Help. Speedometer reading crazy and erratic shifting???

[Merc1973]

Truck: 03 Yukon 4x4, 266k miles, reman trans installed at 250k, fresh ATF fluid last summer (drain and refill only).

Just started this morning on the way to work while stuck in traffic. It's not the step motors in the gauges, but the speedometer will move up and down erratically reading much faster than my actual speed and the transmission will try to adjust for gearing and RPM erratically as well. Is there a bad speed sensor somewhere in the trans??? Or worse??? Now Im stuck at work, unsure If I can get it home. There are no warning lights or CEL? I will check for codes at lunch.

 

[iamdub]

Crawl under there and make sure everything is plugged in securely and the wire harnesses aren't damaged (rubbed through, etc.).

 

[Fless]

If you have the Auto 4x4 transfer case there are several speed sensors. I don't recall how many, but there's at least one for the front propshaft and one for the vehicle speed. Might be one on the tranny, too.

 

[Merc1973]

I got it home last night at about 9 PM, drove fine for 90% of the drive then went into limp mode (0 mph on speedo, abs lights, service brake booster, stuck in 3rd gear). I prayed my ass off too.

I forgot that I had pulled my ABS fuse since I was trying to track down an intermittent activation of the ABS at slow speed, even replaced the front ABS sensors. I installed the fuse and also found my Autel Code scanner that has Engine/ABS/SRS capability in the back of the truck. I had a bunch of ABS codes:

C0237 (rear wheel speed signal erratic),
C0269 (Excessive dump time),
C0265 (Electroc brake control module-EBCM- motor relay circuit),
C0236 (Rear wheel speed signal missing).

I cleared the codes, drove and shifted like normal, SO i just kept going home, as soon as I slowed on my off ramp, it freaked out again. I can understand the ABS component since without ABS module working, it was unable to send the VSS signal to the ECM, after fuse install and clearing codes, it could send again. The brake booster service light is probably on since when first driving from turning it on from 0-10 the electric booster motor self tests, without VSS signal, it will fault i guess. My Yukon does not have Hydro Boost.

Lots and lots of reading last night, points to bad bad ground somewhere, I live in the rust belt and the underside is rusty as ****, previous owner launched a sailboat with this truck in salt water for many years. I will clean up all grounds that I can find and spray electrics cleaner and dielectric grease in ECM/PCM and trans harness plugs. Another issue could be a broken wire from the VSS (3 of them) to the the ECM since the trans and TC have been removed prior so wire damage could have occurred. I highly doubt all 3 of the VSS died, I will switch out the hard to get front one now that its back home and I have access to all my tools, but I wont hold my breath.

 

[Fless]

Don't miss the one that is under the body, behind the left front tire. There should be two bolts and I think three ground wires/lugs. One of those is very important to the 4x4 (not your issue) but it's a low-lying ground and gets corroded easily. Lots of threads here on other grounds, too.

Let us know what you find.

 

[Merc1973]

Cleared codes, took it for a drive around the neighborhood and Autozone for a new can of quick dry electrical cleaner spray and dielectric grease It acted up again after about 45 mins of driving. It coded Erractic VSS signal with ABS lights lit and beeping, but I was in my neighborhood at that point and got it home, it did shift fine though. Afterwards, I cleaned and greased the TB, PCM, multiple ABS and Trans/TC harness connectors, and battery terminals. The negative terminal was actually pretty crusty. While looking for connectors to clean I think I found the culprit. On top of the trans, a purple and green wire, the green one was mangled and hanging by a thread (literally). It doesn't look like rodent damage but probably from removal and installation of the reman transmission. I do not see the Purple/Green wires on the ABS schematic above... Its pretty tight up there, I will have to get more slack to solder it back together or get on of those nice Klein wire strippers from HomeDepot. I hope to fix it tonight after work. Wife is getting pissed she is car-less and borrowing neighbors minivan.

 

[Merc1973]

Solved!!! (I hope, lol)

That broken green wire is an extension to the Left Rear VSS, same color combo (green/purple). This has to the be issue, right? no problems with a 40 min test drive last night. I bought the one handed Klein wire strippers and they worked great. Soldered and heat shrink wrapped, not fun having beads of molten solder roll down your arm into your sleave, but it wasn't too hot. Lol. Took a 70 mile drive today, good to go...

 

[retiredsparky]

Congratulations on doing a good job troubleshooting and fixing it!

 

[YukonXLMan]

Wow, I'm impressed! You have some serious tenacity! Congratulations!