I've owned another basket-case cummins a few years back which finally came to health after months of reading.

Last week I picked up a '94 3/4 cummins 2wd single cab 47RH automatic. The owner complained the transmission was going out and sold it for a pretty good price. I knew the TPS was acting up from 3-4 downshifts and 200RPM jumps. I installed a potentiometer in place of the TPS, set to 2.00v (where it stays) and she ran much better to fine for a day, steady 3-4 shift at 38MPH and OD lockup around 50MPH or so. I sprayed the engine and some of the lower chassis down with foaming degreaser and did a few small spot blasts at the car wash to find a small mystery leak and "The gremlin" appeared

At almost exactly 38MPH, the ABS and Brake light come on, it shifts to 4th for about 30 seconds or so then for no apparent reason, down shifts to 3rd, and acts like the (non-existant) TPS is bad. After a in-neutral "rolling restart", everything resets and starts all over again at 38MPH. I did not particularly look for it, but I don't remember those lights being on for the full-day drive home.

I have cleaned every single ground and battery cable (except the 2 that go from the battery to the block, which are for tomorrow), heavily doused them with di-electric grease and reassembled, took apart every connector, greased and reassembled including the PCM harness (Does the 94 PCM have a PCM-specific ground? I didn't see one and even took it off the truck to look).

The ABS issue stopped today for some reason, but my shifting issue is acting like the TPS again ?? but it's not there, verified the POT is at 2.00v and holding steady. I bridged the tranny sensor with a 1/4w 1Kohm resistor and have seen no change???

Anything ?? please??? I have only the 2 block grounds and the in-cab dash grounds to clean before I suspect the tranny itself or the PCM. It has had 2 rough-running starts over the last 40 or so, like a too-low idle/rough dieseling, but an on-off in the parking lot, and she ran fine.

I'm lost LOL. Where do I start

 

almost sounds like a "limp" mode. there are a few trans sites that have fixes for gov pressure problems that can cause the trans to go into a limp mode. i have no clue if this applies to you but its worth looking into. take it to a trans shop, most places will take it for a test drive and let you know initial diagnostics for free.

 

I haven't heard of the governer being an issue with the older trannys. I'll look into it, thanks. The previous owner had a tranny shop look at it, said the PCM, TPS, and valve bodies were all possibles, but they didn't sound too knowledgeable. I have heard of this type needing a band adjustment. When that's needed (I'll be doing a service soon) would the symtoms mirror what I've having?

Is the 47RH a in-truck valve body replacement, or am I looking at a pull-out and have a shop do it? I'm looking to swap in a rebuilt enging and trans within the next 2 years, so I just need her to live a while longer .


Thanks,

-Chris.

 

you should try pulling the crank position sensor off, clean it, re gap it and see if that helps. just my thoughts for a easy fix.

 

crank position sensor ??? i thought these were mechanical P7100 trucks ???

do we have tps and cps sensors and whatnot ??

 

lodal, thank you. What distance is the senor to be gapped to? I'm at my whits ends. I'm researching the Mystery switch now as well, sounds like a good passive upgrade anyway for towing. Anyone have a sticky for that? I'll look more tonight.

Angelic0, yes even a mechanical truck has a crank sensor. In our case it drives the tachometer and possibly something else, though I believe that's it.

I readjusted the POT to 2.5v which gave a 45ish 4th gear shift, and still electrical gremlin and at 3.0v 4th comes in at about 53ish, and he's still there, though I only had a few minutes drive and couldn't get the tranny hot enough to see if it'd hold 4th or not.

 

hmmm i just re-read your symptoms... sounds like your OD clutch pack is bad. our 98 would shift to 4th and randomly unlock. sometimes itd hold other times not so much and eventually it wouldnt shift to 4th at all.

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actually if i remember correctly, the previous trans shop assembled the clutch pack backwards. causing everything to flex and result in premature failure. eitherway sounds like an identical situation minus the dash lights