LB7: Transmission Limp and Speed Sensor

[3500 LB7]

There’s been a few issues on my truck in the last few months that I can’t figure out nor can the dealer figure out. They want to charge me a bunch of money to take apart the transfer case and transmission just to diagnose, then maybe they can figure out what’s wrong and then determine the fix and cost. I’m hoping I can get some help here first on narrowing a few things down that could be causing the problems before handing over my wallet to the dealer.
1. Only at speeds between 40-60 mph, the speedometer jumps around sporadically and the Service Brake Light and ABS light goes on. Once in a while the brake warning lights would turn off but would come back on eventually. (I’ve recently had the entire instrument cluster replaced under the extended service so I don’t think it’s that.)
2. The truck makes hard shifts between the 2nd and 3rd gear sometimes. I’ll explain. If I accelerate smoothly from stop to cruising highway speeds and get up to the full cruising speed, everything shifts pretty smoothly. But if I’m in traffic and let off on the accelerator just a bit before 3rd gear, the truck would make a hard shift and jerk forward.
3. The truck goes into limp mode sometimes, but more often if I step on the accelerator hard to get on a freeway ramp or merge onto traffic.

 

[Mike L.]

Need codes and list them in the order they showed up.

 

[3500 LB7]

That's another strange thing, it doesn't always throw a code. When it does go into limp mode, the check engine light does come on, but after a few restarts, it would disappear again. The next time it happens, I'll check to see if there are any codes.

The harsh shifting from 2nd to 3rd happens all the time with no code. The sporadic speed sensor, ABS and Service Brake System light happens all the time with no code either.

 

[Chevy1925]

If the abs light and brake light are on, there is a code in the ABS modual and will need to be pulled with either a tech 2 or some other good scanner that can read that modual

 

[DIESELMAFIAPER.LB7]

Im fighting this problem now as well on a brand new gmax trans the truck said it limped 5th when it didnt i was driving it but the speedo will jump around after 40mph says c0035 front left wheel spees sensor i believe and a c0055 i cant find but id imagine its a speed sensor in tcase? Any input

 

[Mike L.]

Wheel sensors will make the Alli malfunction.

 

[DIESELMAFIAPER.LB7]

That's what i thought I've just never had it happen to me :hug: thabks mike we'll replace the front hubs check the speed sensor out go from.there

 

[graystonelbz]

Wheel sensors will make the Alli malfunction.

what types of malfunctions for future reference?

 

[3500 LB7]

I replaced the speed sensor on top of the transfer case but it didn't help. I'll check the speed sensor in the front wheels next. Are there speed sensors in both front wheels or just 1 side? I'd imagine both, but have no idea, so wanted to make sure.

 

[jc1843]

Be sure to jack it up and check the wheel bearings for being loose-- common problem with many trucks.

 

[770JOHNNYW]

Had this problem before ...its the t case the bushing wears out do to lack of oil to bushing therefore causing the reluctor to pull away from the speed sensor. Rebuild the transfer case with a new oil pump or replace it

 

[3500 LB7]

That's what I'm afraid of. I was hoping it wasn't the transfer case, cuz that's gonna cost a pretty penny that I can't afford right now. Especially when I just had to pay Uncle Sam some tax $$$. Dang it!!!

I wanted to fix the possible cheaper fixes first, like the speed sensors.

 

[DIESELMAFIAPER.LB7]

ours lost the bearings in the tcase replaced the tcase and sensor (it was beat to shit) and were good no more codes for the wheel speed sensors either

 

[Florida Cracker]

can you clean the speed sensors

 

[Mike L.]

can you clean the speed sensors

Yes you can and then you can wait for the next piece of metal to stick to the magnet and screw things up again.