I have a 1998 one ton with diesel. My batteries were old and the other morning it would not start. Charged the batteries and headed to the store. Noticed that the tach was not working and alternator appeared to be not charging. Replace both batteries (5 years old!). Everything worked great for a day and then while driving, the tach quit and noticed the amp meter was below 14 volts. Took the alternator off and replaced it. Same problem, not charging, according to amp meter and tach not working. Driving to town and thinking about having to take the alternator off and exchange it, it all started working again. Now sometimes the tach works (always when the voltage on amp meter is above 14) but suddenly tach will stop and voltage is below 14. Beginning to think I have a ground problem but can't understand why the tach is affected. Everything else working great. Anyone have a similar problem?

 

first thing i would check are grounds. than go from there.

 

From what I have read here it could be the CPS (crank position sensor). It is supposed to tell the computer the engine is running so the tach will work and the alternator will charge. Try a search - I don't remember all of the details.

 

Thanks Sidewinder, I think you have the right answer. I'll have the Dodge place check the CPS. I always felt something had to connect the charging and the tach problem. Appreciate your help.

 

Hum!!
1998
No tach == no charging.
Believe sensor is mounted above vib damper.

 

That sensor is called the Engine Speed Sensor or ESS.

 

Thanks for all the replies. For your information in case this happens to your diesel, I took it to the Dodge dealer. There were no codes in the computer and as I said before, it ran fine. The tach, alternator and cruise control would not work. They hooked it up to the diagnostic machine and found no info coming from the cam position sensor. Replaced it ($197) and everything is fine.
Thanks again and good luck.
Ron - 98 Dodge 1 ton dually w/cummins - 93 3/4 ton Dodge with cummins - 96 VW Passat with TDI (40 mpg since the day I bought it new - now over 130,000 miles)