LB7 computer thinks the truck is driving at 35mph in Park

lb7dirtymax

Hello everyone, I hope you can help me
I have a 2001 chevy silverado 2500HD Duramax and I thought I only had one problem but when I took it to the dealership I have another problem on top of the first so here we go:

1). when driving between 1,000 and 2,000 rpms my truck will lose power for a split second. The rpms will drop about 500 and the car will pull forward because there is no power. I have not found any answers for this..

2). The dealership informed me that my computer in the truck thinks that it is going 35MPH when I am not moving. They said it could be a chaffed wire, dead wire or a bad sensor. I am fully out of hope now but I cant let it end like this, im just a college student.

thecardoc3

lb7dirtymax said:

Hello everyone, I hope you can help me
I have a 2001 chevy silverado 2500HD Duramax and I thought I only had one problem but when I took it to the dealership I have another problem on top of the first so here we go:

1). when driving between 1,000 and 2,000 rpms my truck will lose power for a split second. The rpms will drop about 500 and the car will pull forward because there is no power. I have not found any answers for this..

Did they report what codes (if any) were pulled and which computer? How frequently does this occur? Is there a specific set of operating conditions under which this is more likely to occur? (Time, distance, temperature, weather?)

lb7dirtymax said:

2). The dealership informed me that my computer in the truck thinks that it is going 35MPH when I am not moving. They said it could be a chaffed wire, dead wire or a bad sensor. I am fully out of hope now but I cant let it end like this, im just a college student.

Why out of hope? That's not that difficult of an issue to figure out. The vehicle speed sensor (either in the tailshaft of the transmission or in the transfer case if four wheel drive) reports to the transmission control module and that in turn relays that information onto the other modules in the vehicle that need it. So the next question a tech has to consider is:

Does the speedometer also reflect the false vehicle speed?

In many cases the electrical noise that is being falsely recognized as a vehicle speed is easily visible on a digital storage oscilloscope. Noise observed on the return (ground) and signal side of the circuit should change with the sensor disconnected and depending on how exactly that noise changes the next step in the diagnostics can be chosen.

lb7dirtymax

@thecardoc3
1). the truck  does not throw any codes but I have started to notice that I think it only drops RPMs once it is warmed up.  I live in MA so right now it’s like mid 70s every day. 

2).  The truck speedometer is almost accurate. It is about give-and-take 7 mph slower.  I’m not exactly sure how they found the false speed on the computer because it does not show it on the speedometer