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Boozebag
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I have 2 of the fatory tach clusters. This is great info, but IIRC, mine have different color wiring. I'll have to look when I go home. The tach plug in the back of the cluster has a green wire (tach pickup) a black wire (ground) and a red wire (power). I picked them up in Oklahoma, don't remember the years of the Blazers, but they are both mechanical speedos. Almost every aftermarket tach I've seen has this wiring color code, although the illumination wires can be other colors.
Maybe the beer is affecting my memory or something...
 

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4x4 v6 blazer
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87 GM factory wiring manual:

Shows for the tach wiring a white wire that goes from under the hood through a grommet to a plug in the back of the tach, brown/white wire from fuse block cavity b to tach, black wire from tach to bus bar ground.

My computer is dead or I would post a scan, I can take pics with my cell and post them if needed.
 

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Alright, time for the final chapter on this write up.

I investigated to find out that I was correct, there are no pre-cut knock outs in the fire wall, but I did discover there are drill centering 'punch' dimples in the firewall where the wire(s) and grommets go and there is a cutout in the insulation on the inside of the truck. You can see below the dimple where the grommet and wire go for the tach. If you inspect the firewall you will see several dimples marking holes for other wiring options such as cruise control.



Here is the hole drilled and the wire and grommet installed and the wire hooked up to the tach hookup. The hole size is 7/8". If you are like me and do not have a drill this size or a bi-metal hole bore this step drill from Harbor Freight is what I picked up today and it worked perfectly. And drilling through the sheet metal knocked out the insulation cut outs on the other side.



I plugged the ground wire to the ground buss above the L/S of the steering column, the hot lead plugs into an open 'ACC' outlet on the fuse block. Plugged it into the back of the tach, installed the new instrument cluster and reassembled my dash and the tach works and looks great!
 

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You should have always had the shift light, it's ECM controlled.

That is of course, unless you added or changed a burnt out shift light bulb, while you had the cluster out. lol
 

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It's a new cluster since I wanted to add the tach.

The bulb was probably out in the old cluster since I got it, never worked in that one.
 

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hi everyone i will be putting a new motor in my 89 chevy s10 tahoe 4x4 4.3 v6 and it would be nice to have a tech. do i just go to junk yard a pull a dash apart and get wires.

thank for your time
 

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I have some good info on this subject so I figured I'd revive this old and somewhat entertaining thread.
I picked up an instrument cluster with the factory tach from an '87 S10 Tahoe and I did some searching here for help on wiring it, but couldn't locate anything definite so I went back to the yard and pulled the tach wiring too. Here's what I learned:

The design of the wiring in these trucks is a plus, there seems to be a base wiring harness and then separate and smaller harnesses are added with the options. Meaning the tach wiring is not part of your main harness and the cruise control wiring is exactly the same, either or both options can be added without any intrusion into your factory harness. I'll document the cruise in another thread, but here's the scoop on the tach wiring:

This is the factory harness for the tach:

Yep, that's all there is to it. The plug to the tach and three leads off of it.

The brown wire plugs into the fuse box. I'll have to take a picture of the proper plug on the box and post it here later.
The white wire connects to the wire next to the brake booster.
The black wire is a ground. Proper placement is the ground strap behind the instrument panel on the L/S that provides ground hookups for various accessories.


Tach lead wire with rubber grommet for where it passes through fire wall:


Plug into back side of tach:



Factory tach lead:



I haven't studied mine too closely yet, but it looks like if you want to use the factory firewall grommets you'll have to cut some holes, it doesn't look like they are cut and plugged. The same for the factory cruise control. Again, I'll update as I perform the work.

My truck does not have that connection piece...?
 

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my '91 4 banger doesn't have that wire by the brake booster, where does it run to at the other end?
never mind... got it working, when one of my buddies spliced that wire in to the coil he didn't do it right, right wire, shoddy workmanship I attacked it with a pair of channel locks and it works now.
 

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Administrator
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You should start your own thread. Chances of any of these guys still being active is slim.

Let dead threads rest in peace...
 

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I agree with Rhotpursuit...

Probably should leave an old thread to die, this one was revived too many time already. lol

But to answer your question, which was posted years ago on the first page of this thread...

The pink is positive and the white is negative.
 
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