[Fritz]

So I installed the Tachometer for the 3.0 last night. It blips a bit when started, but doesn't do much past that. I realized that I had a permatune installed for the 2.7, but used the original Bosch unit that came with the 3.0. Since the wiring for the tach comes from the ignition timing, is there a chance that it is wired correctly, but that a resistor or something was installed for the permatune? If the tach connection wasn't wired correctly would it jump at all, or is that just a function of it having the positive and negative connected?

Also, there is a small metal box with a 3 wire T connector just to the left of the Bosch box. What is this? I thought it was something essential, but the engine started just fine without it connected. I have two possible connections for it as the 3.0 wiring harness had one and the 2.7 had it wired to the chassis somehow. I'm assuming I should use the one from the engine harness, but odd how it does need to be plugged in at all.

Thanks/

[smdubovsky]

IIRC a 3.0 CDI runs the tach directly. Its one of the 6 pins coming out the bottom. Its a blue/green striped wire (again IIRC - though that might be the color code the 914 uses since I had to mess w/ its CDI to get the tach to work too.)

[Vicegrip]

If not blue green look for black/purple trace. Black/Purple is a fairly standard convention for Porsche from 912 to 993.

[Fritz]

I was at one with my multimeter this weekend.

I traced through the whole system from the lead on the back of the tach all the way to the engine harness. It turns out it was an issue of 1974 speed sensor and RPM relay that are not needed for the SC motor. While the signal was passing through the older equipment, it was degraded and spread across multiple pins on the Bosch CDI. The Violet/Black from the gauge went to the harness connector under the CDI. From there it went through the old electronics to the green/yellow on the CDI/Engine wiring harness. The biggest problem was that the pinout for the 74S engine and 79SC engine were off by a pin on the tach sensor. I think this had to do with the fact that the old relays required powering and the order was different. So I move the pin placement and jumpered the needed path through the old circuitry and the gauge came to life. Hopefully I didn't gimp anything else up in the process.

Now if I could just figure out why the RPM's keep increasing. On startup, the engine sound great and idles around 1200. But after a few minutes it begins to idle at 1800-2000. While that's not a major problem, it's not ideal. Almost like the WUR or something similar is failing or wired incorrectly. Any thoughts?

[Cliff Claven]

measure your control pressures to determine if you WUR is working correctly. but maybe your auxiliary air valve is not closing after warm up. does the idle ever come down, after the motor is driven hard? are you getting current to the heating element in the AAV on startup? is it sticking?