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#1 |
Registered User
Join Date: Oct 2010
Location: Indianapolis, IN
Posts: 134
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1968 GM Truck Factory Tach Wire Location
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!
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#2 |
Registered User
Join Date: Apr 2014
Location: California
Posts: 946
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Re: 1968 GM Truck Factory Tach Wire Location
"What is this used for" grommet nipple is the other end of the retainers that hold the interior firewall insulation pads in, I believe.
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#3 |
Registered User
Join Date: Oct 2010
Location: Indianapolis, IN
Posts: 134
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Re: 1968 GM Truck Factory Tach Wire Location
Ah, gotcha. That makes sense, thanks!
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#4 |
Senior Member
![]() ![]() Join Date: Jan 2005
Location: CA
Posts: 5,480
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Re: 1968 GM Truck Factory Tach Wire Location
I believe the "WTH" wire is the power wire for a carburetor-mounted TH400 transmission kickdown switch.
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#5 |
Registered User
Join Date: Oct 2010
Location: Indianapolis, IN
Posts: 134
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Re: 1968 GM Truck Factory Tach Wire Location
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?
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#6 | |
Senior Member
![]() ![]() Join Date: Jan 2005
Location: CA
Posts: 5,480
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Re: 1968 GM Truck Factory Tach Wire Location
Quote:
http://67-72chevytrucks.com/vboard/s...d.php?t=703538 |
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#7 | |
Registered User
Join Date: Oct 2010
Location: Indianapolis, IN
Posts: 134
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Re: 1968 GM Truck Factory Tach Wire Location
Quote:
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?
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#8 |
Senior Member
![]() ![]() Join Date: Jan 2005
Location: CA
Posts: 5,480
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Re: 1968 GM Truck Factory Tach Wire Location
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 |
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#9 | |
Registered User
Join Date: Oct 2010
Location: Indianapolis, IN
Posts: 134
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Re: 1968 GM Truck Factory Tach Wire Location
Quote:
Thank you for all of the info so far though!
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#10 |
Registered User
Join Date: Apr 2010
Location: So Cal
Posts: 1,437
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Re: 1968 GM Truck Factory Tach Wire Location
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.
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'67 GMC 2500, 292, 4spd, AC |
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#11 |
Registered User
![]() Join Date: Aug 2019
Location: Cheraw
Posts: 1,305
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Re: 1968 GM Truck Factory Tach Wire Location
I came across this picture in the 72 service manual.
Last edited by BigBird05; 09-10-2021 at 01:15 PM. Reason: add photo |
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