Wacky Tach - Team Camaro Tech
Electrical & Wiring Troubleshooting electrical.

 
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post #1 of 9 (permalink) Old May 29th, 11, 03:54 PM Thread Starter
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Wacky Tach

Installed a repro factory tach, which has a few problems.

First of all - the dist is an MSD E-Curve with a tach feed wire. Stock alt with external reg (electronic) Batt in trunk

- Tach needle bounces all over the place at idle. At approx 1500 RPM it smooths out

- At idle - with needle bouncing - if I turn on the park lights, the courtesy lights, or push the brake pedal, the needle smooths out.

- disconnect alt belt - works fine

- with tach feed hooked to coil neg, it works fine but quits @ 4000 RPM

Any ideas?

Thanks!


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post #2 of 9 (permalink) Old May 30th, 11, 03:58 PM Thread Starter
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Re: Wacky Tach

Figured it out w/ help from my brother. Bad VR, which is weird, because it was a newish electronic unit. Oh well, I had a spare electronic one and now all is well. He measured "frequency" with his multi meter and it was all goofy (I obviously don't know what frequency is)


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post #3 of 9 (permalink) Old May 30th, 11, 07:03 PM
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Re: Wacky Tach

Frequency: Number of waves passing a fixed point per unit time; also, the number of cycles or vibrations undergone in unit time by a body in periodic motion. Frequency f is the reciprocal of the time T taken to complete one cycle (the period), or 1/T. The frequency with which earth rotates is once per 24 hours. Frequency is usually expressed in units called hertz (Hz). One hertz is equal to one cycle per second; one kilohertz (kHz) is 1,000 Hz, and one megahertz (MHz) is 1,000,000 Hz. The musical pitch A above middle C (the A string of a violin) has been widely standardized as 440 Hz.

Two places freq can come from:
Alternator on the BATT stud due to an open/shorted diode,
Or induced from the field winding as the field is controlled by a pulsed signal rather than a linear DC voltage.
Alt BATT stud should show a small AC ripple, less then 0.2 VAC. Any huge pulses would be diode problem.

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Last edited by Everett#2390; May 30th, 11 at 07:19 PM.
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post #4 of 9 (permalink) Old May 30th, 11, 07:43 PM Thread Starter
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Re: Wacky Tach

Well.....I kinda got part of that, sort of, Everett. Thanks for the reply!

So I notice now - with the other Voltage Reg installed - that the tach works perfectly, but now the gen light is on. Switched back to the original reg, and then gen light is off but the tach is wacky. Time for a new alternator?


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post #5 of 9 (permalink) Old May 30th, 11, 07:56 PM
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Re: Wacky Tach

Once alt starts to run, field relay closes with power from alt R terminal to VR #2 and places battery power from VR #3 onto VR #4 to turn off the GEN light as GEN light gets IGN power on the other side of the bulb filament. Battery power on both sides of filament, GEN light goes out, hopefully.

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post #6 of 9 (permalink) Old May 30th, 11, 08:04 PM Thread Starter
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Re: Wacky Tach

So which one do I whack with a rubber mallet - the alt or the VR?


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post #7 of 9 (permalink) Old May 31st, 11, 04:09 AM
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Re: Wacky Tach

I'd do both just for the satisfaction.

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post #8 of 9 (permalink) Old May 31st, 11, 04:48 PM Thread Starter
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Re: Wacky Tach

Replaced the alt with a 63A 10si - all is well. Thanks for the help Everett!


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post #9 of 9 (permalink) Old May 31st, 11, 07:14 PM
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Re: Wacky Tach

You're welcome. Always nice to help another with Anger Management.

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