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post #1 of (permalink) Old 05-18-2001, 02:00 PM
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Stock Tach and TBI Revisited


was searching past archives and noticed a thread on Stock Tach and TBI and made me start thinking about how i wired my HEI and Howell TBI kit. so here's the scenerio -

i installed the HEI before the Howell and and my stock tach was actually working perfectly (didn't think much about why at that time). i was drawing power to the HEI power directly from the 13F red wire. according to what JAFFER said regarding the 13F red wire being connected to the 13C red wire into the tach, i think the stock tach was still reading the Power pulses from the 13F/13C wire when the HEI fires the sparkplugs hence allowing the tach to work just fine.

now since i installed the Howell kit, my tach has been an order of magnitude off in the RPM readings. tach needle movement still correlates with accel/deceleration but i don't "idle at 1200 RPM and cruise at 3200 RPM". only diff in wiring is that i'm also drawing 12V power for the TBI Computer ECU from the #13F red wire along with the HEI.

the question: does the ECU also "pulse" the #13F wire (maybe pulse correlates with injectors firing?) or does it draw Power at a constant rate? i'm wondering if connecting the ECU to #13F wire is causing the "extra" RPM readings i'm seeing on my factory tach and if i wired the ECU to some other 12V ignition triggered power source, i woudl revert to pre-Howell install scenerio where the tach only "sees" the Power pulses from the HEI firing the plugs...

btw, if you do a search on "factory tach and TBI problems" as subject, you'll see the bbs thread i'm referring to...


83 CJ7 258/T5
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post #2 of (permalink) Old 05-18-2001, 11:23 PM
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Re: Stock Tach and TBI Revisited

I haven't done TBI myself, but it sounds to me like your ECU should not be hooked into your tach wiring. The pulsing from the distributor firing the cylinders tells the tach how fast the engine is running (rpm), but your TBI unit is independent from the distributor. The ECU should either be on an ignition 12v power feed, or perhaps an always-on 12v power, check your Howell documentation.

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post #3 of (permalink) Old 05-19-2001, 11:22 AM
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Re: Stock Tach and TBI Revisited

hi mark,

the ECU is not hooked up to the TACH wire from the HEI. it's hooked up to the #13F 12V power wire that's hot when ignition's on. the stock 258 TACH is the type that reads pulses in current which is why i still see a TACH signal - it's meant to be wired inline with the ignition 12V power source to your distributor.

just wanted to know if anyone out there knows if the GM TBI ECU "pulses" the current from the wire it's drawing power from or if it draws power at a constant rate? cause with the Howell install, i wired spliced the HEI 12V power wire to the ECU as well hence if there are any other "pulses", my TACH would probably be detecting it.

for more background on what i'm talking about, refer to the old thread that was mentioned in my last post.



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post #4 of (permalink) Old 05-19-2001, 11:58 PM
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Re: Stock Tach and TBI Revisited

I recall that the ignition modulated early '80's 6cyl tachs work on the 4X pluse scheme.
... something like ... 4 X 3 = 12 pluses divided by 2 (intake and exhaust stokes?) = 6

... and that is supposed to be why you need to set an after market tach hooked up to an MPI on an I-6 motor to the 4 cylinder mode instead of 6 cyls ...
Thus subject makes my head hurt ...

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