Erratic Cranking Tach Signal / High rpm dropout Issue

yarin · Post by **yarin** » Sat Apr 29, 2006 7:22 pm

Porsche 914 2.0. Mechanical advance dizzy with Pertronix. No Ed Capactor or extra filtering. Coil powered off fuel pump relay.

MS II V.3.0 firmware 2.33, Fuel Only. RPM triggered off negative side of coil.

I finally solved most of my injector issues and another problem creeped up on me today.

Last couple of days I've been having issues starting the car. I had the coil powered by the fuel pump relay. Whenever the relay would latch the coil would energize and issue a false RPM signal to MS. Then it would think the engine is running and inject some fuel along. Sometimes i could hear the fuel pump cycle 6- 7 times before it settles. I believe I solved this by moving the +12V power for the coil to the switchen ignition output. Presently more often than not the engine cranks, catches for a few seconds(>~1000rpm) and then and it stops sending fuel. I attached data logs ofthe issue. I can see huge spikes in my RPM and drops to zero.

2nd problem(possibly related): I found that exceeding 3500rpm generates a false tach signal and shuts down, probably hits fuel cut and kills fuel. I attached are data logs of this as well. This is my new issue that just creeped up today. I tried revving the engine with no load, same issue. I tried moving the coil +12V from the fuel pump relay to a constant+12V source, same difference. I tried a new dizzy rotor, same issue. Check the logs.. my tach signal drops to zero and the engine dies.

I don't have any input filtering(Ed capacitor, R57..) and haven't had issues like this up until now. The tach signal looks clean otherwise... maybe its noise above a certain RPM... i really dont know. I replaced the coil 6 months ago, its possible that after a few hours of driving time it became a little noisy. OH yeh.. one last thing.. Pertronix, could that be giving me problems?

Where do i go from here?

I hit my maximum quota of 1MB so here is a link to the data plots from Megalogviewer illustrating the problem.

http://coewww.rutgers.edu/~yarin/datapl ... oblems.zip

yarin · Post by **yarin** » Mon May 01, 2006 7:16 am

R57 is not installed. Should I install ~750Ohms to dampen the feedback from the coil?

yarin · Post by **yarin** » Tue May 02, 2006 7:31 am

Installed a 1K pullup from (-) coil to +12V. Fixed problem.

Powering the coil off the fuel pump relay causes erratic fake tach output that fools MS into thinking the engine is running. I'm going to move the tach power to the ignition key switch.

Anyone read this stuff?

PSIG · Post by **PSIG** » Tue May 02, 2006 7:46 am

Yes, thanks. I have a possible future MS project with Pertronix so I'm following. Glad it's coming together! Do you see issues with the Pertronix signalling MS (Extra) only and MS firing the coil directly?

yarin · Post by **yarin** » Tue May 02, 2006 8:40 am

PSIG wrote:Yes, thanks. I have a possible future MS project with Pertronix so I'm following. Glad it's coming together! Do you see issues with the Pertronix signalling MS (Extra) only and MS firing the coil directly?

No issues with pertronix. For now i'm only getting a tach signal off the (-) coil. Running MS II fuel only.