While coming home from the Cruise-In, I had realized that my tach is giving me an intermittent problem. It will sometimes read high in the upper RPM's. At idle, it reads fine, but as I increase RPM's, the tach reads increasingly higher. Sometimes it's perfectly fine, other times it will be off. Also, I have a shift light that comes on at the proper RPM regardless of the tach. If my tach is off, the shift light is still accurate. Any ideas?

 

BTW, my vette is a '79

 

I had a problem with mine after I changed ignition systems on my 1980. It would read fine up until 2,000 - then it would start running backwards back down to 0 and then back up again.

I replaced the tach board, which could be your problem, but it did not help me. I bypassed the tach filter and my problem went away. The only issue I have now is a small fluctuation around 2200 - 2400 rpm. I don't have another tach filter to test with, so I am just going to leave it as is.

 

I looked at a '79 yesterday where the tach read at the end of the range - the needle was all the way to the right. What's up with that?

 

I have a capacitor with the numbers 913 on the can that is connected to my HEI distributor on my 75. It has a small welded on bracket and is bolted to the intake manifold. It has GM connectors but they are attached to the capacitor wires with aftermarket crimp connectors.

I have checked in my 1975 Chevrolet Service and Overhaul Manual Supplement and the Corvette wiring diagram does not show a capacitor in the distributor wiring. (Of course, this is my car that had a capacitor wired across the brake light switch under the dash.) Any ideas if this is correct or not?

 

What you describe may very well be the tach filter. It looks just like a metal can cap but actually has this circuit inside:




You can test it by disconnecting the wires and putting an ohmmeter across the end terminals. If it reads in the neighborhood of 15-16K ohms, +/- 20%, then what you are describing is not a cap but in fact the tach filter.

Hope this helps. Good Luck!

 

I put an ohmmeter across the two wires and the meter jumps around from 70, 35, and settles down from 0.6 to 0.1 ohms.

Now what? Doesn't look good to me.

 

In re-reading my previous post, pehaps I wasn't clear. You need to put the ohmmeter on each of the posts/terminals on the metal can/filter, not the wires themselves.

Based upon the circuit above, the only thing that's supposed to connect the "in" and "out" terminals is a 5.6K and a 10K resistor in series. So you should get approx 15-16K if everything is OK. If not, then perhaps the filter is damaged and needs replacement.

Of course MAYBE someone previous to your ownership did replace the filter with a cap???? Bubba has done stranger things!

Good Luck!

 

I am still a bit confused. I found the filter in my 1975 AIM book (UPC 12J page #8). The drawing shows a part very similar to mine (i.e. a small cylinder with a welded bracket that bolts to a boss on the intake manifold.) The drawing shows a wire coming out of each end connecting to the distributor and the engine wiring harness respectively. My filter has two wires coming out of one end covered with a plastic boot. There are no posts on the filter.

Also, my GM/Packard type connectors are connected to the filter wires with aftermarket crimp tubes (metal cylinder with a plastic cover). I have never seen GM use those tubes in production. (Maybe a dealer service installation or the part replaced under warranty?) My 75 is a late June early July 75 build so the part should have been installed in production at St. Louis.

I disconnected the wires from the distributor and from the wiring harness before I touched both connectors with my ohmmeter probes and got my readings.

 

OK, forgive my memory. I checked my AIM and yep, that's the filter. My vette is temp parked in a garage not at my house as I broke my foot a few weeks ago and can't drive it for at least a few more. The previous owner of mine had installed a couple aftermarket filter caps in an attempt to eliminate the alternator whine in the radio and these have the "stud" at each end. Must have confused this in my mind as I have no visual reference at this moment. And I agree, I don't think anything on the car came with barrel crimp type connectors from GM.

The physical configuration of the filter doesn't matter. If you are connecting the ohmmeter across the "in" and "out" wires with nothing else connected to it, then you should get 15-16K ohms resistance based upon the circuit diagram. If not then I'd have to speculate the filter is bad.

I could send you a replacement "home built" unit for you to try if you'd like. It doesn't look like the original but the circuit is the same. The components are on a circuit board with heatshrink around it and three wires coming off one end. Two are the "in" and "out" and the third is a ground wire. I had built a couple up last winter as I had the parts lying around and needed one for my car. As they only took a couple minutes to build and I knew someone would need one sooner or later, I threw a couple extra together just because.

If you want me to send you one, please IM me with your shipping address and I'll put one in the mail asap.

Hope this helps. Good Luck!

 

Jim,

You have IM.

 

Anybody have any ideas as to why my tach is reading normal at low RPM's but high as the RPM's increase? Someone mentioned that it could be the tach board, any confirmations from others? This is an intermittent problem. Sometimes it works fine, sometimes it reads high.