Help With Stock Tach

302mustanglx · Dec 10, 2013

Hello fellas I want my stock rpm in my cluster to work again. I did a carb swap and would like my bbk cluster to work like it was. I have gone and spliced into the green and yellow wire behind my cluster #11 and ran it to my negative side in the coil and on my msd box tach signal hook up and still nothing plz help I already have after market gauges but hate jumping in the car and having all my stock gauges dead except my fuel. The car is a 88 mustang 5.0 gt what am I doing wrong here guys ur help is much appreciated

madspeed · Dec 10, 2013

Moved to tech

jrichker · Dec 13, 2013

Basically you shot yourself in both feet by doing a carb conversion.
Unless this is a track only car, a carb conversion is a very bad idea. Track only is OK for carb, it just isn't as versatile and can't be tuned on the fly. It's take the carb apart, chase all the little tiny pieces around while your hands get soaked in gas. Then change the jets and hope you don't lose any of the pieces in the process of putting it back together.

If you still had a working EFI system, I could tell you exactly how to connect the tach and what color wire to look for. Since you have done who knows what to the car and wiring, there is no telling where the proper connection point is now.

liljoe07 · Dec 14, 2013

302mustanglx said:
Hello fellas I want my stock rpm in my cluster to work again. I did a carb swap and would like my bbk cluster to work like it was. I have gone and spliced into the green and yellow wire behind my cluster #11 and ran it to my negative side in the coil and on my msd box tach signal hook up and still nothing plz help I already have after market gauges but hate jumping in the car and having all my stock gauges dead except my fuel. The car is a 88 mustang 5.0 gt what am I doing wrong here guys ur help is much appreciated

Check your fuses first. You are using the correct wire to tap into. Fuse number should be 18.

jrichker · Dec 15, 2013

liljoe07 said:
Check your fuses first. You are using the correct wire to tap into. Fuse number should be 18.

The Tach feed comes off the TFI circuit and expects to see ~ 10-14 volts signal input. How do I know that? I attempted to check the calibration on a stock tach to see how accurate It was. See http://www.stangnet.com/mustang-for...ach-to-be-accurate.805785/page-2#post-8333134 if you want to know more.

That definitely will not work well when connecting to the negative side of the coil. There is a very large voltage spike when the TFI module turns off between cylinders. The stock wiring contains a 22k resistor in the dark green/yellow IDM signal line. That probably isn't a high enough resistance if the tach was connected to the negative side of the coil.

liljoe07 · Dec 15, 2013

If I remember correctly. The 22kohm resister is only inline with the ECM and the coil, it may be inline with the TFI as well. I'll have to check when I get back in a few days. I may be wrong. But I don't recall seeing anything between the coil and tach.

jrichker · Dec 16, 2013

liljoe07 said:
If I remember correctly. The 22kohm resister is only inline with the ECM and the coil, it may be inline with the TFI as well. I'll have to check when I get back in a few days. I may be wrong. But I don't recall seeing anything between the coil and tach.

Diagram courtesy of Tmoss & Stang&2birds

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