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  My tach caused the motor to run bad!!

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Author Topic:   My tach caused the motor to run bad!!
65_289
Gearhead

Posts: 761
From:
Registered: Jul 2001

posted 07-29-2001 09:19 PM     Click Here to See the Profile for 65_289   Click Here to Email 65_289     Edit/Delete Message   Reply w/Quote
Don't you love newbies with many posts in a row?

My motor had been "missing" around 4k rpm. We thought it was the old ignition, so we put in a Pertronix unit & coil.

As it turns out, my dad unhooked the green tach wire from the coil and the car ran fine. The green tach wire was hooked to the negative part of the coil, BTW.

So now what could be causing this? I need to get the tach working before I hit the drag strip!!

------------------
1965 Coupe, Tremec 3550, Steeda Tri-Ax, Cable Clutch conversion, 3.25 gears, Ford XB3 longblock, x303 heads, B-Cam, 9:1 comp., Ford Racing high-torque starter, Pertronix Ignitor & Flamethrower coil, Accel 8.8mm wires, 600 cfm Edelbrock carb, Edelbrock RPM intake, K&N air, Holley electric fuel pump & AFPR, dual 2-chamber Flowmasters, KYB shocks, 5-leaf springs, Chrome Export Brace, Griffen Radiator, Comp. Eng. subs.

[This message has been edited by 65_289 (edited 07-29-2001).]

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steve'66
Gearhead

Posts: 8796
From: Sonoma,CA,USA
Registered: Mar 2000

posted 07-29-2001 09:41 PM     Click Here to See the Profile for steve'66   Click Here to Email steve'66     Edit/Delete Message   Reply w/Quote
Make sure the green wire isn't shorting against metal on its way to the tach. If it's ok, then it's an internal problem with the tach. I've seen it happen a few times. Replace the tach.

SteveW

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65_289
Gearhead

Posts: 761
From:
Registered: Jul 2001

posted 07-29-2001 09:43 PM     Click Here to See the Profile for 65_289   Click Here to Email 65_289     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by steve'66:
Make sure the green wire isn't shorting against metal on its way to the tach. If it's ok, then it's an internal problem with the tach. I've seen it happen a few times. Replace the tach.
SteveW

I sure hope the tach ain't bad. I just paid $140 for it a few months ago.

------------------
1965 Coupe, Tremec 3550, Steeda Tri-Ax, Cable Clutch conversion, 3.25 gears, Ford XB3 longblock, x303 heads, B-Cam, 9:1 comp., Ford Racing high-torque starter, Pertronix Ignitor & Flamethrower coil, Accel 8.8mm wires, 600 cfm Edelbrock carb, Edelbrock RPM intake, K&N air, Holley electric fuel pump & AFPR, dual 2-chamber Flowmasters, KYB shocks, 5-leaf springs, Chrome Export Brace, Griffen Radiator, Comp. Eng. subs.

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mvierow
Gearhead

Posts: 209
From: Bay Area, Ca
Registered: Apr 2001

posted 07-31-2001 07:52 PM     Click Here to See the Profile for mvierow   Click Here to Email mvierow     Edit/Delete Message   Reply w/Quote
Very similar thing happened to a friends 89 GT. His Autometer sportcomp was causing some kind of electrical failure that would cause his entire car to quit, almost randomly. Ofcourse it was probably caused by some kind of interference with the computer. Just goes to show you that these things should never be trusted when determining what may be causing a problem. He ended up replacing his tach and the problems went away.

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Jeff
Gearhead

Posts: 306
From: Moore, OK USA
Registered: Mar 2001

posted 08-11-2001 10:35 PM     Click Here to See the Profile for Jeff   Click Here to Email Jeff     Edit/Delete Message   Reply w/Quote
The chances are the tach is arcing over inside when the engine is running. This is likely why the tach drags down the ignition. You need to build a voltage divider to limit the amount of voltage and current going to the tach.

Go to a Radio/TV supply. Get two resistors, one 100K ohms and the other 10K ohm. Both need to be 2 watt resistors.

Mount them in a small box on a stand off solder lug terminals (three terminals). Hook one resistor between the outer and middle terminals. Hook the other resistor between the middle and remaining terminal. Connect the outer end of the 100K ohm resistor with a wire to the coil (where the tach wire now connects). Connect the outer end of the 10K resistor with a wire to the car chassis (ground connection). Attach the tach wire to the middle connection between the two resistors. Make sure the middle connection is not grounded. This will look something like:

^----100K----^------10K----^

Thus:

1. Center connection to tach.
2. Outer 100K connection to coil (Dist) terminal.
3. Outer 10K connection to chassis (ground).

What this will do is to divide the voltage at the distributor terminal by approximately 10 times. This will limit the voltage and current to the tach to an acceptable level while still giving it sufficient voltage to operate.

If the tach does not read correctly using this voltage divider, then try successively replacing the 10K resistor with 22K ohms or 33K ohm 2 watt resistors. These will change the division ratio to 5 times or 3 times respectively. One of these choices should work for you.

Mount the components in an aluminum box and bolt the box to the fender apron. Make sure the only connection to ground is the outer connection of the lower value component, e.g. 10K, 22K or 33K resistor.

Hope this helps.


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