Alright so after reading everything I can find on the internet Im at a loss so here goes. When I go to start my truck and place the key into the run position Im not getting my normal lights like check engine, GEN, trans temp, wait to start and water in fuel.(ocasionally the flicker). Then when I start my truck i get no tach. If i drive the truck by manually shifting and it gets to normal temp sometimes the tach comes on and everything else starts working like normal or once its warm I can shut the engine off and restart it and everything works. (This is not a deffinate eveytime but most of the time when the truck is warm it works) I have changed the ESS and the ignition switch with no change. So I started checking voltage and continuity between the PCM and sensors and everything seem to check out ok. So the next thing i did was with the engine running I pulled the connector on the PCM closest to the motor then reconnected it and when i ddid this it starts working and so far everytime i do this it fixes the problem. What do yall think? PCM or bad connection really hate to spend the money on a PCM if its not going to fix the issue. Truck is a 97 3500 dually 12v auto stock 256k milles 2wd.

 

Sounds like a crank sensor. If the computer doesnt see the engine turning the tach doesnt work nor will the voltage regulator.

 

As Mopar2ya mentioned, Engine Speed Sensor is very often the cause of the no-tach/no-charge/no-shift symptoms you mentioned. It's interesting that you don't get the normal pre-start lights, though. I wouldn't think those require a flat-line ESS signal to be activated.

I would ask if you were certain you have the correct gap on your ESS, but the fact that resetting the harness/PCM connecter temporarily fixes the problem would point in other directions. Have you tried a "wiggle test" with the harness disconnected from the PCM? Do a continuity test between the PCM connector and each wire in the ESS connector, and wiggle the PCM end and see if it breaks continuity? The best way would be to have banana leads and backprobe connectors for best continuity, but if you have alligator clips that go over the tips of your meter probes then you can do almost as well with those and some sewing pins from the dollar store or wherever....

Also make sure the ESS/harness connector is solid and crud-free. There's a big connector down around the toebox on the driver's side. I don't have wiring diagrams available at the moment so I don't know if anything ESS-related goes through that, but if so - check it and make sure it's tightly connected.

If none of those yield anything, then it seems like there may be a bad connection in the PCM itself - perhaps cracked/broken solder joints? There are places that repair PCMs - a member on another forum just got his back from being repaired. He was having very similar issues to yours. I can send you a link if needed (not sure what the forum rules are about posting links to a business or other forums).

Do you have a factory service manual?

 

Thanks for the reply, I have done the wiggle test had good contnuity to all my sensors then checked for the correct voltage at the PCM and the ESS everything checked out with the key on so i started the motor and nothing. Then i stumbled onto the fact that if I unplug the pcm while its running and plug it back in it comes back to life (everytime so far). I do have a service manual and wiring schematic. Everthing so far is pointing at the PCM, if you can post that link.

 

Here you are: SIA I have no experience with them - just passing on a place used by a member on another forum. So far, he's happy with his repaired PCM.