2005 GMC Sierra V8 Four Wheel Drive Automatic 71,000 miles
Yesterday all of my brake warning lights came on with a Service Brake System Message in my display under the speedometer. Speedometer began bobbing up and down and not reading correctly. Truck didnt wanna shift, felt like a rev limiter was engaging. The truck didnt wanna get out of its own way. If I came to a complete stop all of the messages would turn off, and the truck would act fine. Then I would drive maybe 100 ft. And everything would go off again. I have been having problems with the speedometer acting funny for a few months. It would act up then be fine again. I changed my front brake pads the day before yesterday. Everything looked fine. Took truck to mechanic yesterday after everything went crazy. He said everything looked fine with the brakes and he replaced my vehicle speed sensor on the transmission. Drove truck home last night it acted fine. Drove it about 20 miles today and it did the same thing as it did yesterday.
Actually, the speed sensor was my first guess. However, since there are so many things happening at once, I recommend having the computer scanned to identify what is happening. Most parts stores will do it for free.
Have that done and let me know what you find.
April, 3, 2010 AT 5:23 PM
Mechanic did a diagnostic scan. I'm assuming the speed sensor is all that came up. I picked the truck up after hours so I didnt get a chance to talk to him. Would the rev limiter kicking in be a saftey mechanism if the truck thinks you have no brakes I would assume it wouldnt wanna let you go very fast. Would the brakes warnings also be related to the speed sensor? I was wondering if the computer thinks you are breaking but the sensor isnt saying you are slowing down the computer might also read it as a brake malfunction. Also is it possible that this could be related to the GMC technical service bulletin regarding faulty speedometers? I will be speaking with my mechanic about it he is my uncle so he cant run and hide. Lol I will let you know what I find out.
April, 3, 2010 AT 11:25 PM
Actually, they are all interconnected. I miss the good old days. Things were much easier. Mechanics today have to basically have engineering degrees.
As far as the speed sensor, it controls several things. The speedo, traction control, ABS is tied in, transmission shifting patterns.
Let me know what you find. Also, as far as a rev limiter, it wouldn't do this.
Have a good Easter and I look forward to hearing from you.
April, 6, 2010 AT 12:59 PM
Well I took truck back to the mechanic and he accidentally changed the 4wd speed sensor. He changed them back and installed the new sensor on the vehicle speed side. He explained how the sensor controls all of your shifting, braking, and how the truck would interpret the bad sensor as a braking problem and would screw with my transmission. Anyways drove truck home and seemed fine. Started it this afternoon and all the bells and whistles started goin off again. It is a sensor problem cuz it just wont do it then everything goes crazy and it might shut off and be fine 2 seconds later. I have all the same symptoms as before. If this controls the shifting I am assuming the truck is just going in and out of nuetral and that it is not a limiter. Other then the speed sensor is there anything else that could be causing this problem. I heard there are alot of speedometer guage problems with this year of truck could that be causing the trouble. I also see that gm has a technical service bulletin out on the speedometers.
April, 6, 2010 AT 10:35 PM
The TSB is regarding the replacement of the cluster due to a faulty speedo. That wouldn't cause all of the problems. When all the lights come on and the problems start, has a code ever been set in the computer that the mechanic told you about? In one of the earlier postings, you mentioned he scanned it, but you didn't get to ask him.
April, 7, 2010 AT 7:35 AM
It came back as a vehicle speed sensor code. Is it possible the new sensor was junk right out of the box. Is there a way to test a sensor to find out? I spent a few hours replacing a wheel bearing hub assembly on a truck I had before this one only to get everything back together and find out the wheel speed sensor was junk on the new hub.
April, 7, 2010 AT 9:27 AM
Yes it can be bad. Believe it or not, it isn't that uncommon. It seems like everything is coming from countries where they pay employees a dollar a week to work. As a result, you get what you pay for.
April, 7, 2010 AT 7:02 PM
Picked up truck again tonight. Mechanic replaced sensor again. While driving it home when I came to a complete stop it takes the speedometer a sec to go all the way down to zero from ten mph. Did the same thing the other day when I picked up the truck and my troubles started again the following day. Hopefully my major troubles are over with with the new speed sensor. Any ideas about the speedometer lag? I got a feeling I havent seen the end of this yet.
April, 7, 2010 AT 8:54 PM
The lag could be a resistance problem or the insturment panel itself. Let me know if it helped.