Tachometer Filter For HEI ?

[54blackhornet]

I installed a HEI distributor in my 71 C/20 with standard gauge package. Trying to set idle RPM , proper advance but get no signal from the Tach connection on the distributor + Connection at distributor, - black to battery ground, Nothing . So I tried another tachometer thinking first might be broken, still nothing. After some research I read you need a tachometer filter to get the proper signal. Anyone confirm this is correct ?

[Southcity]

May need more info to assist you:

1.What kind of HEI?
2.Stock tach gauge or aftermarket repro?

My understanding is that a filter is not needed. The best way to test is to first ensure you have a good working tachometer, you can mock this up in the engine bay first for testing. Green wire goes to the "tach" hookup on HEI. Keep us posted.

[54blackhornet]

The two different tachometers are diagnostic types to use underneath the hood. Only two leads + red, - black. I connected the red to the HEI connection marked tach. Black directly to battery negative post. I really only need the tachometer to get everything dialed in properly after the HEI install and carburetor rebuild. Do I need to get a late model tachometer with more compatible with the HEI. The two I have used are very old, one from Sears, one from Wards…thanks for your help..Jack

[Southcity]

Thanks for clarifying. When a tachometer is connected to the tach connector on an HEI, it's measuring the signal frequency (on/off signal) which then converts it to a voltage to drive the tachometer. It may be that these older style diagnostic tachs are incompatible with HEI, so I understand why you were asking about a filter. Filters generally help "smooth" the signal which may then make these older tachs you have work. I guess you have some options:

1.Buy a used tach that will work with HEI
2.Purchase a filter that should make the tachs you have work (Amazon actually carries some)
3.Permanently install the HEI compatible tach of your choice

Hope this is helpful

[54blackhornet]

Thanks for your input. I guess I will just buy a tachometer compatible with the HEI and install it in the cab. Was hoping to use one of the ones I have underneath the hood while tunneling up the little old 307…

[Southcity]

Someone may still chime in with a way to do this. Posting a pic of the two tachs you have (front/back) might be helpful.

[chevmn]

Not sure about aftermarket tachs, but I have connected a few with factory tachs to mid 70's factory HEI, and no issue, and tachs work fine. I know some have reported they used a filter, but others say no issues.

[Fbks_Cruiser]

Many digital multimeters have a tach function. That's what I use for diagnostics. Red lead of multimeter to the tach terminal on the HEI, black lead to ground. Works great. (besides, a decent multimeter can be used for so much more! ).

[54blackhornet]

I have a multimeter but I don’t believe it has a tachometer setting ?

[Arnulfo]

You need to connect to the NEGATIVE side of the coil with your tach lead not positive. Thats how mine works with hei and points.

[54blackhornet]

The distributor (HEI) has a terminal marked Tach ! HEI’s only work with three wire tachometers, the forth wire is for illumination. The tachometers I have are two wire designed for points type distributor.

[RichardJ]

https://67-72chevytrucks.com/vboard/...ch#post5302734

[tdangle]

I keep seeing this pop up and to be honest don't understand why so many have issues. I've had 3 different HEI units hooked to my repo tach with no issues. No filter installed either. Maybe there is something I'm not aware of or just lucky.

[54blackhornet]

Hi Terry the issue I have is the two tachometers I have tried only have two wire connections. Black - and Red +. These are diagnostic meter for tune up purposes. For a tachometer to work with a HEI you must have three wires - negative, + to 12 Volts and the third wire to the HEI, forth is for illumination. I confirmed this with Sun Pro technician.

[roboticjunky]

The 2 old tachometers could be current style tachometers. Just like in the diagrams on Holley's page.

https://www.holley.com/blog/post/tachometer_operation/

This is a pic from that webpage.

[Palf70Step]

My 70 has factory tach, but can't trust it. Never know what it will read and how close it is to real RPMs. My friend said it worked great till he went HEI. Tried all the tricks to get it right, but it just never was right. Since the ammeter and the tach don't help at all, I got new aftermarket gauges, to include a tach, that I am going to build a new dash with and get all my engine monitoring back on line.

[PbFut]

Originally Posted by tdangle

I keep seeing this pop up and to be honest don't understand why so many have issues. I've had 3 different HEI units hooked to my repo tach with no issues. No filter installed either. Maybe there is something I'm not aware of or just lucky.

Is it possible that the resister wire is being used by those with issues with their factory tach? Or possibly visa-versa. I saw a post warning about the factory yellow lead most use for HEI power was a resistor wire and can burn out a HEI unit over time. In fact previous owner of my truck hooked on the yellow lead and I read 9.5 volts at the distributor even those the engine ran quite well. A rewire as recommended fixed the voltage issue. I do not have a tach so can't answer my own question

[tdangle]

Originally Posted by PbFut

Is it possible that the resister wire is being used by those with issues with their factory tach? Or possibly visa-versa. I saw a post warning about the factory yellow lead most use for HEI power was a resistor wire and can burn out a HEI unit over time. In fact previous owner of my truck hooked on the yellow lead and I read 9.5 volts at the distributor even those the engine ran quite well. A rewire as recommended fixed the voltage issue. I do not have a tach so can't answer my own question

its possible. I rewired mine replacing the resistor wire and removed the wire from the starter solenoid to the coil.

[ricurt69]

I purchased a "stock type" tach from Classic Chev,. I have a Accel HEI distributor, Don't want to damage the new tach, will add a fuse. I wonder if you installed one of those replacement HEI compatabe tach circit boards I see advertised would that be the answer? Can not seam to get a straight or any answer from any sourse, except from Dynacorn who just told me that their repo tach's would not work with HEI!

[body bolt]

My factory tach and GM HEI always worked fine with the pink wire. I suppose it depends on the module in the HEI.

[ricurt69]

Its a Accel 52000 street billet dist, has no "tach" connection.