Strange Tach wiring issue - help!

[megerian]

Last fall I fried some wires leading to my speedo, and decided that wiring was not my area of expertise, so I took it to a well respected Porsche shop to fix my speedo wiring.

Driving it home in late fall, the speedo and cruise were working fine but I noticed that the shift indicator light on the tach (also known as CASIS) was no longer working. It didn't concern me too much, since that light isn't of much use anyway, but it struck me a little odd since I didn't think they would have had to touch the tach wiring to fix the speedo wiring.

Today I was out for a long drive, tach and speedo are fine, CASIS light still off same as before. Then I turn on the headlights and things got weird:

1) The tach needle swept all the way right, then all the way back, and stopped working
2) The CASIS light came on, and stayed on
3) The tach gauge illumination lights never came on. The other 4 gauges were fine.
4) When I turned the lights off, the tach needle did the big sweep again.
5) When I stopped, turned off the engine and lights, and restarted the car, the tach worked again, then again stopped once I turned on the lights.

What wires got crossed? Anyone have something similar occur?

[juanbenae]

somebody much smarted will chime in here,,, but until then I offer this.

have you checked the ground at the battery to the body yet? with so many differing symptoms id start crossing the simplest things off the list 1st.

ive got like 3 or 4 more ideas, but doordash has rung and im eating..

[spoke]

It almost sounds like a ground is missing somewhere. The elements you speak of should be independent. The only connection I can think of between the headlights and the tach and CASIS light would be the illumination lights.

Does the weirdness happen with parking lights on or only on headlights? The tach illumination lights should come on when the headlight switch goes to parking lights before the second notch for headlights.

Pop the the tach out and inspect the wires. Take a picture or 2 if you can and post them.

[megerian]

Ok, I just tried it. The same weirdness happens when I just turn on the parking lights.

Here is the sequence:

start car, tach works fine
turn on parking lights, tach needle sweeps right then stop working entirely
CASIS light comes on and stays on
if i turn off the lights, the tach doesnt start working again, I have to kill it and restart the car

[spoke]

OK, something is wrong with the wires going to the tach illumination lights and/or the tach itself. Pull the tach out and take a picture of the wires.

It could be that to fix the speedo they had to remove the tach for access to the wires. You could try fixing yourself or take it back to the shop and have them fix it. They did cause the issues you are having.

[megerian]

Turns out I fried the tach, so it's being rebuilt... sigh

[LIRS6]

Originally Posted by megerian

Turns out I fried the tach, so it's being rebuilt... sigh

There must be more to your circumstances ... you fried the speedo, and now you've fried the tach. What's causing that to happen? I would check wiring thoroughly before you fry some other gauge.

Jason