'79 Tachometer reading wrong...

[vetteboy79]

Hello -

The tachometer in my '79 L-82 is reading a bit high...going down the highway at around 70mph or so it's reading almost 5000 RPM with the stock trans and rear gearing. I've heard that this could be caused by a failure with an in-line resistor between the distributor and the electric tach - does anyone know what the value of this resistor should be and also where it's located? Thanks...

Chris

 

[sscam69]

I hear ya.

I am curious to. My tach is possesed!!:crazy :Silly I am idling at 800 rpm, I can tell because of the sound, but it reads 400rpm:confused. And sometimes it will jump all over the place. I could be idling and the tach will read 5K rpm. Maybe my car is just anxious to burn mustangs I dunno L

I saw a really nice aftermarket tach. for $129 with a shift light but at the moment don't have the funds for it

I shift at 6K when I race but it could be 7K for all I know.

How precise and accurate are these Stock tachs on these cars?

Is there any way on improving it?

What could cause the above symptoms?

Frank

 

[sscam69]

Not letting this one get away

TTT

 

[sscam69]

Does anybody have any suggestions or clues as to the culprit of these kinds of problems.

Frank

 

[klb76]

Although I can't provide a definitive answer, I am having the same problems on my 76... from what I have gathered is alot of times the circuit board for the tachometer gets burnt and causing wierd readings... but until i get around to looking I can't really help... the way you would be able to tell is a brown spot on the green circuit board in there...

klb

 

[page62]

You most likely need a replacement tachometer filter, for which an OEM replacement is no longer available. But here's something cheaper and better:

http://www.rodneydickman.com/retail.html

Choose the Electrical tab, and click on Tachometer Filter. Cheap!!!

 

[vetteboy79]

Will work with V8?

That looks pretty good, but will it work with a V8? All that are mentioned are the 4- and 6-cylinder Fieros...I don't know if the resistor/capacitor/choke coil (whatever's in the package) had a different value for an 8-cylinder. But I'm pretty sure that my vette is missing the filter completely because I can't find it anywhere in the engine compartment. Thanks for the help!

Chris

 

[page62]

I wouldn't worry. Filters is filters...

 

[vetteboy79]

Thanks!

I'll try and take a look at it this weekend - I'll let you know how it turns out!

Chris

 

[widewhites]

Keep us posted, I am having the same problem, my tack does the same, driving 50 MPH I am at 3000, I step on it and I hit 5000 rpms at 70.

On a side note: This started to happen when I opened up my spark plug gap, i was running .35 with HEI and opened it up to .45 (recommended), retimed it, but noticed my gas mileage was going down the drain, and still was lacking that uumph, removed the air filter, let her breath and wwhhoaaa. But then I noticed this RPM difference. I understand the change in vacum pressure becasue of the gap change, but that much change in RPM?

 

[ccflorida]

Chris,
If you have the original distributor look on the top and it will say Tach and the filter plugs into the side of the distributor. Also the filter should be mounted on the intake manifold right by the distributor. Hope this helps.

 

[Vette 76]

Tach problem

I have just recently rebuilt two tachs, one for a 76, the other for a 72. Be very careful if you ever put a battery charger or boost your battery. A friend arced the terminals on the 76 and fried the tach circuit board. You can run just fine w/o the small condenser next to the distributer though it isn't a great idea . One problem to check is the tach needle needing to be rezeroed. If you need help with this or need to get other instructions for your tach contact J. Gardner 1-800-484-7597 ext.code 4048. I bought my circuit board from them and they sent a troubleshooting guide (75-77 vette) that was really helpful. I bought the board from one the major vette suppliers (can't remeber as I use several) and they subed it to these people. Good Luck, Vette 76

 

[4spd350]

re: Tach problem

I had the same problem with my 80 vette. The tach was working real erattic between 1500-4500. It as all over the place. Sometimes bounce up to 6000 at times. If you remove your tachometer you will see that there is a printed circuit board on the back of it. If it has a burnt spot on it thats your problem. I installed a new circuit card on mine and it works great. Idle to max rpm's. It is smooth, reponsive and very accurate. It's nice to see it working properly.Mid America sells the tach rebuild kit for $100.00. If you go to www.madvet.com, go to the technical tab, then instructions, put in the part # 602781 and that will give you the instructions. that # now is for an 80 model, the 79 is different. The install was an easy 20 minute operation with a very satisfactory result. Check it out, most likely the problem.

 

[GerryLP]

Vetteboy,

I, like others, had this problem, and I had suspected the signal filter; however, I could not find the filter anywhere. I decided to replace the tach board (P/N 393128 for a 79 Vette), and now it works fine. At the time I had paid $69.00 dollars for the kit, but now lists for $61.00 dollars at Corvette Central.

I am still using the same tach filter I had -the problem was the circuit board