HEI dizzy

[Darrell Norquay]

Dan wrote:

>      Hopefully there are a few out there that know about the early Chev HEI 
>      distributers with the built in coil.  Basically I am after some sort 
>      of an idea what the tacho output waveform should look like, since I'm 
>      not sure if the module in my dizzy is OK.  It is a 4-pin module.  The 
>      waveform I get goes to about 45V for the 1st 1/3 of the pulse, it then 
>      drops to about 15V for the remaining of the pulse.
>      Obviously the pulse is not quite as square as above, but I am 
>      wondering if this seems correct, because it seems to drive the tacho 
>      in the computer crazy and the car seems to idle roughly while 
>      misfiring above 1500rpm.

I have a 78 Camaro with HEI (for now).  All the Tach output drives is the
tach in the cockpit, and I've never scoped it.  It does, however, have a large
feedthrough capacitor (looks a lot like the old points ignition cap, but the 
wire goes straight through) in the line just before the wire goes thru the 
firewall.  You could start by ensuring that this cap is present, and mounted
close to your computer on the engine side, and that the mounting tab is well 
grounded.

You didn't mention what kind of computer you have, but I get the impression
you just installed it, since early HEI's weren't made to tie into computers.  
In any case, most computers expect a 5 volt square wave (well, nearly) input.
No wonder the thing's going nuts if you're dumping 45 volt pulses into it.
Check the dox on your computer, if it has any, and see if they say what the
tach input want's for voltage.  

If it's 5V, as I suspect, you'll need a resistor, cap, and zener diode to
connect in the lead between the tach output and the computer.

                         10000 Ohms
       from HEI    >------/\/\/------o---------o----------------> to CPU
                         1/2 Watt    |         |
                                     |         |
                                    ___/     _____
                         5V Zener  // \      _____  .001 uF Cap
                         1 W       /___\       |    100V
                                     |         |
                                     O----------
                                     |
                                   -----
                                    ---
                                     -

This circuit will convert your input into a relatively clean square wave at
a voltage which should be compatible with the computer.  Solder the 
components together, insulating them from each other, and attach 3 wire 
leads (in, out, and ground).  Heatshrink the whole thing together and wire
it in series with the tach lead close to the computer, making sure it has 
a good ground.  If the input requires some other voltage than 5V, simply use 
the appropriate value zener.

It's also quite possible that your module is bad.  Happens fairly often.
Hopefully, you haven't toasted anything in your computer.  You should be 
able to get a new 4 pin for around $20-$30.

>      Also what is a good size spark plug gap to use with these, I have read 
>      anything from 0.028" - 0.075", I'm using a 0.04" gap with a 350Chev.

Gap em at .045" - .050".  Works for me.

regards
dn



--

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 Darrell A. Norquay              Internet: dn at dlogtech.cuc.ab.ca     
 Datalog Technology Inc.         Voice: (403) 243-2220
 Calgary, Alberta, Canada        Fax:   (403) 243-2872            
 
       "Absolutum Obsoletum" - If it works, it's obsolete    
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