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Old 07-19-2006, 06:13 AM
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Distributor differences

Hey Guys,
What is the difference between a GM HEI distributor and a breakerless dizzy with a hot coil? I have a 71000 series Accel breakerless distributor and an Accel 140001 Super Coil.
Thanks.

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Old 07-20-2006, 05:51 PM
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Doc here,

There are many answers to that question..depending on mechanical and electronic curving Ect..Speed of the electronics, limiters etc..

The Basic story is they operate in the same principal..exceptions being, no in cap coil .. (smaller body) , I believe the lobe pickup is optical, and the module is externally mounted..where on the stock HEI, its a magnetic pickup, module inside the base of the dizzy, and Coil mounted in the cap..(more heat generated in the coil and Dizzy internals)..

Was there something specific you needed to know?


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Old 07-20-2006, 06:53 PM
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Thanks for the reply. I have an odd application. SBC in a Datsun 240Z (definitely not a ricer, although it is a Japanese body, read sleeper). All the information that I have is geared towards using a Nissan 3-wire tachometer from a 280z, which had a transistorized ignition and connecting it to a GM HEI distributor. The trouble is, I do not have an HEI Dist or the transistorized ignition. Of course, the original tach is toast, and all its requisite wiring is MIA.
I currently have an old Autometer tach that works fine, but I'd prefer to mount an original looking one in the dash if possible. As far as I know, most normal tachs are basically a frequency counter and work off changes in freq., but is it possible that the 3 wire tach works off pulse width modulation as opposed to freq?
Also, what type of a signal does the HEI tach output produce?
I'm sorry to be running on, but I'm pretty sure this can be done, but I lack some of the expertise to put all the pieces together.
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Old 07-20-2006, 08:10 PM
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Doc here,

The easiest way to convert and retain the stock head..

Remove the stock tack and open the back up. Next find the calibration card (the electronics) And remove them carefully from the head unit. So that ONLY the meter and it's red and Black wires remain.

NOW Do the same for your aftermarket Tach, Preserving the electronics..See on both Tach's if the METER movement has a stamp "0 to XX Ma" They should be the same , if not close..

Now take your electronics board to Rat Shack, and get a small plastic project box that will house the board, and stand off's..some 24 gauge hook up wire, Grommets for in / out holes on the wires and some shrink tubing.

Locate where your RED / Black wires go on the new board, and add enough wire to allow hookup to the head from the project box, solder those and heat shrink them on the meter leads..

Out of the other side of the project box , your power, ground and coil pickup will exit..If needed , extend these wires to get them to where they need to go.

Close up and reinstall the Tach Remote head, and mount the project box anywhere that is handy, and hook the power, ground and pick up where they need to be..Get a hand tach, and hook it up, then fire it up and check it for operation, then for calibration..If it is slightly off, you can try and calibrate by adding low value resistors across the meter movement OR a potentiometer , about 0 to 500 ohms..

That should get you an operable tach, with your stock head unit, and nobody will know but you..

You are correct, The Tach circuit is a Freq/Time referenced input, through a Divide by N chip output, the input is shaped and buffered to a usable square wave, and provides an (almost) Analog output to drive the head unit (gauge movement).

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Old 07-21-2006, 08:44 AM
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Nice idea, I'm off to the lab...I'll let you know how this works out..... Thanks for your help!
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Old 07-28-2006, 08:51 PM
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Well Doc, I did my best so far with the tach conversion. First, I tried to find out what drives the movement in the original Nissan tach and found that the meter moves in approximately 250 mV / 1000 rpm increments, so that 4 K rpm is approx. 1 VDC.

Next, I gutted the Autometer tach and attached the output leads, with the movement still attached, to the oscilloscope. The output waveform of the newer meter's electronics indicated that the the input waveform is directly in synch with it. However, the output wave appears to be a synthetic DC pulse with a long positive cycle and a very short off cycle. The amplitude is a constant 6 VDC, as opposed to the input wave's 12V.

The output wave looks like it was caused by an inverting buffer circuit, which would pull up the negative half of the square input wave, so that the output wave appears to have the same frequency as the input, but half the amplitude. So with a 100 Hz square wave input@ 12v, the output is still 100Hz, but the voltage is 6v on a square wave of double the positive duration and very short negative duration.

I guess my question this time is if the newer meter's movement is voltage driven, how come the amplitude of the output stays constant. Is this a pulse-width modulation circuit, as opposed to a variable-voltage circuit?

I hope this is comprehensible to you, if not I can send you the scope's output if you wish. I am also in the process of breadboarding up a new tach driver circuit using a tried and true frequency-to-voltage converter chip,which will probably work, but I'm very curious as to why these two units are so seemingly dissimilar.

I'm still a relative newbie to electronics, so any input will be greatly appreciated.
Thanks for your time.
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Old 07-28-2006, 09:11 PM
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Doc here,

The pulse width will look like an analog voltage to an analog meter..ergo, shorter the on / longer the off, will vary the movement (voltage) Your Tach signal is produced in the HEI module..is why the indication of a "Pulse".

On the Square-wave, were you referencing the IRE screen units to 0, or peak to peak? ...0 (centerline) to pos peak is 6 volts.. 0 (centerline) to neg peak is 6 volts...neg peak to pos peak is 12 volts..

Have you yet tried driving the old head? It should be close..

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