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No Low beam headlights (Intermittent)


kgornek

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Ok. This is going to be one of those lengthy posts but I think it’s worth it. I’ve found a lot of excellent information on this site and I really hope someone can help me with this elusive problem I am having.

 

The Problem: Low beam headlamps not working. (Sometimes)

 

I own a 2001 Tahoe and am having a problem with the low beams. Sometimes they just decide not to work. It’s like the truck is haunted. Its night. I get in, turn the key on (before I actually start it) and no headlamps. I start the truck… still no headlamps. I turn the switch to turn them on manually… no headlights. The weird thing is everything else is on and working properly. Dash lights, parking lights, taillights… everything. I hit the dimmer switch and the highs come on. Hit it again, and no low beams.

 

Next day I get into the truck and everything works fine. Low and high beams work. (night time)

 

3-4 days later the problem reoccurs.

 

Since I never know when this is going to happen it’s getting to the point that I basically have to drive home with my high beams on, with no headlights, or get a ride home.

 

Ideas to the problem:

 

Faulty light sensor(s) that automatically turns on the lights – I can’t turn on the lights manually with the switch. I would think this would rule this part out. (?)

 

Faulty relay – Replaced. No change in problem. (This is the relay in the fuse box under the hood)

 

Dimmer switch – Called the dealership and they didn’t seem to think this is the problem. (Although they couldn’t offer me another solution) They also informed me that a new dimmer switch is around $350!! Yikes. If I pay this much, I want to make sure this is the problem.

----Update: This has been replaced and problem still exists.

 

Body Control Module- There have a few people to tell me that this may be the problem. Although I’m not sure how to prove it or disprove it before spending $375 for a new one.

 

Has anyone got any ideas here? I mean excluding all the automatic electronic stuff, I would think that I should be able to turn on the switch, and the lights should come on. But their not. Something is not allowing that. What do you think it could be? Remember, the problem is intermittent.

 

I appreciate any input or advice. I’m not an electrician, but understand the basic fundamentals. And this problem is really start to be a stone in my shoe. :chevy:

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Well I wanted to give you an update on what I found. I took apart the UBEC and took a look at the terminal locations you told me about. I didn't see any physical damage and moving the wires around didn't create any interruptions in the lights not working.

 

I asked a friend of mine to lend a hand and he seem to think you suggestion was a great idea and told me that he was pretty sure there was a tool to get those terminals out of the UBEC. We couldn't seem to get them out and didn't want to damage any of the other wires. Anyway, I just figured I'd take it to the garage sometime this week and ask them to replace all the terminals you mentioned.

 

So we put everything back together and home I went. Later that night I got into the truck and started it up, but again... no headlights. So just on an idea, I popped the hood and gave a little tap on the top of that UBEC.... *WHAHLA* the headlights came on!! So I would say that your guess was right on!!

 

I will make an appointment to replace both terminals sometime this week. As I believe you stated, I should replace connector 1, terminal D3 and Connector 4, terminal B3 in the UBEC correct?

 

And last but not least... I want to say thank you very, very much!!! Due to your knowledge, you saved me a lot of frustration and money. I actually considered trading in the truck due to this nagging issue. So here's to you. Thank you again for all your help.

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I will make an appointment to replace both terminals sometime this week. As I believe you stated, I should replace connector 1, terminal D3 and Connector 4, terminal B3 in the UBEC correct? 

 

Yep, it's going to be one of those, though I've never seen it be both of them. *Usually* it's the terminal in D3, Connector 4.

 

The connectors come out fairly easy. Once you have the UBEC off and turned over, just loosen the bolt in the center of the connector and pull it off. Once you can see the face of the connector, the overheated terminal is usually pretty obvious. The plastic around it will be a little burned from the heat. You can't see that from the backside with the connector still attached to the UBEC.

 

Once the connector is disconnected from the UBEC, there's what is called a TPA (terminal position assurance) comb that has to be removed. It's usually a blue plastic piece with long fingers for each row of teminals that goes all the way from one end of the connector to the other. This is what locks the terminals in place and prevents them from backing out, though they each lock in place on their own. That comb has to be removed before you can get the terminal out of the connector to replace it. You don't really need any special tools. It's easy if you know how it all goes together.

 

Best of luck and let us know the final outcome.

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Loss of power or ground? How exactly would I determine that? Thanks for your help!!! I will look into this right away.

 

I left out an important first step.  Determine whether the low beams don't work due to a loss of power or a loss of ground.  This will really narrow it down as to what the cause could be.

 

When they're not working, pull one of the headlight assemblies out and check this.  I suspect you'll have power, but no ground, in which case the previously mentioned check of the connections at the UBEC would be my first step.  Those connections provide ground for the low beams.

 

 

 

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Loss of power or ground?  How exactly would I determine that? 

 

No offense here, but if you don't know how to do the simple test at the headlamp that I explained, perhaps you should take it somewhere that can accurately diagnose it.

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I have. And they have repeatedly failed to fix the issue. I'm not trying to take shortcuts or save a dollar here. All I want is the problem fixed. Trust me, if someone offered to fix the problem for sure, I would have it done in a minute!

 

Ok, so I'll figure out the power/ground thing. Let me ask you this, if the problem does indeed reside in the UBEC with a loose terminal, what's the best way to remedy the issue? Can you buy a new terminal connection or do I have to go out and buy a whole new box? (UBEC)

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The Problem: Low beam headlamps not working. (Sometimes)

 

Connector 1, terminal D3 or Connector 4, terminal B3 in the UBEC (Under Hood Bussed Electrical Center = honkin' big fuse box under the hood) loose terminal and subsequently burned by high resistance.

 

I've seen a number of these. Both low beams are turned on, in part, by the connection that these terminals provide between the headlamps and the dimmer switch. The wire from the dimmer goes to Conn. 1, term D3 and the two yellow low beam grounds come from the headlamps and terminal at B3 in conn. 4. This is connected to D3 through an internal buss bar.

 

A lot of current flows through those single connection points. If one has even the slightest loss of tension, the terminal eventually overheats and you loose the headlamps, though intermittently to begin with.

 

Remove the UBEC and turn it over and look for an overheated appearance in those cavities. You probably won't be able to see any signs of this unless you pull the connector from the box and can see the opposite side. There's a LOT of wires under there, so just look for yellow wires. The ones from the headlamps come together and you'll have two wires into one cavity.

 

Alternately, if you can't see anything visual, try moving the wiring around under the UBEC when the headlights aren't working and see if that affects them.

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You would just replace the problem terminal within the UBEC. Replacing the UBEC wouldn't fix it, because the terminals are in the harnesses that plug into the UBEC, and they're part of the various wiring harnesses. They're not part of the UBEC.

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You would just replace the problem terminal within the UBEC.  Replacing the UBEC wouldn't fix it, because the terminals are in the harnesses

 

Okay, I can see where that's probably confusing. First I'm telling you to replace the terminal in the UBEC and then it's not in the UBEC but the harness. :confused:

 

There are several large harness connectors that plug into the bottom of the UBEC. The low beam circuit goes in one connector and out another. So the terminals are actually in the harness connector, but those are plugged into the UBEC. Hope that clarifies.

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I left out an important first step. Determine whether the low beams don't work due to a loss of power or a loss of ground. This will really narrow it down as to what the cause could be.

 

When they're not working, pull one of the headlight assemblies out and check this. I suspect you'll have power, but no ground, in which case the previously mentioned check of the connections at the UBEC would be my first step. Those connections provide ground for the low beams.

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Is there anything I should get in advance before starting this project?

 

Not really. If you find a bad terminal, it'll need replacing. There's one problem with that... you can't just walk up to a GM parts counter and say you need a terminal. We have about 1,000 different kinds of terminals and we usually use the "match 'em up" method to figure out which one it is. I have about 20 trays, each with a dozen or more compartments, each with a different terminal type. Okay, so that's only 240 different terminals, but you get the picture.

 

So, you'll probably need the terminal in your hand for them to find a match.

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