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Take it to someplace such as autozone and have them try retrieving any codes it might have stored, see if they can establish a connection with the computer.
Might know nothing about it but give you a starting point, what system/s to look at to correct the problem if it will respond.
One thing I do like over OBD II the KOEO and KOER self tests OBD I offers. OBD II has its strong points yea but those couple tests do come in handy.
OBD I (EEC-IV) computers this vintage are suffering board level failures, small but real chance your problems are computer related.
If all attempts fail to connect with the computer? I'd suspect it might be suffering like issues the EEC-IV before it can and do suffer from these days.
Might be as simple as a failed sensor or chafed wires someplace in the harness.
I vote for bad rear end bearings.
By the way does the odometer go off and on when this happens.
Note that the VSS is source of the voltage for the PSOM and the RABS. It generates its own voltage.
I read that as couldn't connect with the computer, rather than connected with it just fine but no codes where found.
There should be something, did you take it for a ride with it connected viewing the live data stream looking for anything amiss? Check Freeze frame if supported for that model?
Corresponding blips in the TPS readout as speed output as logged when the trucks speedo weirds out while in gear for example?
OBD II and the software I run in my laptop will do that (What is logged varies by Make and model but yet to find one that the basics are not all covered), does his reader support live data logging?
If so it might give you something solid to go on.
I don't have a powertrain circuit diagram for a 1996 half ton to see what coloration/effect its being in gear or not as far as the computer goes for that model but doubt its anything beyond neutral safety input? What I mean is doubt it knows what gear has been selected only that it is not in neutral.
"OBD 2" yea but likely stuck in nowheres land someplace between old and new....modern a better choice?
I read that as couldn't connect with the computer, rather than connected with it just fine but no codes where found.
There should be something, did you take it for a ride with it connected viewing the live data stream looking for anything amiss? Check Freeze frame if supported for that model?
Corresponding blips in the TPS readout as speed output as logged when the trucks speedo weirds out while in gear for example?
OBD II and the software I run in my laptop will do that (What is logged varies by Make and model but yet to find one that the basics are not all covered), does his reader support live data logging?
If so it might give you something solid to go on.
I don't have a powertrain circuit diagram for a 1996 half ton to see what coloration/effect its being in gear or not as far as the computer goes for that model but doubt its anything beyond neutral safety input? What I mean is doubt it knows what gear has been selected only that it is not in neutral.
"OBD 2" yea but likely stuck in nowheres land someplace between old and new....modern a better choice?
It connected fine, but we did not do a live stream road test. I'll try that the next time I have off days. Very good input and ideas.
Does a manual trans even have a neutral switch? Even if it does, the speedometer reads fine if it's in any gear with the clutch in and coasting. I tried that in a road test last night. Clutch in, slight revs, speedo reads normal. In neutral, slight revs, speedo reads normal. Only when in gear, on the gas and worse (as in 0 mph reading) when under a load do the symptoms pop up.
Yea check subfords suggestion first, was my second thought reading your OP but you didn't say anything about clunks, bangs or grinds or anything at all about rear end noises.
I figured electrical in nature and hung with it.
If had enough play in the bearings, the carrier moving around to cause what you describe I tend to think you'd suspect rear end issues due to the noises coming from it and before it got to the point the speedo was flipping out.
It does fit though, rolling no load it's fine but then under load things migrating in the cage even just a little bit can cause those problems.
I vote for bad rear end bearings.
By the way does the odometer go off and on when this happens.
Note that the VSS is source of the voltage for the PSOM and the RABS. It generates its own voltage.
2X on possable bad rear end bearings. Under load the VSS tone wheel could move away from the VSS sensor. That will cause a speedo needle to hit the stop quickly as the sensor is not reading the tone wheel.
Other problem is bad connections, somewhere in the VSS circuit is either opening up(most probable) or shorting out.
Best way to check for that is testing the VSS signal going to the PSOM with a voltage meter set to AC mode for voltages under 20v and watch the meter when problem happens.
I forgot the voltages per MPH, there is a thread about it but I am not finding it.
I found a receipt in the glove compartment for a rear end rebuild in May of 2015. I wouldn't suspect that, but at this point I'm out of ideas. No unusual noises, clunks, play etc., but I'll check it out on my off days.
Other than this issue the truck drives great and feels really solid. It doesn't even have interior rattles over bumps in the road.
I vote for bad rear end bearings.
By the way does the odometer go off and on when this happens.
Note that the VSS is source of the voltage for the PSOM and the RABS. It generates its own voltage.
The odometer never blinks off, it just doesn't calculate the mileage correctly.
Ok, so we finally got the rear end apart tonight. Lots of stuff happening, life etc. Sorry for the long down time. At any rate, this is what we found. One of the shims was broke and in the other pic the bottom bolt on that cap was completely loose, and when I say completely loose, I mean not even threaded in.
So here are my new questions. I thought the arrows on the caps were supposed to point out toward the wheels and not in toward the diff? Maybe I'm thinking wrong, but after finding what I found, I'm questioning a lot of stuff that was done during the above posted "rebuild"
After looking closer, the bolt on the drivers side cap that was actually tightened (top bolt), is warped. So now I have to get new bolts. I also noticed that the cap on that side looks almost bent. I don't know if my mind is playing tricks on me, but I don't think it's out of the question considering only the top bolt was "torqued".
So now my next question is, is it even possible to replace caps?
Lead Head, thank you for the reply. My only concern is after posting those pics, I did a search for the arrow direction and every picture I pulled up, the caps had the arrows facing out toward the axle tubes.
I think it's going to be easier to set the garage and truck on fire.......
Oh, and the ring gear is chipped in places. Assuming this rear end can even be rebuilt, I have all the parts for the rebuild minus the bearings. I was waiting to make sure I got the right axle bearings. Little did I know Curious George was the one that did the rebuild last year.
Yes the arrows point out on each side.
You need to go to a salvage yard and get a another rear end.
Make sure you open it up before you buy it and make sure it looks like a good one.
