Classic tachometer quit.

Behind the tach gauge and under the center counsel.

The alternator you took off, was probably ok. Mine does what you talking about and has done it since new. My tach has never worked right either but my dads works fine. Drives me nuts!

 

At one time I had a eratic speedo prob. turned out to be a lose conection at the back of alt where the hot & ground wires connect to the alt. check the eye terminal where the wires are crimped. make sutre they are real tight. also check all cable connection & cable jumpers for lose or corroted cables.

 

Mine works fine for a week or two and then the next week all over the place. Same goes for the speedometer.

 

if all the other connections check ok, check the sensors on the tailshaft of the trans, if I'm not mistaken in 98 they still used 2 sensors, 1 for tach, 1 for speedo.

 

I have a DD60 in a 93 pete, had all kinds of erratic electrical bugs. The alternater tested fine, but gauge would show low, the same at the batteries, Turns out the starter was not grounding to the block, ran a cable from the starter to the block, that cured the problem. I have 2 sensors at the output shaft on the tranny, speedometer and the cruise.

 

Tach sensor for freightliner is on the passenger side of the bell housing(for the flywheel) at 4 or 5 o' clock.

 

On that model S-60 the tach gets its signal from the ECM. You didn't say if the engine runs ok or not. Assuming it runs fine then the ECM is seeing the crank sensor and the cam sensor. If one of those dies the ECM will not let the truck start or it will run horribly. Which means the problem lies between the ECM and the tach head.

Note- you said you changed the tach head. Does the new one have DIP switches on the back and did you set them correctly?