Last Sunday on the way to the rifle match the speedometer, tachometer, and air conditioning quit, as did the alternator - voltmeter read battery voltage instead of 14 v charging voltage.

Truck kept running and it got me home (55 mile) after the match.

I bought the truck new in 1995 and it has 693,991 miles on the odometer.

No fuses blown or anything simple like that. Started to worry that the computer - PCM / ECM whatever it is called - was fried.

Logged in here and started searching. Several threads pointed to the crankshaft position sensor (AKA engine speed sensor). With no signal from that, the PCM does not believe the engine is running and so will not allow speedo/tach/AC/charging.

One post said that of the three wires, one should be ground, one should be 8 volts sent from PCM, and the third would read output of the crankshaft position sensor. I got 8 V and ground on the two correct-colored wires so I took a chance.

Found one online at Pure Diesel Power and ordered it. Came late Thursday, direct from Cummins, in a Cummins box. Swapped it in bright and early yesterday morning and everything works as it should.

Gamble paid off. Not tearing hair out trying to get a re-built PCM or figuring how to make the speedo, tach, and so forth work with the PCM out of business.

Anyway I am writing this to say a big THANK YOU! to y'all. You bailed me out yet again and I'm deeply grateful.

I feel like a bit of a parasite. I come here for guidance when it hits the fan, but knowing so much less than most people here, I usually have no information or help to contribute.

So - next time someone searches for a thread about speedo tach alternator air conditioning PCM CPS this thread will pop up and maybe be useful to them.

 

Did you also inspect the harness wiring to the ESS? It has been known to rub through and short to ground - it's possible that happened in this case, and you moved the wiring enough in the process of replacing the ESS that you removed the short. If so, then the opening in the insulation is still there. So even though the new ESS cured your ills (well, your truck's ills), I suggest inspecting the harness wiring to the ESS to verify there are no rub-throughs.

 

Thanks for the tip!

I did look at all the wiring I could reach, which is quite a lot. Also checked all the grounds, batteries to frame and to body and other component ground wires. In retrospect I'm glad I did it but at the time I was looking very hard for something wrong, and worrying that it was the computer and thinking how I could not let a bad circuit board do my truck in.

In one of the threads that came up when I was searching the Forum for leads someone made reference to a powertrain diagnosis manual, not the same as the Factory Service Manual that I got in 1995 when I got the truck. I was able to find a copy of that manual and bought it, but it has not arrived yet. Reading it will give me a better idea how all the sensors and wires work, which should improve my confidence that I can fix whatever goes wrong, and maybe even keep the truck on the road if the PCM does indeed fry and I can't repair it or find another one.

But yes, a chafed wire / short to ground would be exactly the kind of thing that could flummox a guy like me. Learned over the years to look - really look - at stuff, to make sure the problem is not immediately visible.

Not knowing what is inside the crankshaft position sensor - but thinking there are no moving parts - I can not really understand how one could wear out. I suppose a wire could break (internally) from vibration or flexing, but somehow I wish it was possible to see inside the thing to understand the failure mode. Maybe I can dig up a description of what is inside it and how it works. The picture of me looking at it after taking it out the first time (put it back in to see if it was magically ok - no luck) - reminds me of a cat picking up a dead snake he'd just killed, to get a better look at it. "What the heck is this thing?"

 

Although sometimes/often referred to as a crankshaft position sensor, the sensor has nothing to do with crankshaft position. The correct name is Engine Speed Sensor (ESS). The harmonic dampener (a.k.a. "balancer") can be bolted on in any of four positions relative to the crankshaft, so the dampener (and thus the notches) has no relationship to crankshaft position and the sensor (a Hall-Effect sensor, I believe) just "reports" when the notch in the dampener passes under it so the PCM can calculate engine RPM.

If the factory service manual doesn't have troubleshooting procedures for the ESS, the powertrain manual should - I expect the manual calls for resistance testing across the pins (in absence of a 'scope). I keep thinking I should backprobe the ESS connection with my scope so I can confirm for my own knowledge that the wave-form is a square-wave, but I've never gotten around to it.

Check your PMs...

 

Yes, you are right about it not really telling the crankshaft position; the manual calls it that though. - No, I'm wrong. In the Diesel section the manual says Engine Speed Sensor, in the gas chapter it says CPS. Sorry!
Thanks for the link about Hall Effect sensors! It is exactly what I needed and will re-read it and anything else I can find about Hall Effect, because I want to understand how it works and how it can 'go bad'.
Seems like it performs the same function as the cam in a distributor, pushing the breaker points open at the right time to break the primary circuit in the coil and induce high voltage in the secondary.
As a young person I did not "believe' in things that I could not see - like electrons - but getting a shock from an ignition coil made a believer out of me.
So, I can see burned contact points and know why there is no spark, but looking at the engine speed sensor without proper instruments (eyes only) is not enlightening.
Thanks for your help!

 

Gas engines care about crankshaft position so the ECM/PCM knows which injector to activate and, if distributor-less ignition, which plug to fire. That applies especially to individual-cylinder-injection systems but also if it's a throttle-body injector like the 80s-90s GMs (I don't know if Dodge had these). As you know, our diesel PCMs need to know only engine RPM for controlling alternator charging, air conditioning, and transmission/torque converter control - thus the difference between "CPS" for gassers and "ESS" for diesels.

 

Thanks, makes sense!

And - "As you (I) now know our diesel PCMs need to know only engine RPM for controlling alternator charging, air conditioning, and transmission/torque converter control..." - because I flat out didn't know until I read it here on the Forum. There are no words to express my gratefulness to people like y'all, who are willing to share knowledge and get a guy out of a jam.