Don't know if I am putting this in the correct forum, but here goes....My company has a '95 2500 12 valve, bone stock with 375,000 miles. About a month ago I drove it downtown to do a job, and when I went to leave the a/c compressor clutch wouldn't engage, and at the same time the alternator quit charging. I dropped it off at our mechanic's shop the next day and he dug into it and found out that the PCM was causing both problems. Apparently the voltage regulator is built into the PCM and the PCM provides the ground for the a/c clutch as a way of controlling it. We decided to replace the PCM instead of rigging the truck to work......I haven't rigged anything on the truck yet. We ordered rebuilt unit from somewhere in Pa. and installed it, and had the same 2 problems. He went over everything again and came to the conclusion we got a problem unit and ordered another one. The guys in Pa. must have checked the first returned unit because it came with a note saying sorry we sent a bum unit. We received and installed the second PCM and had the the same 2 problems plus the tach and speedo aren't working now. My mechanic I know is top notch.....I've known him 20+ years and been in the business for longer than that. I'm a fairly decent mechanic myself and have been around the block a time or two. I guess my question is, is there something we are missing here? Battery temp sensor checks o.k. He has checked various other senders and sensors attached to the PCM....all o.k. Any help or advice would be greatly appreciated!

 

Sounds like the problem is the engine speed sensor or it wiring. It's located at the lower engine pulley.
If it doesn't see rpm it tells the PCM the engine isn't running, nothing electronic will work.
Sounds like you probably have two good PCMs now.

 

Hope this helps

The CPS is a Crank Position Senser and is a magnetic type senser mounted on the block below the water pump and just above the crank damper. It has about a 24 inch pigtail attached to it and plugs into the wire loom at the top front of the block. It senses the gap in the turning damper and sends these pulses to the PCM. The gap between the rotating damper and the CPS is .050 inch and usually is set with a business card or a bronze (non-magnetic) feeler gauge.

When the engine turns over and starts, the crank sensor sends a signal to the PCM that says the engine is running. The PCM turns on and it controls the alternator charging, speedo, odometer, tach, a/c. If the signal from the crank sensor is not sent... the pcm will not turn on. If you have a multimeter that reads hertz (frequency), you can verify the crank sensor signal at the pcm. On my 97 the PCM sends 5 volts DC to the CPS and the CPS outputs pulses to PCM. I've heard some trucks use 12 vdc or 8 vdc so you need to check the schematic.

No pulses from CPS to PCM and the PCM doesn't work.I just replaced mine and it fixed my problem. Cost about $101 from NAPA and $76 from Cummins. I got mine from NAPA cause Cummins could not get it quick enough for me at the time.

You need to first verify you are getting the proper voltages and gnds to the PCM. That is not the case for the trouble in this thread...there was a fuse blowing. On my problem I had all the voltages and grounds at the pcm but the 5 volts to the CPS (from the PCM) was being shorted out by my bad CPS.

Note: The ESS (Engine Speed Sensor) many people mistakenly call CPS (Crank Position Sensor) or VSS (Vehicle Speed Senser. The Crank Position Sensor is the equivelant Engine Speed Sensor part on the gas engines. When I asked for a CPS at Cummins they could not find it. I closely looked at the schematics and determined it was called an ESS. When I called Cummins back asking for an ESS (Engine Speed Sensor) they found it. However NAPA listed my ESS as a CPS (Crank Position Sensor). I know...its confusing.

 

Thank you for clarifying, I get really tried of the ESS being called the CPS.
Besides gassers the CPS is used on the 24 valve VP pumped engines to vary timing. It's located on the back driver's side of the engine.
Since it takes a gear puller to vary 12 valve timing there is no need for a CPS.

 

Thanks for the replies! I'm passing this info. along to my mechanic. I don't know for sure if he has checked the ESS already or not. I will post again with the final resolution of my problem.

 

Third PCM and still same problems. Mechanic thinks PCM people sent him back the same PCM he just returned....it had dielectric grease on terminals where as all previous ones were clean. I got in a pinch and had to drive the truck the last few days, so he put the original PCM back in. I installed the voltage regulator rig temporarily so I could use the truck. I've been noticing some interesting things while driving it....Original problems were no charge/no ac compressor(not sure about O.D.)....Problems now are no charge/no ac compressor/no tach/no speedometer/no O.D./no dash lights(check engine/WTS/water in fuel/trans temp)......Now for the things I've been noticing...friday a.m. going to work, tach starts bouncing for about 20 seconds and goes back to zero.....monday a.m. going to work dash lights previously mentioned as not working are popping on and off over bumps all the way to work.....monday lunchtime intake heater relays are popping on and off while driving, engine at operating temp and temps approaching 90. I noticed this by the volt gauge being dragged down and verified with meter and pulled one coil wire off of each relay to disable for the rest of the day....Picked up new ESS I ordered from Cummins and installed last night. Started truck and noticed still no tach and gave up for the night.....today going to work I notice the speedo is working! not intermittantly but just like new! It felt good to see something working right on this truck lately! I reconnected the alternator and everything back to factory just to see if the ESS had fixed anything or everything. I had tach and dash lights briefly working and speedo still works but nothing else. I think the tach and dash lights working briefly was just a coincidence. Drove it around and looks like I only gained the speedo which I also can't explain as I only changed the ESS yet unplugging the ESS doesn't disable the speedo? also tach bounced with ESS unplugged? I have one big question...I have read the spec for the gap for the ESS should be .050" or about the thickness of a business card. I wanted to use the card method just to make sure I didn't hurt the magnetic sensor of the ESS so I measured a business card with a dial caliper. My card was about .012" so I folded it to 4 card thicknesses which measured .049" and set the gap to this... I wanted to comfirm I was reading the caliper correctly so I used 2 feeler gauges (.035" and .015") which when stacked equals .050" and folded the business card until I had that.....Am I doing something wrong here? I can't imagine anyone has a business card that thick??? Thanks in advance and sorry for the lond post!

 

Every time I've swapped in a known good PCM it's never corrected the problem even when good mechanics and the dealer swear up and down it's bad.
Most common problem, and I'll bet it's yours too, is bad connections at the PCM. That's why bumps are making things work intermittently.
The female connectors on the PCM plug are super cheap, break and bend easily just by plugging and unplugging them. Get enlarged and cause a poor connection.
It's time consuming but has worked several times for me to go over every female connector carefully and bend them smaller with a needle.
With any broken ones you have to get creative.
I think the only way to replace the connector is to buy the entire main wiring harness for close to a thousand bucks if it's even still available.

 

WOOOOHOOOO! The truck is fixed finally, thanks to all here who pitched in to help! We actually fixed it last friday, a week ago today, I just haven't had time to post the end results until now. After Infidel's last post I started going over the pin connections at the PCM, tightening and inspecting them. I found an orange wire on pin #9 that when I applied pressure to it, the relays in the power dist. box would click, and I knew that when the auto shutdown relay switched on, the problems went away. So I figure poor connection at the PCM connector for that wire and I inspected, tightened and checked the connection to no avail.(original PCM in truck) In the end we plugged in the rebuilt PCM and presto! problems solved! Apparently the PCM had a break in the circuit feeding that pin and would make/break continuity when tweaked or over bumps. We checked with a meter from the backside of the connector(connector pulled apart) so we could see whether the pin connection was poor or if the poor connection was within the PCM. In the end, I think the ESS shorted in such a way that it killed the PCM or vice-versa. I saved the old ESS and will take some contiuity readings and compare with the new one when I get time. Again, thanks for all the help!