Joseph Parsons

Hello Corvette friends... done a ton repairs to my girl in preparation for winter... two things left for this season... replace the targa lens... mines broken and lets moisture in. And two get back my OD gears. I've had the transmission serviced and inspected this summer. It came back with a clean bill of health... yet my OD gears don't engage correctly or my OD light don't light... I get no errors in service mode either.. system checks out fine... so why no OD then... the transmission specialists had no clue why...
I think I know why, let's see if I'm right, ready?
my tach has never worked correctly since my wife bought me the car.... either stays off, or reads crazy high like 77 an 95, then flips all the way back 15, .... so according to the service manual.... the ECM controls the OD... and engine speed is a key factor.... so I think the crazy tach is causing my od woes... No it's not the dash board.. already been serviced by battee.com... dash is good...

I replaced the plugs and wires yesterday... still the same... I got a new cap and rotor ready to go on... should I replace the coil too? I'm stumped.... I need help... any one ever have a crazy tach issue like mine... I'm running low on time, as winter comes quickly in Maine... love you guys. Thanks in advance....

SLVRSHRK

I don't know if the coil is your problem, so I will say that up front, but I would like to relay my experience to you IF you do decide to change the coil.

On my 86 (late 86, aluminum head), I had a bad coil issue and changed out the coil to the same MSD model I use in my 78. The car ran perfectly, so the coil was the issue in my case, but it did cause an issue with the tach. At lower speeds, the tach would read correctly, but as rpms built, the tach would get flaky and misread by A LOT. It was all over the place. After some research, I saw that other had this experience as well.

Replacing the tach with an AC/Delco unit cured the tach reading problem.

You might just check and see if your coil is an AC/Delco or if you have an aftermarket unit.

I can't explain it, I just know what I (and others apparently) have experienced)

Good Luck

Joseph Parsons

Thanks for the zippy reply.... I will check to see if it is an aftermarket coil... the wires were cheapos, the plugs were champions... the cap looks aftermarket too... so I'm sure the coil is too... thanks

I have also heard the tach filter could be bad but where is it... I looked for it when I replaced the wiper motor but didn't see one? Anywho thanks again

jv9999

Maybe sure your tech filter is grounded (bolted to a good ground). Also see batee.com for other common problems.

ctmccloskey

I believe that the Tach Filter is mounted on the top of the bell housing on my 1988 C4. I would certainly check it before I bought a whole new tachometer. When the Tach filter acts up you get a "Wacky-Tach" like what you are describing. Electric Tachometers make a lot of sense when you deal with my 1968 C3 and it's mechanical tachometer with that darn cable that has the hard cover which doesn't like to bend so you have to keep moving the Distributor to get the best run for the Tach cable. One problem with "electric" Tachometers is that they are susceptible to electrical noise so be sure the lines to and from the tach don't go near any high voltage wires like spark plug wires. I have never heard of a cheap spark plug making that kind of noise but I guess it is possible. The Tach wire should be treated carefully and routed so it does not come into contact or touch anything connected to the Ignition system's Secondary side. Also be sure that the Tach output has a good connection to the wire heading for the Tach Filter. I had a weird intermittent issue with my C4 after a year or so of ownership, turns out some fool broke the locking clip of the power wire going to the Distributor and it would make/break connections whenever it felt like it.

Check your Grounds on your car, especially the engine itself. It is not uncommon to find corrosion on the ground straps and this will make more problems for the fuel systems and the ECM as well. Check your ground between battery negative and engine Block, you should have a solid connection here. Also check your Alternator's output with both the DC voltage scale and also with the AC voltage scale, there should be no AC on the lines going to your battery from the Alternator. Sometimes the Diode's fail and the alternator will produce a lot of AC ripple in the DC lines. This too can cause havoc with your battery and anything that has access to the DC power like your ECM and fuel pump and on and on from here. It is just a guess but it should be checked anyway since you are getting weird data from your tachometer.

It is likely the Tachometer Filter is giving you all the problems. Make sure that all the connections are good and solid and as jv9999 suggested be sure that the tach filter is grounded properly! If all else fails call Bryan at BATEE.com and he will know what to do! Bryan is "The Man" when it comes to anything related to our digital dashboards and the C68 Electronic A/C controls.

I wish you the very best in solving the "Mystery of the Wacky Tach" so you can once again cruise the highways in your beautiful Corvette and be able to accurately see your engine's true speed!

Joseph Parsons

Thanks for all pointers... I kinda figured it was the filter...I just dread removing the windshield wiper motor to check it... I have already replaced the motor once... no fun... so removing it to to check the filter is not on my agenda... but I guess that's what need to be done....
I know Bryan is amazing , he repaired my cluster already... great work worth every penny... worst comes to worst I'll call him for advice... thanks again

FOURSPEEDVETTE

Simply cut the two wires going to the tach filter and leave the old filter in place as it's barely noticeable and difficult to remove. It's located just below the wiper motor. Install a new tach filter attaching it to a good ground source anywhere on the block. I did this repair and attached it to the thermostat mounting bolt. It has a threaded end on the end facing upwards which made the installation very easy. Tach problem solved.