1968 GM Truck Factory Tach Wire Location

[Six862m6]

Hi all, been awhile since I last posted but have been a member here for over 10 years. Have a question that I'm hoping someone can help me out with. I have finally gotten around to neatening/cleaning up my '68 GMC's engine bay and am now starting to work on what is left of the wiring..... the previous owner(s) did some real questionable things wiring wise before I got ahold of the truck some 12 or so years ago and I didn't do much to the wiring under the hood until now other than making sure it was functional (which some of it wasn't when I first got the truck, such as having an internally regulated alternator physically installed with no signal wires ran to it and leaving the original regulator still plugged in.... safe to say it wasn't charging.).

I will be acquiring some new engine bay harnesses from American Autowire that will be correct for my truck's current setup (HEI, SI Style Alt on the passenger side, etc) as I am over looking at and trying to use what is left of the factory harnesses. That being said, here are my questions for you guys:

Can anyone here tell me/show a picture of where the factory tach wire location for a '68 (probably '67 as well) would be? I have already seen where it is located for the later trucks but their firewalls and firewall holes are slightly different than a '68. The harness directions are even 90 degrees different: 68's are arranged "vertically" and the later ones are "horizontal". My truck has what may be a factory tach wire already installed. It is currently to the lower left of the main firewall connection as you are looking at the firewall. I've marked it in one of the pictures that I have attached to this post.

Next, can anyone tell me what the grommet/hole is for that I have marked "what is this used for" in one of the other pics? Perhaps this was actually the factory tach wire location for 68's and the current one was just added by a previous owner?

In the same picture I have marked one other oddity as "WTH???". I believe this was definitely added by a PO, probably to control/power something that was added. It is no longer attached to anything inside or out (and wasn't when I purchased the truck) I have not seen this hole/wire in any of my searches, including in the one attached picture that is of another forum member's '68 GMC. However, you do see the item I labeled "what is this used for" on his truck. His truck being a 6 cylinder, I highly doubt he had a factory tach though.

Ironically, I used the factory manual choke cable hole to run the electronic choke wires through, (I was unaware of what this hole was for at the time). as well as the control switch wires for the fogs in my front bumper. I will of course be cleaning this up as I replace an rerun the rest of the engine bay harnesses.

Thank you in advance to anyone who can shed some light on these items for me!

[jumpsoffrock]

"What is this used for" grommet nipple is the other end of the retainers that hold the interior firewall insulation pads in, I believe.

[Six862m6]

Originally Posted by jumpsoffrock

"What is this used for" grommet nipple is the other end of the retainers that hold the interior firewall insulation pads in, I believe.

Ah, gotcha. That makes sense, thanks!

[pjmoreland]

I believe the "WTH" wire is the power wire for a carburetor-mounted TH400 transmission kickdown switch.

[Six862m6]

Originally Posted by pjmoreland

I believe the "WTH" wire is the power wire for a carburetor-mounted TH400 transmission kickdown switch.

Interesting.... my truck has a TH350 in it. May or may not be original, the 400 SBC sure isn't. I didn't think the '68s were available with a TH400? Definitely could be wrong though. Still.... wouldn't that hole be present on other 68s and just have a "plug" if that were the case?

[pjmoreland]

Originally Posted by Six862m6

Interesting.... my truck has a TH350 in it. May or may not be original, the 400 SBC sure isn't. I didn't think the '68s were available with a TH400? Definitely could be wrong though. Still.... wouldn't that hole be present on other 68s and just have a "plug" if that were the case?

In 1967 and 1968 your choices for automatic transmissions were either the Powerglide or the Turbo-Hydramatic 400. The TH350 wasn't introduced until 1969.

http://67-72chevytrucks.com/vboard/s...d.php?t=703538

[Six862m6]

Originally Posted by pjmoreland

In 1967 and 1968 your choices for automatic transmissions were either the Powerglide or the Turbo-Hydramatic 400. The TH350 wasn't introduced until 1969.

http://67-72chevytrucks.com/vboard/s...d.php?t=703538

Well damn, I never realized that the first year for a TH350 was 1969 period! I always thought that the TH400 was developed/used after or at the exact same time as the TH350 but after some research after reading your post, you are indeed correct sir! Thanks for the info. So I now know that neither my engine nor my transmission are original.

I took a look at the leftover grommet/wire again last night and sure enough, on the inside of the truck I still have the connector attached to the wire going through the fire wall shown in the picture I've attached as well as just the barest piece of the orange wire sticking out of it as well. The picture is for a '71 cable operated gas pedal vs my 68's mechanical linkage but the connector and wires for the switch match.

I guess it is safe to say that the "WTH???" hole is indeed a factory hole meant for the kickdown wires! I wonder why the other guy's '68 didn't have the hole as well (plugged of course since his is probably a 3 on the tree)? I would think they would all have the same holes and just plug the ones not used.

Think I'll use this hole/grommet to run my bumper fog control switch wires! I certainly don't need it for the kickdown wires anymore.

That's 2 of the 3 items down, only one to go. Anyone else out there have any info on a 1968's stock tach wire firewall hole location?

[pjmoreland]

It seems the 67-68 factory tach wire passed through the oil pressure line grommet over behind the engine.

http://67-72chevytrucks.com/vboard/s...d.php?t=447536

http://67-72chevytrucks.com/vboard/s...d.php?t=658229

[Six862m6]

Originally Posted by pjmoreland

It seems the 67-68 factory tach wire passed through the oil pressure line grommet over behind the engine.

http://67-72chevytrucks.com/vboard/s...d.php?t=447536

http://67-72chevytrucks.com/vboard/s...d.php?t=658229

Hmmm..... after reading through those it sounds as if the '67s did indeed pass through with the oil line. The 68's and up did not. Kind of unclear on the 68 itself, seems to be the exact same harness as the later trucks. Was thinking it might be in a slightly different location than the 69 and up trucks because of the firewall difference and positioning of the junction block (being clocked 90 degrees different). Was hoping someone on the forum that has a for sure stock tach option in a '68 might come across this thread and be able to tell me or snap a picture of what they have.

Thank you for all of the info so far though!

[RichardJ]

Your Bulkhead connector will probably have several unused holes. In this schematic drawing there is an empty hole next to a 20DG - Temp Sw and a 20DBL - Oil Press. The empty holes are usually filled with old, dried up putty. Remove the connector from the firewall and separate the two connector halves. You can poke the gunk out with a stiff wire or paper clip.
On the inside half of the bulkhead connector is a flipped mirror image of the outside. The matching holes are marked with a RED Arrow.

The male/female Packard terminals are available at most auto parts and allow much neater wiring than random holes poked through the firewall.

[BigBird05]

I came across this picture in the 72 service manual.