1972 GMC tachometer problem

[tmarvin]

Hey guys this is my first attempt at this forum so please bare with me. I have a 1972 GMC 4x4 and recently purchased a stock tach from LMC. I removed the old bezel ( Instrument panel) and installed the tach. On the rear of the tach are three electrical connections. The middle one is a nut and is obviously the ground. The other two one goes to the distributer and the other to a power source. I do not know which one goes where. I contacted LMC and the tach did not come with instructions. My concern is if I hook it up wrong I could damage the " expensive" tach. Can anyone help me a pic of the rear of the tach would help tks in advance

[hugger6933]

I'll go dig out my service book. I had it on some notes here at my desk but can't find it. welcome

[Coley]

Ok, I've got one of mine sitting in front of me so here we go.
First, as you look at this from the back (important!!)
As viewed there are 3 connections.
The top centre one in the 'triangle is the chassis ground
The bottom RIGHT connector is the 12Volt power
The bottom LEFT connector is the COIL (-) connector.
Again, these directions are based on 'as viewed from the back'....so keep all of those orientations in mind when you spin it around and mount it up into the dash and then start to view things from the opposite 'needle' side as viewed from the drivers point of view.

Coley

[hugger6933]

According to my book looking at the front of the tach, the lug in the 9 o'clock position goes to the neg side of the coil. The one in the 6 o'clock position goes to the # 3 spot in the fuse box. I have like I said another handwritten tach /dash wire diagram in my notes if I find it I will let you know if you have not been answered , but you will be. There are tons of people here to help you out. Jim

[TBONE1964]

Coley is right on the money. One other thing to remember is that the tach housing should ground to the cluster tin. How do the other gauges work? Does your truck have a factory wiring harness for a tach? The pink/red wire is the 12V wire and the brown (coil) wire should be on the negative side of the coil for a point style distributor or on the "tach terminal for HEI.

It is OK to add the ground to the center lug if you suspect a ground issue. Do you have a DVOM or a 12V test light to ensure power and ground?

What is wrong with the old tach?

Keep us posted.
Tom

[Coley]

Here's a quick pic for ya'.
If you look closely you can see the terminal/post I.D stampings that say 'COIL' and '12V'.
Have you removed and/or installed one before? (if not P.M me....I have a pretty good set of instructions that I did up to limit not only mistakes but also the time it can take if you haven't done it before)
Keep us posted on the installation.
All Good
Coley

...also, how' the weather up there in Oliver? I have relatives in that area that have a vineyard...nice place!

[Andy4639]

Nothing hooked to the screw the tach is grounded through the dash. Just the factory v connector. This is mine.

[TBONE1964]

Originally Posted by Andy4639

Nothing hooked to the screw the tach is grounded through the dash. Just the factory v connector. This is mine.

That is correct Andy and if the rest of the gauges work OK then the ground should be good. Don't over think this, it is not very complicated to figure out. make sure if you have an HEI that you are getting a good connection at the "tach" terminal on the distributor.

[Already Gone]

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[Andy4639]

Mine was in my 71 for over 20 years and it worked great straight out of a junkyard. Didn't do anything to it just ran the wires as suppose to. One to 12 volts and the other to the HEI tach connection. I used regular spade connector on the tach.