taking it apart is easy compared to getting it back together.
I was told it is probably a bad board. I will replace it the next time I open the dash up for another issue as I'm not tearing that apart again just for that circuit board. Taking it apart is easy compared to getting it back together.
thats what they said when I called in for technical support, so they sent me a new one... Same story different day, I think the circuit boards from MA are bad
Racer
Joined: Aug 2005
Posts: 435
Likes: 2
From: Boise Idaho
I guess I should have called them when I had the chance.
This was about a year ago now. I have learned to live with it and mentally adjust by feel of engine.
If I blow the engine I guess I'll be able to justify a 'modest' upgrade.
This was about a year ago now. I have learned to live with it and mentally adjust by feel of engine.
If I blow the engine I guess I'll be able to justify a 'modest' upgrade.
3rd Gear
Joined: Apr 2007
Posts: 3
Likes: 0
From: Newport Rhode Island
Tach Nightmare(s)
I went the board replacement route TWICE and could never get the stock Tach to work. I refused to rip the dash out again so I just installed a good after-market
Tach on the steering column. Someone did mention to me that sometimes the problem can be as simple as replacing the Tach Filter, but I see another reply that said he tried that.
I actually had the vendor bench test both of the boards I tried and they both worked fine when he tested them, so why they wouldn't work when I installed each of them is still a mystery, but I wasn't going to let it make me go mental, so the after-market unit suits me just fine because the car is a daily driver and I'm not sweating the fact that the original Tach refuses to work. Good Luck, mick
Tach on the steering column. Someone did mention to me that sometimes the problem can be as simple as replacing the Tach Filter, but I see another reply that said he tried that.
I actually had the vendor bench test both of the boards I tried and they both worked fine when he tested them, so why they wouldn't work when I installed each of them is still a mystery, but I wasn't going to let it make me go mental, so the after-market unit suits me just fine because the car is a daily driver and I'm not sweating the fact that the original Tach refuses to work. Good Luck, mick
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
I suspect that the tach signal line needs to be filtered to "clean up" unwanted pulses. Extraneous pulses from ignition, etc. can trick the tach into counting more pulses than those sent by the tachometer, alone. If the circuit board is new, the filter is what I would try first. Find the tach filter in your system and replace it with the circuit presented above. That's a lot easier than tearing the dash apart.