HEI Distributor Tach issues, Issue has been resolved

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68 Coronet RT

68 Coronet RT

68 GTS # 508 in Registry
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Just wondering if anyone has installed an HEI distributor, if so what did you do to correct the tach from bouncing all over the place. I bought a filter wired it in as per instructions and it didn't work. Looking for some input.
Thanks
 
Should not cause an issue. Breaker points, HEI, the Mopar OEM ECU, and the so called "ready to run" Chinese dist's using the whatever it is Auzzie module are all negative coil switched ignitions.

WHAT tach what coil and how do you have it wired? EXACTLY. Grounding? Power?

In other words, "no information." (Not enough)
 
67Dart273 said:
Should not cause an issue. Breaker points, HEI, the Mopar OEM ECU, and the so called "ready to run" Chinese dist's using the whatever it is Auzzie module are all negative coil switched ignitions.

WHAT tach what coil and how do you have it wired? EXACTLY. Grounding? Power?

In other words, "no information." (Not enough)
Click to expand...
The Tach is a Sun Super 2 Had a Mopar Performance Electronic distributor, went with a 440 Source HEI street distributor. The car performs so much better except the tach bounces all over. I bought a filter, filter has 3 wires, Red to negative side of coil black ground and yellow is to tach filter post. I believe that the yellow should be tied into the green tach wire some where below where I tied the red wire into the green tach wire to the negative side of the coil. I'm going to try to wire the red filter wire directly to the negative side of the coil to see if that makes a difference. Working on a honey do list at the moment.
 
You lost me on the filter wiring. I have no idea what filter this is, or how it is supposed to be connected. Post a link or brand/ model/ part number?

Don't discount the possibility that the old Sun has something wrong
 
Yes, the original GM style "big cap" distributors ALL had a filter from GM so the tach would operate correctly. I had to get one for my F250 to straighten it out. It worked fine. What distributor do you have and what filter did you use? The GM number for the filter is 14020025. They all had it if they had the big cap HEI. Be advised, NO OTHER distributor is HEI unless it is the big cap unit and uses the GM module. They are only imitations using the HEI name, so that could be an issue.
 
This is the bonafide GM part.
TACH FILTER.jpg
 
Some points tachs require a filter when used with HEI. This one works. R7 needs to be changed to 1.8k ohm for HEI. I get an old plastic medicine bottle & put the components inside, with 4 wires exiting. I pump the bottle full of neutral cure silicon [ it is not conductive ]. Bottle is tie wrapped to harness under the dash.

img292.jpg
 
I just wanted to weigh on this with a couple of recommendations for anyone building circuit, from my 40yrs in electrical design.

  1. Most RTV's will corrode copper so if using RTV places like Newark electronics would be where to get the DOW Corning RTV for electronic circuit boards.
  2. A better choice is to use a Automotive grade epoxy to encapsulate parts, This common practice for US Navy, Even Automakers. Aerosol spray on epoxy 2-3 heavy coats.
  3. Newark Electronics sells component, project boxes and circuit boards recommend Automotive grade components they better thermally and handle high temperatures found in a Automotive enviroment.
That's just my 2 cents to the discussion. -Speedy Pedi
 
The directions were very vague, the filter I bought is a Classics Instruments # CIN-SN20. The instruction on how to install it was printed on 2.5 X 2.5 piece of paper and hardly readable. It was just trial and error but now it works perfectly. The key was running the red wire from the filter directly to the negative side of the coil and the yellow signal wire from the filter to the green tach wire and black to ground. I didn't like the idea of running the filter under the hood with all the heat so I mounted it under the dash. The filter supposably has a 2% acracy
 
I have used neutral cure silicon numerous times as described in post #7. It has no chemical in it that causes corrosion or conductivity.
And I just buy the cheap stuff from the hardware store because nothing special is needed for this job.
 
68 Coronet RT said:
The directions were very vague, the filter I bought is a Classics Instruments # CIN-SN20. The instruction on how to install it was printed on 2.5 X 2.5 piece of paper and hardly readable. It was just trial and error but now it works perfectly. The key was running the red wire from the filter directly to the negative side of the coil and the yellow signal wire from the filter to the green tach wire and black to ground. I didn't like the idea of running the filter under the hood with all the heat so I mounted it under the dash. The filter supposably has a 2% acracy
Click to expand...
I just zip tied mine around the low side rubber AC hose. lol
 
I thought the extra capacitor in an HEI setup was a radio supression cap
stops your AM radio buzzing.

i.e its not used as a condenser like with points

presume it will take out a small part of the spike cuased by switch coil off which may or may not avoid overload of the timing circuitry in a basic tacho.
I belive there is some benefit in using one. although i don't on mine.....too much clutter around my coil already

I had the Tach problem, and overcame it mostly with a clamp, a Zener diode between coil - and earth. it probably achieves much the same impact

i have some idea how it works but not enough to fully explain
it limits a volatge spike from the coil primary while its creating a spark via the secondary, to 400volts, at one of the transistor junctions in the module
and of couse you tacho has to deal with this back emf as well.

basically HEI changed a bit over the years when it first came out it had the capcitor for radio noise superssion, and the diode for protection, The modules could handle 350V back emf on the coil primary side, so those 2 components had multiple jobs one of which was keeping the module alive, the other making AM radio and CB usage possible.

these days the modules are good for 500V and nobody uses AM radio so the diode and the capacitor are not necessarliy used when retro fitting this stuff to another car

i use a diode and my tacho works 99% of the time, previously it was 50% of the time.

presume i don't spike my tacho with some horrible blast of EMF when i switch on now, and i did before.

VS-40HFR40 Diode

VS-40HFR40 Diode for sale | eBay

potentially total overkill but i like robust....

Bolted into the body or bracket with a wire from it to the coil negative may help. IT ALSO MAY NOT... but its a feature of the factory HEI set up used on 6 and V12 jaguars from the 80s and 90s... they persisted with the origianl specification of capacitor and diode well after GM stopped...lazy design or sensible move i don't know but the jag used a small head distributor in a proper OEM set up..and that small dizzy cap set up may well have dicated 2 components that act as a limiter at a level obove what is needed to make a very good spark.

I'd suggest either capacitor or diode or both will provide some benefit if your tacho is basic hook up to coil negative tacho, with 50 year old capacitors in it, surely part of my problem.

350V one may be better but finding one is hard it needs to be an R diode so that the correct end of it has the threaded bolt like connector for its body/earth mounting

Dave
 
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There are two types of tach designs used by OEMs. One design connects to the coil + terminal [ mainly on Fords I believe ]. The far more common design connects to the coil - ve terminal.

The filter cct below worked with my GM OEM tach. Should work on Mopar tachs. If using a GM HEI module, change R7 to 1.8kohm.

img292.jpg
 
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