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Banging noise from np241 then no 4wheel drive

We updated the tune for the vss input but I'm running into problems, I changed vss and input sensors, runs better it shifts and what not, but my speedo is off by like a ton when I'm going 5 mph it says I'm going 45, when I get to 50 mph (verified speed with different app verified that my tune has the correct reluctor wheel count in ) or abouts and I'm crusing and if I give it gas to like pass some one or to down shift it just stops accelerating for like 5 seconds then comes back to life. I'm not sure what to look for next. Help!
 
was looking through this thread - mind taking 2 steps back and giving some background on the truck for any of us that are not up to speed?

it was a CUCV... then it was some kind of gas motor (fuel injection?) with a 4l80 and mechanical speedo np241?? THEN it went bang and you lost 4wd...

this issue with the speedo and tone rings... where does all of that fit in? and the transfercase is mechanically ok now? or you still have something major wrong in there and will need to split it?
 
scouthead said:
was looking through this thread - mind taking 2 steps back and giving some background on the truck for any of us that are not up to speed?

it was a CUCV... then it was some kind of gas motor (fuel injection?) with a 4l80 and mechanical speedo np241?? THEN it went bang and you lost 4wd...

this issue with the speedo and tone rings... where does all of that fit in? and the transfercase is mechanically ok now? or you still have something major wrong in there and will need to split it?
Click to expand...
Sure let me start from the beginning.

In July I went to the beach and I was was in 4 high, the truck ran great. At one point I switched to four low and my truck started bucking and I heard some popping and I thought my t case broke. In reality when I got home I found the shift rod linkage broke the cotter pins and my truck popped out of 4low into 2 high ( I drove home from the beach in 2 High no problems)

Further investigation showed that I had never hooked up the vss to the wiring harness which the PCM needs to tell the computer when I am in 4 low (it requires a vss signal from the t case and a grounded switch to let the PCM know I am in 4 low) I hooked up the vss pulse emitter (Dakota Digital aftermarket set up) to the PCM. the truck ran like doo doo and I opted to install a factory VSS set up with a tone ring and a pulse emitter sensor. Doing that improved the truck and it ran but still had issues. I replaced the wiring harness from the new LS harness to the VSS sensor with a new cable and new vss sensor and new input sensor in the 4l80. I test drove it yesterday to see if the sensors made a difference and it did but it still does the fall on its face thing and has no power when I get to about 50 mph. When tried to down shift and gain speed and after a few seconds it comes alive again but it repeats this process over and over so i had to slow down and it drove normal home. I'm not sure where to look next. my tuner thinks its a tone ring or sensor issues (we verified that the tune in the PCM has the correct amount of pulses from the tone ring. its set at 40) I'm going to double check the wiring that goes from the harness plug to the vss plug is good and has secure connections. After that im lost on what to do.
 
So it's an LS swap, then. Stock LS PCM?

Scan the thing and see what speed it's reading.
 
So you had one of these pass-through sensors, with the signal going straight to the PCM? https://www.dakotadigital.com/index.cfm/page/ptype=product/product_id=57/mode=prod/prd57.htm

Then you converted to NP241 40-tooth tone ring and had the tune adjusted from 8000PPM to 40 PPR. The PCM also needs to know the ratio between your driveshaft speed and vehicle speed, which is essentially the differential ratio and tire size.

Assuming speed sensors are the issue, datalog the transmission input speed, engine RPM and vehicle speed. I think you can also log current gear if that helps. If the signal integrity is bad, you would see the VSS dropping out or erratic. If the math is bad, you'll get the wrong ratio between those speeds. It does shifting based on VSS, not RPM, so an error can make it shift too early or you can only reach a shift point as very low throttle position. Does your speedometer work? On your drive, hold some known speed steady for a while so you reference that to a known speed at the same time (like GPS).

1734096757586.png

You've wired the 2 wires from the VR sensor directly to pins 20 and 21 on the PCM or is one of them grounded?
 
Blue85 said:
So you had one of these pass-through sensors, with the signal going straight to the PCM? https://www.dakotadigital.com/index.cfm/page/ptype=product/product_id=57/mode=prod/prd57.htm

Then you converted to NP241 40-tooth tone ring and had the tune adjusted from 8000PPM to 40 PPR. The PCM also needs to know the ratio between your driveshaft speed and vehicle speed, which is essentially the differential ratio and tire size.

Assuming speed sensors are the issue, datalog the transmission input speed, engine RPM and vehicle speed. I think you can also log current gear if that helps. If the signal integrity is bad, you would see the VSS dropping out or erratic. If the math is bad, you'll get the wrong ratio between those speeds. It does shifting based on VSS, not RPM, so an error can make it shift too early or you can only reach a shift point as very low throttle position. Does your speedometer work? On your drive, hold some known speed steady for a while so you reference that to a known speed at the same time (like GPS).

View attachment 493092

You've wired the 2 wires from the VR sensor directly to pins 20 and 21 on the PCM or is one of them grounded?
Click to expand...
I had the emitter that just gave a pulse and not the pass through, I want to run the 1991 vss cluster. as far as I know the harness is pinned correctly (not a cheap e bay harness) I'm using the torque app and I can see the speed bouncing all over to me that makes it seem like a signal issues, I was going to check my harness to vss cable for any issues and a possible loose connection. The tune is set correctly at 40 for the vss, 37 inch tires and 4. 56 gears. (this is from my tuner who I trust) I am looking. Into getting. Hptuners soon though.
 
Yeah, check the harness routing (make sure it's away from ignition), condition of any splices. There is a + and - assigned. I don't know if reversing them is very bad. You could twist the wires to improve noise rejection. If you can, unplug the sensor and measure the resistance of each harness pin to ground. Sometimes the - gets tied to ground and you effectively lose half of the AC signal that the PCM would normally see. A VR sensor makes an amplitude that varies with speed, so the circuit is finding the zero crossings of that signal and tying one-half to ground screws that up.
 
Blue85 said:
Yeah, check the harness routing (make sure it's away from ignition), condition of any splices. There is a + and - assigned. I don't know if reversing them is very bad. You could twist the wires to improve noise rejection. If you can, unplug the sensor and measure the resistance of each harness pin to ground. Sometimes the - gets tied to ground and you effectively lose half of the AC signal that the PCM would normally see. A VR sensor makes an amplitude that varies with speed, so the circuit is finding the zero crossings of that signal and tying one-half to ground screws that up.
Click to expand...
How do I measure resistance? I have a meter
 
Vss wiring in OEM harnesses are always twisted pairs. Sometimes even shielded. So if the harness is not twisted you could be getting EMI interference from other electrical devices near the harness. That could make the value erratic for sure.
 
Cucv_M1008 said:
How do I measure resistance? I have a meter
Click to expand...
Set the meter to Ohms. If it doesn't autorange, you'll want to select something low like 10's of Ohms. Put one lead to a good ground source like a harness ground screw and put the other to the pin in the connector.

Seeing 100's to 1000's of ohms is good - probably a resistor in the PCM. Seeing a couple of ohms is bad - like it's spliced into a ground somewhere.
 
Blue85 said:
Set the meter to Ohms. If it doesn't autorange, you'll want to select something low like 10's of Ohms. Put one lead to a good ground source like a harness ground screw and put the other to the pin in the connector.

Seeing 100's to 1000's of ohms is good - probably a resistor in the PCM. Seeing a couple of ohms is bad - like it's spliced into a ground somewhere.
Click to expand...
Thank you.
 
So after all that. I purchased HP tuners, I was going over my tune with my tuner and he was showing me everything and when we checked the Gear Tire Wizard it was still off now at 185 inches of tire (no wonder it would not accelerate and fall on its face. we fixed the tune and double checked my tires were set to 37 Inches and I re uploaded the tune and now it drives normal (Blazer Bash wooooo) I now need to finish the shifter set up install and get my 4wd back. it drives normal and shifts 1234 gears now and no falling on its face at 45 mph. i guess my speedo wires are good. I'm not mad at my tuner it was a mistake on hp tuners software not his.
 
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