Erratic speedometer readings

[adventr]

ERRATIC, not the other word you guys are thinking. :giggle:



At low speeds, between 0 and 15mph, the speedometer reading can be very "jumpy". Sometimes, even when sitting still, the speedometer needle will randomly jump up to ~10mph and quickly fall back down. Other times, when driving slowly (say 10mph) the speedometer needle will quickly drop down to 0 and stay there. This causes the transmission not to shift out of 1st gear.... Have to come to a complete stop, then when I take off again it's back to working.

Thing's I know:

- It's not "just" the speedometer being weird, as I can monitor VSS on my OBD2 reader, and the random readings appear on there too.
- Hitting bumps, or jerking the vehicle around has no visible effect on the readings (was thinking maybe loose connection at first)
- Just started doing this out of the blue last week
- Once above 15mph, the jumpiness isn't really noticeable (cruising at 70mph is stable, and the speedo doesn't jump around)
- If going slow, and the speedo drops down to 0, no amount of cursing, swerving, accelerating, hitting bumps will bring it back to life. Will not shift out of 1st gear when speedo is sitting at 0. Only coming to complete stop and then taking off again will get it working.

- Thought it may be my crappy OEM battery terminals again, so I replaced them today with good heavy duty terminals that are rock solid. Made no difference.
- Visually inspected VSS on tail shaft of T-case, and re-seated the connector



Anyone had this issue? Anything else I should check before ordering new VSS? No check engine lights or anything like that.

 

[adventr]

May have just found my answer in an older thread, seen here:


http://www.hummer4x4offroad.com/forum/showthread.php?7004-My-truck-is-psychic/page2

Update: I'm told that the "bucking" / hunting for gears actually WAS the speed sensor after all. It apparently would short out when the T case (and thus the sensor) got warm. So far, good news from Mike @ Tustin Hummer

Now that I think about it, when the vehicle has sat for a while, the issues aren't apparent until driving for a while.

 

[LagunaH1]

Sounds like it very well could be the VSS acting up when it gets warm

 

[adventr]

Sounds like it very well could be the VSS acting up when it gets warm

Thanks for the reply!

That was what your problem ended up being, right? Just the VSS?

 

[LagunaH1]

As I recall, yes

 

[adventr]

As I recall, yes

Awesome, I'll toss a new one in there and see what happens!

 

[amrg]

Are you throwing a code?
The H3s have an ISS (internal speed sensor too) inside the trans. My truck shows im doin 50 mph while idling sometimes (but thats at the reader, the gauges are fine) but I have an ISS code and my truxck shifts harshly from first to second.
I noticed I get this issue when the trans gets really warm (like in very hot weather)

 

[adventr]

Are you throwing a code?
The H3s have an ISS (internal speed sensor too) inside the trans. My truck shows im doin 50 mph while idling sometimes (but thats at the reader, the gauges are fine) but I have an ISS code and my truxck shifts harshly from first to second.
I noticed I get this issue when the trans gets really warm (like in very hot weather)

No codes were being thrown that I could tell. Scanned it with my reader and showed no pending codes either.

Local parts store had a VSS in stock for less than $30.
Typically I only use genuine GM parts, but they were 5min down the road and it was cheap enough to try out to see if that may be my issue.
Pulled the old VSS and it looks fine, but if there's an issue it's obviously internal to the sensor.

We'll know in a bit if the new VSS fixes it up!

 

[adventr]

New VSS did not fix the problem.



Backed out of the driveway, started driving, speedo was jerky, hit 10mph and dropped back down to 0. Had to drive to end of my street in 1st gear because the trans doesn't shift when that happens.

Turned around, took off again and speedo was working and trans was shifting like normal.

 

[amrg]

What i forgot to mention earlier, is a P0716/717 code related to the ISS has a condition that needs to be set in order to show up (it has to be faulty for two consecutive drive cycles)

Apparently, there is a wire for that sensor that gets damaged by a sharp edge internally in the trans, and this gets it to keep shorting out. Removing the pan and inspecting the wire can tell.

Offcourse since you have no codes, its too early to tell....

 

[adventr]

Just put the old sensor back in it and messed around with the electrical connector quite a bit.
Pulled back the loom, and trimmed some of the tape right at the connector.
Looks to me like one of the wires probably has a break in it right where it goes into the connector.
The wiring harness for that connector is a twisted pair, and they had it taped really tight at the connector. Which was making one of the wires have a tight jog to it.

After looking the connector over and playing around with it, I went to drive the rig and now the speedo is completely dead. No movement at all.


No check engine light still though...

 

[adventr]

I think it's fixed.

Local Napa parts store had an electrical connector in stock. It was a good quality Delphi unit, and fits the VSS perfectly.
Cut off the old plug and soldered on the new pigtail. This added about 4" to the overall length of the VSS harness, which was a good thing. Was pretty tight before, and I think that contributed to the issue. That, and the wire harness was twisted too tightly at the connector.

After finishing it up, took it for a drive and all was back to normal. Speedo is super smooth and consistent at low speeds now.


Take a look at the pic I've attached of the old connector. I think you'll see the wire that I believe was the issue. It's got a weird jog to it. Reminds me on a broken finger. Lol

I'll update again in a week or so after more driving.

 

[Ghostbuster]

Glad to hear

 

[Bigunit]

Good work, glad you were able to figure it out and thanks for the update!

 

[4speedfunk]

That's great news. Your DIY repair translates to $1200 at the dealer!