EDIS CTO signal - scoped

shauer · Post by **shauer** » Sun Mar 05, 2006 9:55 am

I've read all the EDIS/tach threads I could find and could not find this information so I wanted to post it and see what others thought.

I just got my MSII with EDIS up and running ignition only for the time being. When I first tested EDIS w/o MS connected my tach worked fine using the CTO (EDIS pin 11) signal to drive my VDO tach. When I hooked up MS I got a good RPM signal in MegaTune but my tach stopped working. In the process of bringing up the EDIS functionality I enabled multispark and my tach started working again at idle but would stop working around 1500 RPM.

I put my scope on the CTO signal and found that I had a nice clean square wave of about 13 volts but I found that the pulse width changed. Here is what I measured:
- With MS disconnected, CTO pulse width was 1.0ms
- With MS connected, W/O multispark. CTO pulse width was 500us
- With MS multispark, multispark CTO pulse width was 1.0ms
- With MS multispark, higher RPM (non multispark) CTO pulse width was 500us

It seems pretty obvious to me that my VDO tach works fine with the wider 1ms pulses from CTO but the shorter 500us pulses are too short to trigger the tach.

Has anyone else observed this? I'm planning on backing into a schematic of my VDO tach to see if there is an R/C filter I can modify to make it sensitive to the 500us pulses.

Mike Simard · Post by **Mike Simard** » Mon Mar 06, 2006 3:45 am

I wonder what the IDM pin 2 would show, if you had an EDIS8 you wouldn't have the CTO pin and would have to use pin 2.

shauer · Post by **shauer** » Mon Mar 06, 2006 4:19 am

I did not look at the IDM signal, I might try the next time I'm working on the car. I read somewhere on this site that someone traced the CTO and IDM and found they were connected together within the EDIS module.

I traced my tach input circuit and found a two stage r/c filter going to the input of what I'm assuming is a 555 timer IC. I simply reduced the R values to about 1/3 of the original value and the tach is working great.

I know several people on the board have fixed their tachs the same way, I was just surprised to see the different pulse widths on the CTO line.