VSS signal testing

[Tonyorlo]

How do you all scope signals? Splice into the wire, or probe a connector? I am having a hell of a time getting my speedometer to read, which is effecting the shifting. I?ve rang all the wires out from the vss, and trans connector to the tcm. I?ve also tried multiple sensors. Speedo only shows half speed, and won?t go any higher than 25. What I see in HPtuners matches what the Speedo shows. I?m very desperate, any help would be greatly appreciated.

[DanN]

What is your vehicle config?

If you're getting any speed signal at all, it's likely the wiring is OK. You could have an issue with the sensor or with the speedo config. You should upload your tune and a log.

If you want to scope the speed signal you'll need some sort of scope. At a minimum, you may be able to use a normal AC multimeter to measure AC voltage between the VSS wires. I have used a cheap Aliexpress oscilloscope for looking at various signals, including VSS.

PXL_20210630_162420894.jpg

[Tonyorlo]

The vehicle and hardware is

07 trailblazer ss awd
4l80e converted with bw-4485 t case from Escalade.
E67 ecm
T42 tcm
4l60 to 80 adapter harness from pcmofnc
Ecm and tcm both from pmcofnc set up for my truck and trans.

I have a scope, I’ve never used it before but it was too good of a buy to pass up. I might have a log, I’ll report back when I can.

Thwnks for the reply.

[Tonyorlo]

h2.hplh1.hplh.hpl

Some logs I uploaded the other day.

[DanN]

h2.hplh1.hplh.hpl

Some logs I uploaded the other day.

Is the temperature data accurate? Like, you're running at 235F+?

[Tonyorlo]

It was at that moment. The e fans weren’t kicking on, helps to have the ground landed. That was the first drive after swapping the trans.

[Tonyorlo]

pcmofnc tune.hpt

the tune

[Alvin]

pcmofnc tune.hpt

the tune

This isn't our tune. this is just what is needed to get the 4l80 going.

Is the wiring all stock? The wiring for VSS are normally a twisted pair to cancel noise.

[Tonyorlo]

I’m in no way saying anything you guys did is wrong. I labeled that as pcmofnc tune just for my records, and I posted it just for information.

The wiring is stock. I am going to try to count the teeth on the reluctor wheel today. That’s about all that I have left.

This may be frowned upon, but I’m at the point of needing it to work. Do any of the aftermarket gps senders put out enough current or voltage to signal the ecm?

[gtstorey]

Have you checked the wiring diagrams to confirm where it is getting speeds from? Different applications pull it from and send it to different places.

Have you checked where the vss sensor is reading? Is that a toothed wheel with maybe a different tooth count?

GPS signal would have too likely put out the correct voltage range and frequency also.

[Tonyorlo]

I have checked the diagrams through alldata. It goes trans VSS - TCM - ECM - Speedometer. I have rang every wire out from the trans to the TCM, and load tested the VSS wires. The TCM connector doesn’t seem to hold the pins very well, but I have checked and re checked that the VSS wires are seated correctly in the TCM connector.

The GPS sender does put out the correct 4k ppm sine wave signal, but like it’s been mentioned the signal can be lost. Another thought I have is to run an external sensor on the driveshaft. I haven’t tried scoping the signal, but everything is good to the sensor. I’ve tried multiple sensors, I have a hard time believing that I have that 3 bad sensors. Here’s a video of what it’s doing.

[Alvin]

Are the VSS wires twisted together? They should be. If not it won't work at least reliably.

[Tonyorlo]

They are twisted to both ends. Maybe 4” has been straightened at the tcm, and roughly the same at the sensor for where I had to repair the broken wire at. What’s odd to me is that nothing changed after finding the broken VSS low signal wire. I have not verified the reluctor ring tooth count, but I have pictures of the ring on the shaft when I had the trans apart, so I know it’s there.

[gtstorey]

If you have a transfer case, is it not pulling the speed from there? Or is the gearing different between the transfer cases if the speed is pulling from the transmission.

[Tonyorlo]

If you have a transfer case, is it not pulling the speed from there? Or is the gearing different between the transfer cases if the speed is pulling from the transmission.

Well I think I have messed up. 100% my inexperience. I came across a thread mentioning that no post 96 4wd 4l80es have an output reluctor ring. After searching those terms I am almost positive mine does not have a ring. So this wild goose chase was for nothing. The 4wd trucks vss is in the tail shaft of the transfer case.

Sooo, I am going to have a ring machined for my pinion flange on the axle, and I will use the original sensor. I will look into it much further, but is the original ring a 40 tooth ring with no missing teeth?

I’m sorry to create this thread and bug you folks about this. Thank you very much for the help!

[Tonyorlo]

Question.

Is there a trick to twisting wires for a VSS? I am making a diff mounted reluctor ring and need to extend the wire. I just don’t see twisting it and staying you know? Maybe put a very very little amount of heat on it?

[Alvin]

put two wires in a vise, other side in a drill. Run drill until they are twisted up tight.

[ns158sl]

put two wires in a vise, other side in a drill. Run drill until they are twisted up tight.

There use to be a spec for twists per foot, 25? Also, if you are using cross-link wire, run over it with a heat gun to "relax" the insulation. We use to do this at OEM R&D to make prototype CAN harnesses.

[Alvin]

Probably unknowingly broke some rules lol. Done a lot that way and never had any trouble. Never put much thought into it "eh.. that looks pretty good." We have a in house wiring guy/dept. I think we are using TXL. I can ask if you'd like.