Speedometer reading slow in two-wheel drive?

Hello [email protected],

Wow. Seems like these shops aren't testing anything, just throwing parts at it. Let's see what we can come up with.

Sounds like a wiring issue to me because it is so intermittent. But then completely stops when in four-wheel drive.

I know you had the transmission completely rebuilt with all new electronics, but I have seen new sensors bad out of the box. I have also seen techs keep parts and not install them. Not saying anything about any of the places you have worked with, but we need to think about and eliminate all possibilities.

The speed sensors on the tail shaft of the transferase for the rear drive shaft is the same as the one for the forward shaft.

The front driveshaft speed sensor is a 2-wire sensor that sends an A/C voltage signal to the transfer case shift control module or (TCSCM). Speed sensor signals are used by the (TCSCM) to determine variations in speed or slippage between front and rear axles. When the selector switch is in auto-4WD position the (TCSCM) compares front and rear driveshaft speed signals. The transfer case shift control module will determine rate of transfer case clutch according to speed sensor signal.

I think when it is 4WD, the computer is using the signal from the front driveshaft because the rear signal is unusable for the speedometer.

We can switch the 2 sensors and see if anything changes.

I will include removal and installation of both sensors for you.

The image I am adding is showing 3 sensors, but my information tells me there may only be 2.

So, I am going to add instructions on how to remove all 3. I would remove the front one and remove one of the rear ones, switch them and take it for a ride. See if anything changes.

If not, put the rear one that was removed back where it came from and remove the other rear one and put in front of the transfer case and then move the front on to the other rear sensor we just moved and take it for a ride and see if anything changes.

Let me know if I can clarify anything.


Also, let see if I can get more information.

1) Is the engine light on or any other warning lights?

2) Are there any codes? Some codes won't necessarily turn the light on so if you don't have a scan tool, going to a place like AutoZone or similar will help as they will read the codes for free for you. Have them write them down. We are looking for codes that begin with P-(Powertrain) or U- (For computer data issues, like the wiring or modules.

Any past codes some of these shops have found and told you about would be helpful as well.

Since it works in 4WD but has an issue in 2WD, it is possible it is the sensor or the wiring. Either from the sensor or one of the computer data lines these modules use to talk to each other. There is actually a Technical Service Bulletin that pertains to some of your symptoms.

I will add the link so you can look it over. There is a section in it that describes some of the symptoms associated with this issue. Take a look and see if you have had any of the symptoms listed other than the ones you already told me about.

Here is the link:
https://static.nhtsa.gov/odi/tsbs/2012/SB-10043829-6448.pdf

For the fixes listed in this TSB, I think we should hold off on them for now. This is because those connectors are a pain to get out and put back in without breaking it or pushing a pin out. We also don't know how many other symptoms match this TSB yet and we don't want to cause any other issues from taking that off if we don't need to.

Once we rule out the sensors, we will need to test the wiring, starting at each sensor.

For this you will need a multi-meter and some pins to back probe the connectors so we can test them when they are plugged in.

Here is a link to a multimeter I own. Usually, a good one is over $200 but this one is accurate and easy to use:

https://www.homedepot.com/p/Klein-Tools-600-Volt-Digital-Multi-Meter-TRMS-Auto-Ranging-Temp-MM420/320822810

Takeaway's:
Seems to me like the speed sensor for the front driveshaft works fine since the problem disappears when in 4WD.

After making sure the sensors are completely fine, we will have to make sure we are getting the right voltage to the sensor and the signal going back to the computer is correct.

When the sensors are disconnected, take a look at the condition of the connector. Any corrosion or wires bent, loose, dirty anything but a clean connector that might be causing an issue.

Hopefully, we will have some codes to go off of to lead us in a certain direction as well.

Thank you,
Brendon

Brandon, thank you for the reply. I just got under my truck, and I have attached an edited image. The sensor marked in green (#4) was supposably replaced, the one in red (#3) does not appear to exist on my transfer case, and if i understand you correctly the one marked in yellow (#5) is the one that reads when the 4wd is engaged? So should i try replacing the one in green(#4) again(i can do that myself) and see if they possibly did not replace or maybe got a bad one…I know they said they replaced it but i also know they kept telling me it was the instrument cluster but i had already replaced it with a used one(that has since been tested and worked perfectly), then i send mine out and had it overhauled and still didn’t help…and when they pulled it in to replace the speed sensor(#4) they magically had a new one in stock and it took them less then 5 minutes and i know it never went up on the rack…i can’t tell if it’s new or old. I ask the guy who rebuild my transmission (different shop) and he said that he didn’t replace anything on transfer case except the gasket where it attached to the trans…he is also the one recommended to front 4wd sensor since i told him the other shop had already replaced the one on the rear (#4). He also told me that to replace the 4wd sensor (#5 i am assuming) that he would have to remove the exhaust, crossmember, and lower the transfer case to get to it…which would cost another $200-$300 plus the cost of the part and no promise that would fix it.

Hello [email protected],

I'm sorry I didn't get your image. Could you try to send it again?

If (#4) looks like it wasn't replaced, I would test it and the wiring going to it. However, it is a $40 or so sensor which is the same cost as a decent multimeter which you will need to test it. So, if you don't have a multimeter, you could try replacing it. If it doesn't fix it though you will need the multimeter anyway.

You can replace it on the ground pretty quickly so the shop may have not wanted to put it in the air. Probably had an hourly guy swap it out on the floor. But it is possible it wasn't done.

The other sensor (#5), that sounds odd you shouldn't need to remove all of that to get to that sensor. But that's ok, let's just keep that on the back burner for right now because I think that one is working. So better to leave it alone for now.

If they did replace that sensor, then we need to test the wiring.

Let's see if we can get the right image of the transfer case so I know what we are working with.

To help with that I have added an image of a label with a bunch of codes on it. It should be in your glove box. These are RPO codes or Regular Production Option Codes. See if you can send me an image of this label. It will tell me everything your truck came with.

If you can't send a photo of it, I am looking for one of these codes:
For the transmission:
(MYD, MYC, MW7, MT1, M70, M33, M32 or M30)

Then for the transfer case:
(NR3, NQH, NQG, NQF, NP8, NP2, or NP1)

Also, can you tell me what the 8th digit from the left of your VIN is? It should be a 0, 3, or J for the 5.3L.

I would hope everyone that has worked on your truck wouldn't overlook that sensor (#4) but you never know.

Try removing the connector and inspecting it. Be careful it might be brittle. I have added some images of what you are looking for.

Also look at the wires coming out of the connector. Sometimes they can be bent at an angle, and it breaks or partially breaks the wire, not always the insulation though.

Thank you,
Brendon

Did you get the pictures this time? I’ll have to check the VIN code tomorrow. I do have a multimeter, and i picked up a new sensor today (#4).

Good morning, [email protected],

Thank you I did get the picture this time.

Yes, if you could look for that label for me, I would appreciate it. That way I know exactly what we are working with.

I have added some tests with the multi-meter for the speed sensor #4.

A/C Voltage Test:

So, to test the sensor, it will need to be installed and plugged in. Then back probe the connector with pins, a bobby pin or sewing needle will work. I use t-pins myself, so I can connect an alligator clip to it. They can be found at arts and crafts stores. I will add an image for you. A couple paper clips will work as well.

The truck will need to be jacked up under the rear axle and put on jack stands. This way the axle wont hang. We want the axle pushed up as if it were sitting on the ground, so the driveshaft angle is the same or close to it.

I have included a video on how to jack up the rear of the vehicle using the shock mounts which will be better especially if you are using a smaller jack like the one in this video.

Here is the video for a small jack:
https://youtu.be/W0gyKPUkjI8?si=bKLqoLAbU2yxLPpy

Some wheel chocks behind and in front of the front tires will also be needed to keep it from rolling.

With the vehicle jacked up and secured, you can now connect the multi meter to the back probe pins and turn the meter to A/C voltage.

It is best to have a friend help you since now we will have to start the truck and put into drive.

It should slowly start to accelerate, while it does this, watch your multimeter and the reading should climb slowly and evenly as the vehicle accelerates. The faster the speed this higher the voltage.

Try not to let the tires go too fast. Apply the brakes if it starts to go to fast. Because there is no inertia from the vehicle actually moving you will need to hit the brakes slowly. It will take a lot less braking to stop the wheels with it off the ground.

Resistance Test:

Place your meter on ohms.

This test you will disconnect the sensor connector and put the leads from the multimeter, one on each pin of the sensor.

Temperature will affect the sensor's resistance. The hotter the area is, the higher the reading will be.

I would park it and let it cool off as much as possible. You should be between 1420 ohms at 77 degrees Fahrenheit and 2140 ohms at 302 degrees Fahrenheit.

You can use the new one you have and compare the resistance from the old one to the new one keeping in mind the temperature of the sensor.


***NOTE***
The test I sent have different colors they refer to for the test leads. You meter more than likely has a RED and a BLACK lead. Looking at the tests you will notice a positive (+) and negative (-) symbol under "Connect Test Leads".

For both of these tests you will always put RED on (+) and BLACK on (-).

Vehicle Speed Sensor Pin Assignment: (See Image)

Purple with a white or orange stripe will be positive (+).

Light green with black or white stripe will be the negative (-).


I have added both tests below. Since you already have a new sensor, and it wasn't too expensive, I will leave it up to you whether you want to test the old one or just replace it.

Let me know how it goes and we can take it from there. We just need to eliminate this sensor as a possibility before we test anything else.

Thank you,
Brendon

I just bought a 2007 Suburban with this exact issue. I have replaced all VSS sensors, but the issue persists. I'm starting to think it could be a bad transfer case. Did you find the solution to this?

It sounds like the transfer case control module is not switching the VSS sensor input from 4wd low to 4wd high. I would swap out the module which should fix the problem. Here is the location and how to change it out. Check out the images (below). Please let us know what happens.