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Old 08-07-2009, 10:00 PM
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Dash Bezel TACH, 76 Chev C20 3/4 Ton

Hi all,
I'm switching my dash bezel (NO TACH) out for a stock bezel of same yr. make model, but WITH a TACH.

Q1 - Should I expect to find an unused harness and 3 prong plug in my dash? My guess is "NO" since I can't find one.
Q2 - Assuming "NO", can anyone tell me where each of the 3 prongs in the back of the stock TACH should map to? I have followed previous Hotrodder threads and am aware that standard TACH wire mapping should be:
Red = +12 volts
Black = ground
green = tach signal from the coil. Negative lead on the coil
white = instrumentation light panel (switch on only)
However, since I don't know where the wires map to the back of the TACH, I'm afraid to guess wrong and fry it.
Thanks in advance for any help!!!

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Old 08-09-2009, 08:06 AM
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I have an in-dash tach in my '75 and there are only three leads. in the junkyard I pulled the plug along with the cluster, and the wire lead colors were self explanatory, from what I remember. However, I will try to crawl under there today and see what lead is on what terminal.

By the way, this vintage of GM tach is notorious for not being very reliable or accurate. There is a circuit board on the back with a weak wiring trace on it, and it often fails. Mine sometimes want to read a constant 3700 rpm.

Several Corvette parts places sell replacement circuit boards, but they can't verify that a '75 Vette circuit board will fix a '75 truck tach. At some point I want to take my tach back out of the dash and try to compare parts numbers for the replacement boards.

Bruce
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Old 08-09-2009, 09:43 AM
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Bruce is right about the accuracy and also the reliability of these things.

the tach in my 79 'works' in some modes.

low RPM will only work until rpm reaches about 1500

once it reaches 1500----it will not drop below 1000 until the car is shut off and restarted.

You will also need to get the correct fuel gage for this conversion.

the printed circuit board is different between the gage/lights/and tach dashes.
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Old 08-09-2009, 12:05 PM
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Dash Bezel TACH, 76 Chev C20 3/4 Ton

Thanks Bruce & Bryan.

Bruce, it would be great if you can see which wire color is associated with each of the 3 prongs. Here's a crude drawing of the 3 prongs from the back of the tach as a frame of reference.

Two prongs on top and one on bottom.

____ ____

____


Thanks
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Old 08-09-2009, 09:23 PM
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two prongs on the top (they are angled down a little toward the passenger side)
top, passenger side prong - brown wire , lead to HEI tach terminal
top, driver side - pink wire, 12 volts switched

lower prong - black, ground

I could see and follow the pink and brown wires to the fuse block and the HEI, but I couldn't see where the black wire led to. I can't remember if there was a ground terminal on the cluster itself, but I think there must have been.

Bruce
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Old 08-10-2009, 01:24 AM
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Dash Bezel TACH, 76 Chev C20 3/4 Ton

Bruce, thanks so much!! This saves me the trial and error and distinct possibility I fry something. I hope I can return the favor sometime soon!!
Trikepapa, aka.... Ronnie
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