[Kiejo]

I purchased a 5K tachometer (and console gages) from Classic Industries for my '67 RS 327. I replaced the under dash and engine harness and clipped the appropriate connector to the distributer. The tach worked fine for a few weeks. Now when I rev the engine it is smooth until about 3500 rpm where the needle seems to waiver and can even decrease a bit. I don't have an factory original distributer. It is not an MSD type but I'm not sure what kind it is other than the cap is red. The tach wire connection at the distributer it is tight and the engine raps out very easily without loss of power. Any ideas what would cause this?

 

[77wolf10.85]

Check your 12 volt supply voltage. But first...
Check your ground.
Sounds like a bad ground. People don't like to hear that. So I now say it like this....
Run a new ground wire and see if it behaves.
Later
Tim

 

[Kiejo]

I called Classic Industries on this. The tech said that since I don't have a factory points type ignition (I think I have an HEI system) I need a tach adaptor to adjust for differences in output voltage and waveform. Before I spend the money I wonder if other forum members think this sounds correct.

Here's what's odd. The Classic Industry's web site says the tach I bought is "HEI tested!" and the catalog says "Not for use with HEI systems."

 

[JimM]

Definitely double check your wiring, make sure all the connections are tight and the dash carrier has a good ground.
Is anything else on the dash wierd? High beam light? turn signal indicators?

Check your system voltage. Should be stable at 13.5 to 14.5 volts at any rpm. A little lower at idle is ok, higher than 14.5 is not ok. Tachs are sensitive to voltage.

If all that checks out, build one of these for five bucks and keep your fingers crossed. tach filter

 

[dnult]

I second Jim on this, check the simple stuff first, and then build the tach adapter if you're up to it. They are very simple and inexpensive.

Tachometer circuits (pariticularly the OEM type) are pretty finiky about the signal they see from the ignition system. Most likely, the signal from your igniton is generating a higher voltage with more oscillations on the signal than the original breaker point ignition presented. Diodes in the input circuit can break down and cause irratic readings. The filter should fix you up.

I'd disconnect the tach if you intend to drive the car before you get around to checking the power / ground and experimenting with the filter. Prolonged exposure to voltages high enough to break down the input diode can do damage to the tach circuit.

 

[JimM]

just fyi... my new (several years ago) OER tic toc tach worked fine with an Accel Distributor, did not work well at all when that died and I temporarily put the points distributor back in (without a balast).

When I installed the permanent MSD 8361, I built the filter in the link and it works fine.

Again tho, particularly since it worked initially for a good period of time, you need to check out the wiring and electrical system.

 

[Kiejo]

The problem I had with my tach floating around 3500 - 4000 RPM seems to have disappeared. It now operates smoothly up to redline. The only change I made was to give the battery terminals and clamps a good scrubbing with a new terminal cleaning brush and reseat the tach wire clip on the side of the distributer. In the mean time I talked to the Classic Industries' tech department and they recommended a tach adapter. The tech pointed out that their catalog says, "Not for use with HEI system." He was pretty confused when I told him that their online catalog says the opposite, "HEI tested." (They changed the online catalog the next day.) I was nearly convinced there was a clash between my HEI ignition output and what the stock tach could read but since it decided to work again I can only conclude that the tach it is very sensitive to the quality of wiring connections as others have mentioned.

My distributer (red cap) has no points. Are all pointless ignitions HEI? How many others have original or after market tachs that work fine with HEI?

 

[JimM]

HEI (High energy Ignition) is a new distributor released by GM in 1975 or so. It was the general's first "across the board" electronic ignition. You can spot one by the huge distributor cap with spark plug type terminals and the coil sitting in the middle of it.

To some, HEI has become kinda a generic name for an electronic trigger ignition system.

There are LOTS of electronic ignitons beside the HEI, and many aftermarket manufacturers sell distributors based on the HEI design.