Well, I fired up my 1997 one-ton dually service truck this afternoon, warmed up the cummins diesel and backed up. So far, so good.
But when I put it in Drive, it really struggled to get going up a slight grade. I realized it WASN'T in first gear!

Getting it on the road, I thought the tranny might just need to warm up or something. But it continued being locked into 2nd gear.
Strangely, the Tachometer also crapped out on me. I could hear my 5.9L cummins revving up about 2000 to 2250 or so, but the tach said 0.

Even stranger: both the tranny and the tach came back to life somewhere while I was driving!

I would've chalked that up to Gremlins or some other devil and gotten on with my life. But it happened again and continues to operate in 2nd with no tach.

So far, I've found out that the tach works off a Crankshaft Position Sensor aka Engine Speed Sensor (ESS) and that gadget uses a 5VDC supply (from the Engine Computer?) using 3 wires. And apparently I'm in "Limp Mode" now, stuck in 2nd gear.
Is it possible that these 2 phenomena are related? If that sensor went out, would it kill the tach and ALSO put me in Limp Mode?

Unfortunately, my rig got stolen a couple years ago and they must've only watche the GTA movies, cause they sure didn't know how to PROPERLY hotwire a vehicle. Caused an ENGINE FIRE that burned up the main wiring harness. About $5K later, and 40 solder-splices I had to do myself, I got it running again (though I lost the Speed Control). No problems till now. Maybe one of those solder connections - like going to the ESS - went bad?

Thanks so much for your help!

 

Most certainly. If the PCM doesn't know what engine RPM is, it doesn't know what gear to put you in. 2nd is the default.

 

Good chance the issues are related but probably not from the sensor going bad. I'd guess something is drawing 5V sensor supply down. Try disconnecting the connector at the transmission, the transmission will still work but needs to be shifted manually. If the tach still acts up the problem is likely somewhere in the wiring harness, if not it's something in the transmission.

 

With all that wiring going on I would most certainly check over the wiring, take out junk yard harness's should be super easy to find, i wouldn't even bother trying to diagnose the harness, these trucks are getting cut up everyday for Cummins conversions. There's hardly anything to a 12v harness. I believe a 96 to early 98 harness will work for you

 

Here's one on ebay, i know nothing of the seller or the item, but id call that a fair price

 

Thanks to all of you for the tips. You helped me narrow it down to some wiring fault related to the ESS (Engine Speed Sensor). I'll post the results after my "exploratory surgery".
I've got a telecom tracer (signal generator and probe) so shouldn't be too hard.

 

Found the ESS (Engine Speed Sensor) and traced the wiring using this diagram. Colors matched up and there was a 3-wire connector about a foot up from the sensor, so I unplugged it.
Tested for Hot or Ground on all 3 contacts with engine OFF. All 3 showed continuity to Ground.
Tested with Ignition in ON position: The Hot contact (5VDC) was indeed Hot; the Ground contact was Grounded; nothing on the Signal contact, so...
Traced the Signal wire using a Telecom tester (Signal Generator and Probe) and it traced right back to the PCM, like it's supposed to.

So. Wiring seems ok! Then what in the world is going on?

This wiring checks out. So what's up?

 

Are you getting signal BACK from the sensor??

 

Hmm rethinking my earlier statement... Maybe the engine speed sensor is failing and as a consequence the transmission goes into limp mode because the PCM needs to know the RPM to determine when to shift into what gear.

 

Yeah, I'll have to check that. Prob check the Signal contact on that connector with the pointy end of my test light with the engine running.
If the Hall Sensor is bad, doesn't look like the easiest thing to get to. It's behind the radiator and fan.