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Speedometer reading while parked

O

Oldude

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Hi all. Been looking the forum over a bit, decided to join cause I see a lot of stuff being discussed that I involve myself with, "for fun".

New truck, to me. 99 C3500 dually 7.4l auto trans.

Speedometer reads while parked. Goes up as high as 50mph while revving the engine in neutral, while parked. Drop it in gear, reads 0.

While driving, it reads pretty normal under a load. When coasting or light throttle speed moves around a bit and shifts real late, even hunts gears under light throttle near normal shift point.

To me it seemed like a bad ground, so I cleaned and tightened all ground straps I found, even added one from trans case to frame. No change. I don't think it's a bad cluster because the scanner verified the speedometer. Also changed out the VSS, no change.

Parts that were thrown at it by previous owner and mostly verified by me:
ECM
engine harness
TPS
MAF
EGR valve
Distributor
VSS
Trans internal harness

I verified the ground from the neg batt to fender was solid, and the 2in1 strap near the starter from frame to body and frame to back of cyl head was re seated.

I'd love to hear opinions about this issue, or anything else if you choose...

Dennis
 
Welcome to the forum @Oldude I assume your trans has a 4L80E? there will be two speed sensors on it. what I am thinking is the shielding on the twisted pairs going to the sensors are possibly open where interference could be causing the erratic readings. if you are getting the same erratic readings using the scanner I would inspect the harness up around the trans mainly from the two sensors. you should see the two wires going to each sensor wrapped in some foil type shielding with the wires twisted, will also have some plastic loom covering it all up.
 
dbrannon79 said:
Welcome to the forum @Oldude I assume your trans has a 4L80E? there will be two speed sensors on it. what I am thinking is the shielding on the twisted pairs going to the sensors are possibly open where interference could be causing the erratic readings. if you are getting the same erratic readings using the scanner I would inspect the harness up around the trans mainly from the two sensors. you should see the two wires going to each sensor wrapped in some foil type shielding with the wires twisted, will also have some plastic loom covering it all up.
Click to expand...
Thanks dbrannon79.

It does have a 4L80. I'm not aware of 2 speed sensors. It is a two wheel drive, I addressed the VSS near the tailshaft housing on the driver's side. Where would I look for the second?

I did get a mostly complete donor harness and I was just going over what you talked about, a twisted pair with an apparent shield covering it.

I'll split the cover off the donor harness to see where the shield hooks to. I'm assuming it would be a data ground out of the ECM.
 
Here is a pic of a 4L80E the two speed sensors are on the side of the case on the drivers side. the input is just above the shift shaft and the output is above the main harness connector. hope this will help.

1702428820324.png
 
depending on what scanner you are using, you can possibly look at the live data from each speed sensor and possibly isolate which one is "acting out" it might be simply a bad connection at one of the connectors. or one of the sensors has a cracked plastic housing allowing trans fluid into the sensor. both of the sensors are held in with a single bolt and will pull straight out from the case with the bolt removed. they are a magnetic hall effect type sensor with the trans having a reluctor ring inside. if ether one has any visible signs of a crack or have anything peculiar, replace them.

one might be out of whack but not enough to cause any codes on the ECM.
 
dbrannon79 said:
depending on what scanner you are using, you can possibly look at the live data from each speed sensor and possibly isolate which one is "acting out" it might be simply a bad connection at one of the connectors. or one of the sensors has a cracked plastic housing allowing trans fluid into the sensor. both of the sensors are held in with a single bolt and will pull straight out from the case with the bolt removed. they are a magnetic hall effect type sensor with the trans having a reluctor ring inside. if ether one has any visible signs of a crack or have anything peculiar, replace them.

one might be out of whack but not enough to cause any codes on the ECM.
Click to expand...
@dbrannon79,

Nice info. I didn't realize we had an input VSS. I'll crawl under and check it in the am. I'm guessing they are the same sensor, I only see one part number listed, however it lists the same number for my 99 yukon and the 4x4 transfer case has a different one altogether.

I'm using a Bluetooth dongle with torque pro app. Not sure if I can look at that live data.

Thanks
Dennis
 
the app can read the data as long as you have all the right PID's in the app for your rig. iirc both sensors are the same except for 4x4. one of the sensors are removed from the trans into the transfer case but perform the same function
 
dbrannon79 said:
the app can read the data as long as you have all the right PID's in the app for your rig. iirc both sensors are the same except for 4x4. one of the sensors are removed from the trans into the transfer case but perform the same function
Click to expand...
Still not sure what Torque PIDs are.

In my motion control world (mostly CNC machine tools) a PID defines an algorithm that closes a position loop that you tune so the axis doesn't over or undershoot it's desired location.

Time to dig in a little deeper.
 
I use an app called Car Scanner Pro on my iphone, not sure if it's available for android, but for me it does most things I want. if it's available, you can use the free version to verify it will work with your Bluetooth adapter. it should show live data in a graph form. you might explore other apps and see what works best for you. not all apps will actually show you trans data though. you might end up having to borrow a shop scanner or investing into something that works for most of your vehicles

for GM a Tech2 is the go to. you can find knock-off chinese GM Tech2's on ebay that work just as the real one so long as you don't try updating it. lots of folks rave about them until they update it, then GM zaps them to be non-functional.
 
dbrannon79 said:
I use an app called Car Scanner Pro on my iphone, not sure if it's available for android, but for me it does most things I want. if it's available, you can use the free version to verify it will work with your Bluetooth adapter. it should show live data in a graph form. you might explore other apps and see what works best for you. not all apps will actually show you trans data though. you might end up having to borrow a shop scanner or investing into something that works for most of your vehicles

for GM a Tech2 is the go to. you can find knock-off chinese GM Tech2's on ebay that work just as the real one so long as you don't try updating it. lots of folks rave about them until they update it, then GM zaps them to be non-functional.
Click to expand...
@dbrannon79

Ok, found and installed the standard pids for gm n such. Made a custom display and see the output VSS is giving the bs info. Not sure how to attach a screenshot yet...

@ak diesel driver,

I'll try the serpentine belt removal. Are you thinking alternator noise is getting intp the VSS wires?
 
Problem sorted

Started thinking about the info the input vs output that Torque Pro pids were giving me.

It makes sense the input shaft will spin freely in neutral, stops when dropped in gear before vehicle moves, two identical VSS's on trans ...

VSS' were plugged in backwards!

Now on to calibrate the speedo.
 
Glad you found the issue. I didn't think of that but I suppose the connectors have enough length to reach to ether or!

there is a thread here somewhere on calibrating the speedo after a gear change. not sure if 99 model would have the VSSB behind the glove box like the older ones do. mainly you have to de-solder the bridge pins on the circuit and install a set of dip switches so you can adjust it.

use a GPS on your phone to verify a various speeds.
 
dbrannon79 said:
Glad you found the issue. I didn't think of that but I suppose the connectors have enough length to reach to ether or!

there is a thread here somewhere on calibrating the speedo after a gear change. not sure if 99 model would have the VSSB behind the glove box like the older ones do. mainly you have to de-solder the bridge pins on the circuit and install a set of dip switches so you can adjust it.

use a GPS on your phone to verify a various speeds.
Click to expand...
Thank you for your valuable help.

The key for me was knowing there was 2 speed sensors. I've been building engine and transmissions since the 60's. Obviously never done a 4l80E. I also learned about the wild world of pids and the vast amount of information I was missing, not knowing about them.

This one doesn't have the buffer behind the glove box, fortunately the previous owner had, in his flailing about, trying to shotgun approach throw parts at this problem, bought and included a Hypertech 3 tuner that took care of the tire calibration and shift points. I'm down now to normal problems like leaking EGR actuator and stuff like that common with a truck of this age with nearly 200k miles on it. He gave up on it years ago and stacked it out in his yard after throwing 5k worth of parts and voodoo fixes at it. He was convinced that it had an electrical issue in the harness. In fact it had a pair of lazy downstream O2 sensors causing the issues. After employing lots of hunch fixed, all done wrong, it compounded into a fiasco. Every fix had it's issues like he replaced the injectors and the "mechanic" missed re attaching the pcv hose from the throttle body causing a huge vacuum leak and sending the short term fuel trims swinging into the +40's, and another yahoo tuned the ignition and didn't get one spark plug wire attached firmly causing yet another intermittent miss... It just went on and on.

Anyway, I have a pretty darn nice running 3 owner big block crew cab dually for a budget price.

Again, thank you for sharing your expertise and knowledge with an oldude!
 
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