79 Tach Harness Routing

[79 Silverado/454]

Can someone please post a pic of where the factory tach harness comes thru the firewall ? Also how it is routed by the factory along the firewall to the distributor ? I did a search but, I could only find information on the 67-72 trucks. I assume 73-80 would be the same. Maybe 73-87. My truck is a 79. Any help would be much appreciated.

[79 Silverado/454]

Anyone ?

[ray_mcavoy]

Attached is a pic (I think it may have been from this site) that I saved years ago showing the factory tach wire going through the firewall on a 78. It's the brown wire just below the lower / right corner of the fuse box. GM changed this over to a white wire sometime around 79. I believe there will be a dimple stamped into most firewalls in this location.

As for the routing along the firewall to the distributor, I simply ran the tach wire along side the engine harness when I added a factory tach to Dad's 77. I'm not sure if that's exactly how they were originally routed though.

[79 Silverado/454]

Thanks Ray for posting up the picture. From what I can tell, that looks similar to how the 67-72 trucks are done as well. My harness has the white wire. It also has a capacitor in the white wire with a strap attached around it with a screw or bolt hole in it. I would assume that the capacitor attaches to the firewall somewhere but, not sure ???? Does yours have the capacitor Ray ? I guess the capacitor could have been a 79 & up thing ?

Does anyone have a picture of the wire ran in the engine bay ?

AIM ( assembly instruction manuals ) sure would be handy for these trucks. I have restored some Chevelles over the years and those manuals have helped me a bunch. Hard to believe that the AIM for these trucks were thrown away after the model year ended but, that's what I've also read.

[ray_mcavoy]

You're welcome! Yes, the hole/grommet through the firewall is pretty similar to 69-72 (earlier years had the tach wire going through a hole in the oil pressure line grommet).

The "capacitor" connected to your harness is a noise filter (it contains a couple capacitors and resistors). And you're right, the mounting strap on that filter is supposed to be screwed to the firewall. Unfortunately, the factory wiring diagrams don't show the physical location. In fact, the wiring diagrams don't even show the filter until into the 80's ... I know they were using them earlier than that though. The one I installed (77) didn't have a filter but I bought it used online and therefore didn't get to pull it from the donor truck to know for sure if it originally had one that year or not.

Yes! An AIM for these trucks would be very handy. I have a couple of Novas (74 & 77) and the situation is about the same .. no AIM available for 73+. There's an old post here that was recently brought back up and hatzie posted a link to an 82 truck AIM on ebay. One of those might have the info you're after. Of course that's assuming the tach wire routing didn't change significantly over the years and that the page(s) covering it are not missing from that copy available on ebay.