C5500-No Brake lights

'07 C5500 GMC has no brake lights unless it is in reverse! An alarm is on in the cab. All the fuses have been checked and the brakes do work. Any ideas?

A wiring diagram is probably your best bet here, sounds like a different set up than the lite duty trucks..

Thanks for your reply maxwedge. I asked the question because I haven't been able to locate a wiring diagram for this truck. The dealer wants to fix it but not give out info. $$$ Does anyone have a wiring diagram of this circuit that they can post or email me?

Look along the frame rail towards the rear and you should find a weather pack connector. Check the connector for corroded terminals and use a test light. Another check would be to apply the brake pedal with a sutible prop, key on and recheck all the fuses again.

I'm not sure how you made out but apparently these trucks are experiencing problems with the brake switch when they've been outfitted witha brake controller for a trailer.

The link below may help your situation.

http://www.gmupfitter.com/publicat/bull/bull66.pdf

Thanks for your reply toddman67. I went and looked at the truck instead of going by what my brother-in-law told me. It turns out the right rear light is wired wrong for one thing. The brake light wire goes to the backup lamp and the backup wire goes to the brake lamp. That's why the brake lights supposedly worked in reverse! I talked with a GM tech and he told me about the problems they are having with the trailer controllers so since it was installed by the dealer my b-i-l is taking it back to let them fix it under warranty. The tech also told me you should never probe the new connectors with anything that is larger then the mating connector because you spread open the connecter and you won't get a tight connection afterwards. He has seen intermittent connection problems because of this. He also said that if you break open the wire insulation with a probe it will cause the wire to oxidize and eventually fail. I'm glad I got out of the business!

Good deal, I'm glad things worked out for you. You're fortunate that the dealer owned up to the brake controller issue as well.