Factory Tach Install

[Times Welding]

I am installing a factory tach in my '87 K5 TBI Blazer. Well, not factory for MY truck, but a GM tach, none the less. I have all of the necessary components, including wiring harness with the proper grommet installed. My question is: where, on the firewall, does the grommet pass through for the tach signal wire from the coil? Is there an existing hole that I should be using, or do I need to drill? If I need to drill, where is it supposed to be? I am not trying to build a "by the book" Blazer, but I'd like the hole to be where it belongs.
Thanks,
Doug

 

[Irish1941]

I am installing a factory tach in my '87 K5 TBI Blazer. Well, not factory for MY truck, but a GM tach, none the less. I have all of the necessary components, including wiring harness with the proper grommet installed. My question is: where, on the firewall, does the grommet pass through for the tach signal wire from the coil? Is there an existing hole that I should be using, or do I need to drill? If I need to drill, where is it supposed to be? I am not trying to build a "by the book" Blazer, but I'd like the hole to be where it belongs.
Thanks,
Doug

If I remember right, firewall is dimpled for it right behind the cruise control vacuum but I took path of least resistance.
I put the tach lead in with the bundle that comes thru the firewall on lower left, below fuse panel. No hole to drill and tach lead is more than long enough to reach coil lead.
Here is my factory tach in my '89 Blazer.

 

[Times Welding]

Thanks, Irish. I'll have a look at the firewall behind the cruise servo tonight.

 

[Times Welding]

Well, I finished my tach installation, and I'm very pleased. Thanks for the advice. I do have one issue, however. When the ignition is OFF, the tach reads 1000. When the ignition is ON, it reads accurately at all engine speeds. Is this normal, or have I overlooked something?

 

[max 02]

I think that is Normal.
Mine does it too.

 

[82355]

Mine stay where the engine was running when the ignition was shut off.

Martin

 

[Times Welding]

I think that is Normal.
Mine does it too.

Thanks, Max. Now I don't feel so bad. It works fine and I'm very pleased with the results. I just thought that was kind of odd.

 

[Irish1941]

Mine stay where the engine was running when the ignition was shut off.

Martin

This correct. It will stop where ever idle is at when you shut it off but it should zero when you turn the key on.