Testing the Tach

[Pistol Gripper]

Anyone have a quick and dirty way of testing a tach?  I've about got my 8 Grand Tach out of the dash and I see that the wires were spliced to the harness ( not the original tach ) and then the wires were cut.  ( No wonder it didn't work ! )

Thanks.

[MEK-Dangerfield]

 Hello Pistol Gripper,
  I'm not sure I totally follow you here about the tach. not being original. The quick way to test it is as follows: There will be 5 nuts on the back of it. One will have a tab that connects to a 12 volt source. If the wire is dead, try connecting it to the radio power, or even run a wire to the "+" side of the battery. There will be a longer screw sticking out with a nut on it that should connect to the "-" side of the coil. That's the signal.  You can take any of the 3 remaining screws and wire them to a known ground connection. That would be most screws under the dash. I hope this helps.

  Mike

[Pistol Gripper]

MEKrunner,

Thanks for the info.  My Chally is a 74 that came with standard guages, somone swapped in a Ralleye cluster with the 8 Grand Tach.  I pulled out the cluster and the wires came along with the Tach, they weren't hooked to anything.  Once I get the dash out I'll see if the original harness is intact or if I have to replace it.

Thanks again,

P.G.

[MEK-Dangerfield]

  Hello Pistol Gripper,
  Now I see the light The standard gauges didn't have a tach., so I guess someone didn't bother to wire it up for you. There is still a question of the tach. working or not. Many have crapped out over time. I hope you get lucky with this one, as a working 8 grand tach. is worth big $$$.

   Mike

[Pistol Gripper]

Got the Tach out of the cluster.  I'll try testing it tonight, got my fingers crossed.  Looks like most of the wires were left off all the guages.   Anyone got a good diagram of what wires go where ?

P.G.

Just found an article on using a 12V battery charger as a signal source.  Text follows.  Any comments on using it on a Chrysler Tach ?

PREFERRED SIGNAL SOURCE... an inexpensive 12v battery charger can be used as a deadly accurate signal source for the re-cal of your tach. This signal source is so steady and accurate that there is no need to compare the reading with another V-8 calibrated tach. Battery chargers typically use a rectifier bridge to convert the AC power source into DC. This turns the 60 hertz sine wave input into a 120 hertz ripple wave DC output. The ripple wave output is be read as 3600rpm by the stock rotary calibrated RX-7 tach. For a V-6 application, an RX-7 tach should be adjusted to read this signal as 2700rpm. For a V-8 application, adjust your RX-7 tach to read this ripple signal as 1800rpm. Please note that you must use a 12v battery for a power source, and the battery charger as a signal source only. The ground wires for both the battery and the battery charger should be connected to the ground terminal of the tach.
NOTE: Be sure that the RX-7 tach is about the same orientation as when installed, because the re-calibration may be off if done with the face in a horizontal plane.

[MEK-Dangerfield]

  Wow P.G.
  That sure is an interesting article. I don't have the guts to try it, but it sounds simple enough. Anyone out there ever tried the battery charger test???

  Mike

[Pistol Gripper]

MKErunner,

In case there is any doubt,  I'll try it on any old tach I have lying around, not the 8 Grand     If it works on a couple different ones, I'll try mine.

P.G.

[MEK-Dangerfield]

  Hello P.G.
   I'm real interested in how this will turn out. Here's why I don't like the idea. When my car is running, I measure a little less than 5 volts on the "-" side of the coil, which is the signal for the tachometer. If you connect the tach. to a battery charger, then suddenly you are supplying it with 12 volts, which I would think would cause a state of maximum smoke inside the tach   Now maybe the voltage on my coil goes up when the RPM's go up, but I doubt it. What I'm saying is, I think a 12 volt supply on the "signal" wire of the tach. would fry it.  Just my educated guess.

  Mike

[Pistol Gripper]

Well ............  It is an article about RX-7 tach's.  A little more research is in order before I let any of the magic smoke out of a Tach.

P.G.