Problem 02 GMC 2500 Diesel/Allison 4 by 4

[mgamble]

Just got it a week ago and now a problem already. Brake light comes on along with ABS a couple days ago nothin seemed wrong and it went away. Well today after pulling a 12000 lb wagon i got on the road and about 5 minutes later my speedometer goes wacky and the brake light, ABS and service engine light come on. I turn around and when I am in 2 hi it wont shift out of the first gear which means only about 50mph. I think its a problem in the 4 wheel drive becuase in 4hi when driving now i kinda here a clinking noise. Anybody have problems like this with theres?? Now the service engine light stays on but not the brake or ABS light anymore. I dont know what the hell is goin on. Only 54000 miles on it.

[GM-Tech]

Lack of a speed signal can cause just about everything you mentioned as going wrong. With the speedo going whacky, that's a definite link!

 

Could be something as simple as the output speed sensor and or wiring. In fact, that's probably the most likely cause.

[mgamble]

Lack of a speed signal can cause just about everything you mentioned as going wrong.  With the speedo going whacky, that's a definite link!

 

Could be something as simple as the output speed sensor and or wiring.  In fact, that's probably the most likely cause.

 

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THat could be it but also when the speedometer is going wacky and the brake/ service light it on when i start to accelarate it acts like I am pulling 15000 lbs and doesnt shift correctly at all at the same time the speedmoter is going back and forth. Could that still be something in the wiring??

[GM-Tech]

The trans is shifted electronically and one of the inputs to determine shift points, which gear it should be in, etc., is a speed signal.