Can anyone shed any light on why the (repro) shielded wire for the 1970-72 U21 tach is 65" long? And why does it need to be shielded?

 

The shielding is for spurious interference from such sources as ignition and power lines.

 

Any idea why it's so long? I couldn't believe when I uncoiled it and measured 65 inches! It only needs around 20" to get from the tach to the distributor.

 

Maybe it is supposed to go through the fire wall at the fuse panel. It's a long way to your left foot.

 

The tach wire IS the ignition. It carries the ignition pulses from the coil to the tach. It's shielded because as installed it passes very close to the antenna lead-in wire going to the radio. Without the shielding, that tach wire would turn the radio into an "audio tachometer". The shielding keeps the ignition pulses "in"; it's not protecting the tach wire, it's protecting the rest of the wiring under the dash FROM the tach.

 

So, again. WHY is the shielded wire 65" long??

 

Don't know why other than if there was a specific routing that required that length or its a one size fits all and the length covered all the bases with other GM car models that use the same harness.

 

Ding ding ding! We have a winner. Not sure why I just didn't look in the 72 Cutlass Assembly manual to start with. Yes, the shielded wiring is supposed to go from the tach over to the left side of the firewall, through the accessory grommet (which I don't seem to have) and then back across the firewall using the existing wiring support clips to the coil. Whoever installed this for my Dad obviously took a shortcut through the firewall and direct to the back of the tach. Problem solved!

To validate the AM, I went to the garage and laid out the repro shielded wire as indicated and shazzam! It's spot on. Learning just goes on forever with Oldsmobile. Note to self. Start looking things up before asking questions....

 

Great, now I have to uncoil mine and re-route it the correct way. I was curious as to why it was so long also. Thanks for the info.

 

I'm not going to bother. I'll just leave it coiled and use the shortcut already drilled into the firewall. Leave that re-route for the next owner who might want to do a concours restoration