Tach wire termination if no tach?

[rich weyand]

Does anybody know where the brown tach wire terminates under the dash if you do not have a tach on the truck?

I am buying the combo tach/fuel gauge from these guys, and would like to have some idea of where to look for the other end of that wire. It is there, it comes off the distributor and heads into the cab, I just don't know where it ends up. Yet.

http://www.gmsports.com/content/new-...and-gmc-trucks

[bake74]

     I am not sure where the actual wire terminates or if it is even there if the truck was not equipped with a tach, but your link you provided does not work, it says "page not found".

[rich weyand]

How about this:

http://www.gmsports.com/content/new-tachometer-1973-1986-chevy-and-gmc-trucks

[bd]

Does anybody know where the brown tach wire terminates under the dash if you do not have a tach on the truck?

According to the 1978 Wiring Manual, page 13, the brown wire (Ckt 121) passes through a firewall grommet and terminates in the 3-terminal connector that plugs directly into the tach board.  The diagram shows the tach circuit as an option, but doesn't show an interim connector between the distributor and the tach.  Neither does the diagram show the tach circuit being used for anything else.  You may just have to trace it out.  Maybe you'll get lucky and it's already there.

[sphinx]

From my experience, if the tech was not installed at the factory, the tech harness was not either.  Check the distributor to see if there is wire attach to the TACH terminal, if not you will need the tech harness.  Don't use a plain wire from thru tech terminal, use the tech harness as it has some type of 'filter' capacitor built into it to keep electrical interface from the ignition from reaching the tech.

[Irish_Alley]

now here is a question. i have read that you can get a rpm signal off the alt. if this is true would gm run a separate wire from the distributor or just tap off the alt wire?

[rich weyand]

GM in fact did run a tach wire from the distributor, that connection is part of the distributor cap assembly.  Mine has the brown wire from that connector, but does not have a tach.  It may have before it was rebuilt in the late 90s, but I have a hard time thinking someone would remove the tach if it was already there.

[bake74]

How about this:

http://www.gmsports.com/content/new-tachometer-1973-1986-chevy-and-gmc-trucks

     That one works, thanks.

[sphinx]

Irish-Alley, yes you can get a tech signal off the alternator.  But you got to know the number of poles in your alternator and the pulley ratio between the flywheel and the alternator then set the tech to read that signal.  Every turn of the alternator produces pulses in the voltage out that are converted to the engine rpm.
 

[Captkaos]

I ripped a few of these apart and never found a tach wire.  They were separate.

[sphinx]

The tech has to be designed to use the alternator as the input instead of the distributor.  Not the standard GM tech in our trucks.  Sorry if I confused anyone.

[Irish_Alley]

i did a little research dakota digital makes a unit to use the alt signal(i was looking at it for the cummins) but you need another tach reading to calibrate it. kind of hard on a cummins but there is a Magnetic Sensor (R8912) that you put near the harmonic balancer and epoxy two magnets on the balancer. it gets its reading and sends the signal to the tach. supposedly its universal  waiting on a email that will confirm this

[bake74]

      What happens when your magnets collect a bunch of iron dirt and get covered up ?

[Irish_Alley]

idk i dont make this stuff lol

[bd]

Flying magnet pickups are proven technology.

Rich - did you ever get the answer to your question?