knight1976

I recently put a Weber 45 dcoe on a s4 motor and installed it in a s5 chasis the issue i am having is the tach not working at all. I am running a msd box for my leading and running the trailing off the distributor. I am using s5 coils and cannot get the tach to function. I did a search and found the yellow wire off the coils goes to the yellow with blue striped wire on the driver side harness. I am running the trailing coil ( only need one to fire trailing) off the distributor ignitor; not the ignitor on the trailing coil. I tried connecting the negative terminal of the trailing coil to the yellow with blue striped wire on the 4 wired connector. When the key is turned on and off the tach will flinch then not function when the car is running. What am i missing or am i just getting misinformation on the forum?

Thanks in advance fellas

Icemark

Not quite gettingwhat you are saying for your triggering of the trailing coils (not trailing coil... but "trailing coils"!).

The Tach in the dash uses a summed signal from both trailing coils firing. This is provided by the Yellow/Blue wire at the trailing coils.

It sounds like you are not firing both coils, or they are firing at the same time (from your description) but then if you were firing them at the same time, then you might as well just not fire them, as they are probably doing nothing to help combustion. Either way fired at the same time would only provide half the signal that you would need. In which case its probably easiest to just remove the stock coil and use something aftermarket.

knight1976

Actually it is only one trailing coil (removed one coil since two are not necessary to fire through the distributor). It is set up like a first gen with only one trailing coil. I am using both the leading coils fed by the msd 6a. Make any more sense?

My main question is what wire on the s5 harness do i need to wire to the trailing coil's negative terminal? I need to get my tach working so i dont over rev the motor as i dont have a rev limiter on the car anymore.

Icemark

I thought the MSD6a had a tach output... have you tried that???

Running only one coil is probably not enough signal. You might be able to use a shunt filter (diode and a 4.5 or 4.75k resistor) and ramp up the signal off the neg of the coil... butI still think that the signal will be at least half of what it needs to be.

RETed

The trailing coil (pack) is not going to function correctly and shoot out a proper signal out the yellow tach wire if you're not using the stock ECU!!!

Yeah, the MSD should have a tach output lead, so just connect that.
We've done it before, and the stock tach does respond to the MSD tach outputs.



-Ted

Icemark

Ah there you go.. Ted knows much beter than me on those set ups.

RETed

Yeah, the yellow tach signal only triggers when the toggle triggers, so if you got no toggle in the trailing coil pack, you get no tach output.

Toggle can only be triggered by the stock ECU, or something that can mimic the stock ECU like a Microtech or Haltech.


-Ted

knight1976

Man you guys rock!! THanks for the help ill hook that up straight away

knight1976

tach works great. thx guys

RETed

Congrads!


-Ted