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2005 chevy trailblazer_ls
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Discussion Starter · #1 ·
Hi all! I have the strangest problem. I have an 05 Trailblazer which I love and thanks to all of you for posting what you have learned from fixing your cars. It has save me a ton of money!:hail: The problem that I am having is that my reverse lights stay on while I am driving. I am not sure how to fix this. Does anyone have any suggestions?
 

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2004 gmc
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This is actually a rare problem with our vehicles. The control comes from the BCM (Body Control Module) under the rear seat. And the BCM discovers what gear the vehicle is in by way of a data link from the engine computer (PCM) which gets its information from a switch assembly on the river's side of the transmission.

But if that switch goes bad, you also notice (probably) the gear shift indicator on the gauge cluster also reads "R" all the time. Does it, or does it read properly?

Very rare to get a BCM that goes bad that way. You might need a mechanic with an advanced scan tool to see what the transmission switch is telling the PCM, so you can determine if it's a bad BCM or a bad tranny switch. One is a lot less money than the other. Might also be a wiring issue between the tranny switch and the PCM.
 

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2005 chevy trailblazer_ls
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3 Posts
Discussion Starter · #3 ·
Hi Roadie. I just wanted to thank you so much for all of your wisdom. You personally have saved me so much money with your posts and I really appreciate that! Actually, it shows that I am in the correct gear while I am driving so it is kind of wierd. I guess I will have to take it in and see what is going on. Thanks again...you are awesome! :thumbsup:
 

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2009 chevy
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6,431 Posts
I'm gonna toss an other possibility out there.

Most automotive switches, switch the ground lead and this circuit is like that (the switch on the rivers :dielaugh: side of the transmission that roadie is talking about)

This P/R/N switch provides a ground on one of a number of leads (depends upon gear selected) going to the PCM.

The switch has 4 leads, one labeled P, the others are A, B and C.

I suspect a ground on P indicates "in PARK"

The other positions (R/N/D) are probably indicated by a grounding matrix on the other 3 wires (like binary code).

A problem in the switch might (I say might) cause the PCM to indicate I'm in DRIVE, but I need reverse lights (it's probably not smart enough to know this combo should be mutually exclusive).

Before I spent any big money at a tech shop, I might pull the plug off the switch to see if the lights go out.
 
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