2001 GMC Sierra 1500

A.moruzzi · November 27, 2021

Forgive me if the is the wrong place, I don't know the etiquette of a forum.

I recently replaced my fuel pump by removing the box/bed of the truck.

We unplugged the 4 connectors on the block by the hitch, and we picked up and moved the box back. We noticed there was a wire strained taught (we don't know which wire it was) and we realized the connector for the trailer plight was still connected.

When we got everything back together, we noticed I had no brake lights, even the third on the cab. All other lights work fine, even the same bulb that the brake lights are one. Does anyone have any idea on where I should look first.

Sharpz · November 27, 2021

10 hours ago, A.moruzzi said:

Does anyone have any idea on where I should look first.

yes. Check your fuses 1st, make sure you didnt pop one. If those check out, determine if its a ground problem or a power problem and go from there. Since its all of your break lights you can forgo checking the bulbs as step two. If you have a power problem then you need to trace down those wires, you can get a diagram of what wires do what, and what color they are.

A.moruzzi · November 27, 2021

Yeah, fuses were all good. Where do I find a diagram like that?

I have a fluke multimeter I use for work on HVAC service, how do I determine if it's a ground or power issue?

Sharpz · November 27, 2021

These types of forums are designed to help other users with similar issues as other GM owners. If you ask multiple questions on a thread that says nothing more then the year of your truck it doesnt help any of them, unless they search just the year of the truck. You would be helping the community by asking specific questions such as "Brake light wire diagram 2001 chevy 1500" , "how to test a ground wire" etc.

What you have to determine is how far are you willing to work backwards. You think you pulled a wire when you lifted your box, but dont know which wire it was. The logical thing would be to pull your box again and try to trace that wire down, but obviously not the easiest route. So you are left with removing protective coatings, or harnesses cover to "probe" wires. To test a wire for positive simply put a board or something on the brake to keep it on or have someone hold the brake for you, while you probe wires. You would only need a simple dc wire probe. You can get one for around 5 dollars. place the clip on a good ground spot such as the frame, if the area you inspect after taking the harness coating off has no positive wires when the brake is applied, you will know the wire issue is cab side and will need to find a spot closer to the fuse box. If you find positive wires, you will need to remove whatever you have holding the brake if they still light positive they are not related to the brake. If they go out you will know they are, and can move closer to the brake fixtures. This is what tracking down a broken or loose wire entails.

Before you do the above steps, test your ground wires at the fixtures. You can do this by running a jumper wire from a known power source such as your positive on your battery. Place the probe tester clamp on the now charged wire, and poke one of the wires to the brake fixture, if it lights up, then that is your ground wire, and you wont need to try to find a ground problem, as you eliminated that as a possibility. If you want you can test the positive there doing the reverse. Find a known ground, such as the frame and probe the wires at the brake fixture while having the brake pressed. If you have power, and the bulb is good, the issue is grounding.

This is pretty basic, and hard to explain how easy it is to test wires, but the hard part is the application to do so, you will need to be in places that are hard to access and you absolutely need to make sure that your test is good. I am giving you what is required to test wires. Obviously you should back check the grounds you mentioned removing from the fire wall, as well as the junctions in the back.