I am using a MSD ignition system (controller and dist.) so the ecm is not getting any info from the ignition system, as a result the tach isn't getting a signal. Does any one know which wire in the wire loom I should connect the MSD tach lead to? Any help would be appreciated.

 

Your cars tach actually gets it's signal from the coil. The ECM gets it's signal from the ignition module. Two completly seperate systems, wiring wise. One was not the result of the other. I'm pretty sure this applies to all third-gen f-bodys.

Um...if your fuel injected, your ecm MUST have an rpm signal, for many obvious reasons. And since you've neglected to include ANY information about your vehicle, it's impossible to say "It's the [insert color] wire".

Do you have an igniton module? 4-pin, 7-pin, or 8-pin? I'm pretty sure the purple/white wire is the ECM's rpm signal. On my 7-pin setup, there is a brown wire that goes from the dizzy's cap, to a small capacitor mounted to the firewall, then to the tach gauge.

 

It would help to post information about your car.
But generally, the white wire is what carries the tach signal to the tachometer.

Hope this helps.
Lou

 

Thanks for your quick reply's guys. I am sorry I didn't supply more info. This is a 1988 Formula that was originally a 5.0 TBI car (auto) I have a 383 SBC in it now and the stock ECM has nothing to do with the motor, EFI is controlled by a Megasquirt 1 ECM and the ignition is completely MSD stand alone system (controller and distributor). I am not at my car now but if I remember correctly the original distributor had a 2 pin connector and a 4 pin connector. I just need to know which wire to connect the MSD tach lead to or if I can still run a wire from the coil to the wire loom tach feed. If you need more info I can look at the dist. wires when I get home
Thanks

 

Locate the white wire as mentioned below.. The wire is located in the harness comming through the firewall next to the break booster. Connect the wire to the Tach output on the MSD controller.