alright...either im an idiot or the car is possesed. help me solve my tach mystery

[BOWTIEKILLR]

alright lets see how retarded i am, can anyone figure this out?...85 gt with 89 motor and mass air conversion, kenne bell blower, msd6al(the tach function in the box doesnt work, we are raplacing it next), other than that...mostly stock stuff. the in-dash tach stopped working, tested it with engine analyzer and analyzer read correctly, so we bought another stock one, hooked it up and low and behold, no worky. i take the original tach, wrap some wires on the back and put little alligator clamps on the other end, pop the hood and attached it to the battery, the coil and a ground, exactally as it would be in the dash. no worky. hook up the engine analyzer to the clamps on the tach wires...and the analyzer works. pop the hood of my car(88 gt) hook up the tach to my battery, coil and ground, and it works!!! hook up the engine analyzer to the clamps on the tach, just like on the 85, and it works just fine. go back to the 85 and repeat what i did on my car, and it doesnt work. anybody know if im doing something wrong? we even disconnected the msd box and ran it and it didnt work, as well as hooking it up to the msd and nothing. i must be missing something...

 

[jrichker]

Has the tach worked before or did it suddenly fail?

 

[4dStang]

alright lets see how retarded i am, can anyone figure this out?...85 gt with 89 motor and mass air conversion, kenne bell blower, msd6al(the tach function in the box doesnt work, we are raplacing it next), other than that...mostly stock stuff. the in-dash tach stopped working, tested it with engine analyzer and analyzer read correctly, so we bought another stock one, hooked it up and low and behold, no worky. i take the original tach, wrap some wires on the back and put little alligator clamps on the other end, pop the hood and attached it to the battery, the coil and a ground, exactally as it would be in the dash. no worky. hook up the engine analyzer to the clamps on the tach wires...and the analyzer works. pop the hood of my car(88 gt) hook up the tach to my battery, coil and ground, and it works!!! hook up the engine analyzer to the clamps on the tach, just like on the 85, and it works just fine. go back to the 85 and repeat what i did on my car, and it doesnt work. anybody know if im doing something wrong? we even disconnected the msd box and ran it and it didnt work, as well as hooking it up to the msd and nothing. i must be missing something...

I had a 95 cobra with afunky intermittent tach. Turned out to be the tach itself after replacing it with amonster autometer. Electronics go bad sometimes and replacing the tach will basically swaping the gauge panel with a repaired or used unit. Just my experience. Try a different tack for temp purposed. If it works, I'd bet on a bad factory tach.

Chris

 

[4dStang]

Has the tach worked before or did it suddenly fail?

Good point!

 

[BOWTIEKILLR]

it worked ever since we got the car...about a year, then it died. we tried a second tach and nothing. they both work hooked up on my car, but not oh his...its like the coil doesnt read on tachs...only on the engine analyzer

 

[BOWTIEKILLR]

ttt

 

[Mustang78]

check fuses??

~Mark~

 

[BOWTIEKILLR]

did it. its wierd..its like the tach cant read off the coil but the analyzer can

 

[jrichker]

You have a probable wiring problem. The 89 harness used a wire from the TFI to the computer that also triggered the tach. That wire used a 22,000 ohm resistor in line with the tach lead to drop the voltage down to a level suitable for the tach. I can't seem to locate it on the 89 model diagrams I have, but it used the Dark green/yellow wire to feed signal to the computer & tach.

The 85 tach diagram I have shows the 22 K Ohm resistor as part of the tach assembly. Using both resistors may drop the voltage so much that the tach can’t sense it correctly. It may be worth your effort & time to run a wire directly from the ignition coil negative terminal to the tach.

 

[noslow1986]

reply

i had the same problem i needed a msd tach adapter works fine now dont know if this helps u

 

[BOWTIEKILLR]

. It may be worth your effort & time to run a wire directly from the ignition coil negative terminal to the tach.

THATS WHAT I DID!!!!!!!!! it didnt read hooked up just like you are saying. but the same tach and wires worked hooked up the same on my car

 

[dragnazz5.0]

it might have something to do with the msd box. since the tach runs off the negative side of the coil and the coil is grounded by the msd box to give it the multiple spark it could be screwing with the electronics of the tach. that is unless your car has an msd box and it works fine hooked up to your coil.

just a suggestion