Tach works intermittently
 

The tach in my S4 TII has decided not to work for the first 5-10 minutes of driving first thing in the morning. About half way home from work in the mornings, it randomly starts working again. I checked the trailing coil and all of its connections and everything is fine. Next I checked the ground on the strut tower underneath the trailing coil (and cleaned/dielectric greased before reinstalling) and it hasn't changed. I've already pulled the cluster apart and cleaned up all common cold solders (stock boost gauge "works" again!).

Strangely, it seems to work fine every time I start the car in the evening, regardless the time/humidity/current weather outside.

I'm looking for any help I can get. The car still runs perfect, I just want to be able to see my rpm.

You can jumper the two bullet connectors to each other which are located by both of the coils. This will drive the tach by the lead coil rather than the trailing coil. If that solves the issue then it reduces the existing possibilities.

you can always try another set of wires and plugs too.
Since the coil sees spark,if the plugs are not being ignited then it will not allow the tach to read.

Plugs and wires were replaced when the engine was rebuilt, roughly 300 miles ago (I've had the car back for a week). I decided to check them anyway.

All four plugs turned freely with my fingers, they weren't even finger tight. Leading coil is connected to the trailing plugs, trailing coil is on the leading plugs. All four plugs were slightly black, but didn't look fouled. I cleaned them and reinstalled with the wires in the proper places. The tach worked for a second, then died, then came back on a little ways down the road. I think my plugs are just fouled.

I'm not saying this your problem, but I did have erratic tach behavior (especially above 4k) which did not go away until I replaced the coils. I went with a direct fire setup, not a new-to-me 30 year old set of OEM coils.

At a certain point you can only do so much before part swapping becomes a viable troubleshooting method as well. Perhaps if someone is local with a known working trailing coil you could swap for 30 minutes and observe the behavior.

I think you would have a problem if this were how you have it set up.

The car ran great like this for 300+ miles. I wasn't the one who did this, but now that it is corrected I'm not seeing a change.

This issue usually stems from the Haynes manual, which documents the connection incorrectly.

Just remember Leading = Lower, Trailing = Top

If you had the plug wires running to the incorrect plugs then it might have affected the plugs in a negative manner.

Maybe the tach wire is coming out of the back?

Update:

I cleaned the trailing plugs and reinstalled them. Tach works for a second after the engine starts, then goes back to zero. Now it jumps erratically while I'm driving, sometimes jumping all the way past redline. I'm also getting the two beeps and a pause that indicate a lack of engine speed signal. I'm starting to think it's an electrical issue somewhere.

Your overlooking the tach itself. Try swapping it with another tach.

I don't have another tach to try.

If you jumper the two bullet connectors it will drive the tach from the lead coil. Doing this will tell you more of what is going on w/your car. It's your choice on whether you want to rule things out or not!

There are two connectors on each coil. Which wires do I jumper together? What outcome am I looking for after jumpering them? If the tach works, I have a bad coil? If the tach doesn't work my tach is bad?

In the following link look at the second pic in the first post as it shows the bullet connector. There are one of these on each of the two coils. If the tach works flawlessly then you know the tach is not the problem and points more to the trailing coil.

Sounds simple enough. I don't see a link, though. And thank you for the help.

I jumpered the two bullet connectors together and the tach works! It does jump up and down a little at idle, but it reads true when driving.

How about giving the car some time to be driven during different types of situations such as hot/cold and so on and then tell us if it's still operating as it should to validate some things.

I put around 45 miles on the car last night, including stopping at places and restarting the car. Then I drove it to work this morning. Still reading fine, but bouncing at idle.

How much range is the bouncing? Is it like 100 rpm? You might try checking the TPS to see if the Green/Red wire reads close to 1 volt w/key to on after a 20 minute drive.

It's bouncing between ~900-1800 rpm. As soon as I come off of idle it stops bouncing. I have the Bonzai Racing LED connected to the TPS and it shows that it's adjusted correctly.

And the idle never settles once the car is being driven?

No. Any time the idle drops to 1000rpm or less the tach bounces. I just drove it again to get some lunch, and it didn't seem to be bouncing past 1200rpm. It was a short drive though, less than ten miles round trip.

To clarify, the idle stays steady and does not fluctuate at all. It's only the tach needle that's bouncing.