oem tach wiring
 

Hi, I have a 94 dodge diesel. The tach, alt. and airconditioning quit. Went with external reg. that work's fine now. Jumpered the airconditioning pump and that work's. How do I make my original tach work, and is there a way to get the airconitioning pump to work with in the system. Thank's to ALL you knowledgeable folk's. Skinnysanta

I am having similar issue with my '97. when pcm does not get a signal from tach, charging, cruise, etc shut down...

If you fix your tach, the other stuff should come back on.
Not sure how to do that though... I think there is an engine speed sensor that sends to tach?

Yep, you need the ESS for that stuff to work. Before you go crazy spending money though, locate the connector for the ESS on the front of the engine, if it is covered in oil, take it apart, clean it out thoroughly with contact cleaner, allow to dry and put back together. Many of us have had that solve the problem........ If it is clean, then you need to start diagnosing.

oem tach wiring
 

Hi Guy's, thank's for the info. I am confident the computer gave up, atleast part of it. My cruse control and shifting is fine. The tach, air conditioning and charging quit. I have fix the charging with an external reg. I fed power to the a/c pump and it work's, but I would like to get my tach back and have the a/c work thru the system if possible. Thank's For the help. Skinnysanta.

What you describe does not really point to a computer fault.
I think that you should really take a look at the ESS and also measure if you have +5V on the sensor supply.

Pat and Alpine are correct - you need to look at the ESS and connector. If you are going to check the sensor-supply voltage, be sure you have a high-impedance meter - 10 MegOhm impedance is preferable (certainly no lower than 1 MegOhm if you want to risk going that low). This will prevent the voltage test from possibly drawing too much current through the PCM.

Hi Guy's, do the cruise and shift work off of the ESS? The truck shift's and the cruise works. I have a digital, will that work? Thank's Skinnysanta

Cruise control and transmission shifting do need the VSS (vehicle speed sensor) but not the ESS.

The ESS is used for:
Tachometer in the dash
Charging system
AC system
Intake heater relay control

IIRC the ESS shares the +5V supply with the IAT sensor, and a short to ground of the +5V will lead to the same problems

Skinny, the fact that the meter is digital does not mean it has the proper impedance - you need to look up the specs for the meter. You can get a super-cheap digital multimeter anywhere but that doesn't necessarily mean its good for testing PCM-controlled circuits. I have read that "most" digital multimeters are high-impedance - but I wouldn't take that for granted that where my PCM is concerned. If you don't have the literature for your meter any more, look it up online or contact the manufacturer. Am I overcautious? Maybe - but if I'm troubleshooting something I don't want to take the chance of damaging my PCM with my test equipment and adding that to the issues.

oem tach wiring
 

Hi Guy's, Thank you all for the help, now I have to go to work and use all of you Guy's info. Again, thank's a lot, Skinnysanta