Big Daddy Spo

Need some help with getting the tach in my 64 dialed in. It is a factory console tach that seems to be in working order as it will read at idle. I am running an HEI distributor and have the brown wire from the tach connected to the tach side of the HEI. Like I said, it seems to show good reading at idle, compared to my hand held tach. Its' when I increase the RPM's it stalls out at 2000 RPM. Not sure if there is a calibration screw on these or a shunt. I would think that if it was calibration, I would at least get a full sweep of the dial. Has anyone else had this problem when running these thru an HEI?

Thanks,
Steve

oldcutlass

I have never had any issues hooking up tachs to an HEI. For years I always thought all tachs would work. However I have read some articles within the last few months where stock tachs may have issues. Research tach filters, corvette tach filter, etc... There is a schematic of what gm used for stock tachs in later model cars with HEI's.

joe_padavano

I've never needed to do anything special to hook a tach to an HEI. If the tach needle is sticking at 2000 RPM, the problem is likely the tach. Try hooking it up to another car and seeing it the same thing happens.

jRoofgunner

The only thing I can tell you is that with the Petronix coil and distributor Ignitor 2 FlameThrower (non-HEI) they tell you to install two diodes in line to the tach. (Mine is the original like yours). They address this on their website and I think I down loaded the instructions as to what value diodes and direction they go. This does work in my car and that tach is accurate. Otherwise consult you ignition maker. If you do what they say and your tach doesn't work then maybe it is the tach. If you need diodes PM me. I had to buy a pack of 10 or so and you only need two of them.

joe_padavano

Assuming that when the OP says "HEI", he really means a GM coil-in-cap HEI distributor, then the TACH terminal on the HEI is exactly the same signal as connecting the tach to the "-" side of a points coil. Nothing extra is needed.

64cutlasssurvivor

I was looking at the forum to get information about connecting a 64 console tach to an updated HEI distributor and came across your post of the schematic. Thanks, Oldcutlass, that is very helpful.

That schematic shows that the signal out of the new HEI needs to be "padded down" before it goes to the old tach. The drawing shows about 15,000 ohms of resistances in line with the signal. I'm thinking you could ruin your old tach if you try to run it directly out of the tach terminal on an HEI. Also, there are a couple of capacitors to remove noise or smooth out the signal to the old tach.

Glenn,
Senior Engineering Technician

joe_padavano

From another site:

"...even if you put a filter on that doesn't guaranty it will correct the problem. That is a bandaid so to speak. These filters are not all the same and certainly don't have the same dynamic electrical responses to whatever the modules are putting out. That's the problem....what the module is putting out! This is due to the "overseas" zealots trying to reverse engineer the module (most modules you find at Poop Boys or Auto Zomb which are usually cheap ...cheap...cheap and/or no-name HEI copycats) without real knowledge of the output wave use and/or caring really.

The real underlying problem is the output of the module from the HEI. The tach signal can be distorted either by peak size (causing "bouncing readings at idle and then smooths out around 3000rpms) or funky square wave of various sizes- signals that do all kinds of things to your tach readings, to very low fat retangle waves that don't do hardly anything. It's the modules design and output that is the problem.

Many of the inexpensive HEIs and/or replacement modules will cause these problems to happen, as well as are NOT designed for performance use period. In my experiments with several modules the cheapy and older type will severly reduce voltage output at around 4500rpms too! So, it's not only the distributor...it's the module and coil too that play a part in total performance and certainly tach compatibility.

Most "performance" HEIs have it all....great coil/module output to 6500 rpms and complete tach compatibility. Plus, if you have a performance coil, don't just put a standard module in...you can eat it quickly.

The modules that I know work are the Accel and Echlin ones. Yes, I know they are a tad expensive but when you wind that motor up to 5500 rpms like a song and power under foot, it's worth it.......and you can also read your tach!"

All I can say is that I've never needed any filters or add-ons with a stock HEI. I have no experience with aftermarket or MSD systems.

oldcutlass

It's a shame that people don't come back and state what cured their issues in threads they started.