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Old 07-23-2010, 07:40:07 PM   #1
llskis
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Question Intermittent Tachometer;1970 Z28

Have a problem with my 70 Z28 Tachometer. It only works some of the time
(Not Often). Have taken a continunity test on the Brown wire from Coil to
printed circuit board and everything checks out OK. It seems to me it comes
down to one of two things.

1) Ground is intermittent.
2) Internal problems with the Tach.

Questions:
I believe the ground to the Tach is accomplished by the tach metal
housing itself. (Correct?) How is this done? By screws or is there a circuit
on the printed circuit board that feeds the housing?
If it is the second (Internal Problems); is there a way of testing the tach
w/o taking out? For that matter is there a way to test the ground to the
tach w/o taking out? Need your help on this one; thanks in advance.
All other gauges work fine.FYI
Larry
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Last edited by llskis : 07-23-2010 at 07:46:20 PM.
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Old 07-23-2010, 09:20:46 PM   #2
77RS
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The tach receives ground from the printed circuit board via a screw that runs through the circuit and into the tach mounting plate. It's very possible that the screw has become corroded (or loose) and the tach has lost ground.
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Old 07-23-2010, 11:11:42 PM   #3
Twisted_Metal
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The screws that connect to the circuit panel sometimes corrode at the tach's circuit board too. (The other end of the, which the nuts thread to in attaching the circuit panel, is run through the tach circuit board.)

I have one here which doesn't work and shows signs of corrosion at that junction.... I suspect that is the problem but I haven't gotten around to testing my theory yet.

You would need to remove the cluster and disassemble it to inspect the tach board connections inside the cluster housing.
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Old 07-23-2010, 11:32:38 PM   #4
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Quote:
Originally Posted by Twisted_Metal
The screws that connect to the circuit panel sometimes corrode at the tach's circuit board too. (The other end of the, which the nuts thread to in attaching the circuit panel, is run through the tach circuit board.)

Terry, I think you're talking about the 78-81 tach circuit, right?
The 70-77 tach is completely different, which doesn't have any screws running through the internal circuit
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Old 07-24-2010, 12:02:21 AM   #5
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Quote:
Originally Posted by 77RS
Terry, I think you're talking about the 78-81 tach circuit, right?
The 70-77 tach is completely different, which doesn't have any screws running through the internal circuit

Yes, I was. Thanks for clarifying that point!
(I've never dismantled the older style to see how their connections are made.)

Never mind.... Carry on.
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Old 07-24-2010, 08:33:19 AM   #6
hogg
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the grd for the cluster (i thank )is the black wire to lite switch that comes from the grd.bracket of lite switch and then to wiper switch housing and then to metal behind the wiper switch,,check lite switch to be tight ,and screw at black wire behind wiper switch,
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Old 07-24-2010, 01:53:38 PM   #7
llskis
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Attn: 77 RS

Where would this screw be located at? The one that connects the ground
to the circuit board. Thanks Again. Any pictures of it?
Larry
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Old 07-24-2010, 05:24:57 PM   #8
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Quote:
Originally Posted by llskis
Where would this screw be located at? The one that connects the ground
to the circuit board. Thanks Again. Any pictures of it?
Larry

Yep Also, see the 3 tiny screws in the center circle? Those are the mounting screws for the D'Arsonvol meter used in the 70-77 tach. The 78-81 tach uses a bicoil meter that is very similar to the fuel, volts, and temp gauges.

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  • 66-67 Chevelle and El Camino
  • 71-77 Chevelle, Monte Carlo, and El Camino
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Last edited by 77RS : 07-24-2010 at 05:29:36 PM.
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Old 07-25-2010, 03:02:47 PM   #9
llskis
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Finally a conclusion to my problem!!

We just got the tachometer to work now; for how long I do not know. Had
to take out the ash tray and holder to get to the printed circuit board. I
tighten all the small nuts and screws(for the tachometer) so I don't know which one did the trick. They all needed to be tighten. It appears that the
small screw for the ground may be stripped. I wiggled it a bit with long nose
pliers so this may have done it. Will see what happens. I want to thank all
the responses I had and special thanks to 77RS; you certainly know your
gauges.
Larry
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Old 07-25-2010, 03:35:48 PM   #10
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Old 07-25-2010, 10:26:38 PM   #11
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Glad it worked out for you
Daniel
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Complete gauge Restoration, Calibration and Repair for:
  • 70-79 Camaro (all models)
  • 70-79 Trans Am
  • 66-67 Chevelle and El Camino
  • 71-77 Chevelle, Monte Carlo, and El Camino
home of the 2nd Generation Camaro Oil Pressure Gauge!

Gauge Marks in HPP magazine

Visit the web site, PM, or email info@gaugemarks.com for more info.
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