I gathered some input about this ‘factory tachometer’ issue today. From last weekend’s journal entry, email response, searching on the internet and a couple of phone conversations I have some info to add.
I was able to access another Saturn with factory ICE and get reference tach signals.
Dave Koller, who is working on a 1999 Saturn SL2 conversion, is encouraging & making suggestions to help me to find the answer.
I found this site http://www.uoguelph.ca/~antoon/gadgets/555/555.html today about a 555 timer IC that might work.
I was surfing the internet and found Jim Dawson’s 1994 Saturn EV conversion and its video on Youtube http://www.youtube.com/watch?v=wPYhsfshciw . I noticed the tachometer in one shot was at 3000 RPM. There was a Saturn with the tach running!
I later called Jim in Chicago and he returned my call. He is a very gracious, smart guy, also an auto technician like myself. We talked about the tach and what he did to get the tach up. He explained that the white signal wire from tach to PCM (or to Ignition module on a 1994) was simply wired to a tach drive box he purchased from KTA. http://www.kta-ev.com This box will, take a speed sensor analog signal and change it to a square wave. It can also open or close a circuit if the RPM’s reach a certain threshold. This could be used to shut off the Advanced DC motor if over revved. Sounds good.
Next I called Wistar Rhoads at KTA, the owner and a mechanical engineer. I went over all that has transpired with the tachometer, tests and conversation with Jim Dawson. Wistar explained the “box” was a TD-100. This timer controller is not available anymore. The guy that used to build them stopped putting the TD100 together.
We talked some more about what might work including getting advice from an electronics person or engineer.
My last call today was to a local electrical engineer Chris Whitten. He believes it will be easy to do with a “pick processor” but might even work with a simple transistor. He asked what the hertz were.
Well today I had scoped a 1999 Saturn at my shop and at 1000 RPM (see two images above) the scope shows 10 pulses in 100 milliseconds or 100 pulses in a second. Chris said he will get back to me tomorrow.
I added these last 3 images off the same 99 ICE Saturn tach white wire.
The second scope image is reading KOEO ( key on engine off).
The third image is at 1000 RPM and 10 milliseconds scale.
The forth image is at 2000 RPM.