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post #1 of (permalink) Old 05-14-2001, 12:45 PM
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Stock Tachometer Wiring and HEI

hi,

my stock tachometer has never worked right since i installed my Howell CA kit and an HEI distributor. so i was looking at the wiring yesterday and noticed that there were 4 wires coming out of the gauge - ground, two reds, one orange to fuse box. whenever i disconnected either of the two reds, my engine wouldn't start; it would crank but no juice to the spark plugs i guess.

how the heck does the stock tach work?
do the 2 red wires loop into the tach and back to the power wire powering the HEI distrib, therefore causing the non start problem whenever i disconnect it?
will i cause a engine fire if i just splice the two ends of the 2 reds together if it is a loop?
how can i incorporate the one tach wire from the HEI into the stock tach if the tach expects two wires?

thanks

Jack


83 CJ7 258/T5
Howell/HEI/CenterforceI
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post #2 of (permalink) Old 05-14-2001, 01:25 PM
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Re: Stock Tachometer Wiring and HEI

I run a Howell with a GM HEI and had the same problem.. and overcame it by simply hooking the FOMOCO ignition moudlue back into the circuits and BINGO, the TAC magically works agian.. small price to pay.

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post #3 of (permalink) Old 05-14-2001, 01:34 PM
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Re: Stock Tachometer Wiring and HEI

There are two types of tach's.
There are ones that work by measuring voltage pulses at the coils (-) terminal, and then there are the ones that measure the current pulses flowing into the (+) terminal.
The 6 and 8 cyl tachs are the current pulse type.
Power from the battery flows through the ignition switch, through the tach, in one red wire and out the other, then it flows down to the (+) terminal of the coil, through the primary winding, out the (-) terminal to the module or points, and to ground.
That power only flows when the points are closed. This means that the current, which is a measurement of the flow of electrons, is pulsing every time the points (or module transistor) close and complete the circuit.
The tach uses these pulses to measure the RPM.
Now when you switch modules you might not be getting the same type of pulses. If the module does multi spark then it's producing several pulses for every one pulse the old module produced. If new module may also take current for its own internal circuits which the tach counts as a cyl firing. The width of the pulse (dwell) may also change which could account for the errors in the readings.
The new modules may also change how the coil gets its power. If the coil is getting power directly from the battery by passing the tach circuit then there are no current pulses to measure. If the module is filtering the power to coil there won't be any current pulses. If it's a capacitive discharge unit there won't be any pulses.

Some of the modules provide a simulated tach input but those are usually designed for the voltage type tachs which sence the (-) terminal voltage pulses.


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post #4 of (permalink) Old 05-14-2001, 08:11 PM
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Re: Stock Tachometer Wiring and HEI

hi,

so if the 258 tachs are current pulse types, does that mean it can't be adapted to be used with the (i assume) voltage pulse tach output from the HEI distributor?

and if the stock tach and the HEI won't work together and i need to install a new voltage pulse tach/remove the old stock tach, can i splice together the two red wires which go into the stock tach? the stock 258 tach doesn't reduce the current or anything such that if i splice them together, i'll fry my electrical system... then all i need to do, i assume, is pull the Tach wire from my HEI into the new voltage pulse [email protected]

thanks

Jack



83 CJ7 258/T5
Howell/HEI/CenterforceI
post #5 of (permalink) Old 05-14-2001, 08:48 PM
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Re: Stock Tachometer Wiring and HEI

I'm not a HEI expert, but if the HEI module has a tach output pin it is most likely for a voltage pulse type tach. Don't try and connect the 258 tach to that module pin, you'll cook something. You're virtually connecting the pin to the (+) battery terminal via the tach and ignition switch.

Yes you can remove the 258 tach and splice the two red wires together. Actually the connectors are made to just plug together. My Jeep originally didn't have a tach but already had the two connectors in the wiring harness. They were just plugged together instead of into the tach. Thats the way they came from the factory if they didn't have a tach, so you won't have any problems with current if you disconnect the tach and reconnect the wires.
Most aftermarket tach are the voltage pulse type. They use three wires instead of the two that the current type use, not counting the dash light wires. The three wires are +12V, Ground, and coil(-). Connect the +12V wire to the ignition power, Ground to Ground, and the coil(-) to the HEI tach output.

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