BURNING SMELL? Car won't start? Check the Rear Seat Fuse Block NOW.

[Anthony Cipriano]

The "Melted" Mystery: This is the #1 "mystery failure" on the Buick Lucerne (and Cadillac DTS).

The Symptoms:

The car cranks but will not start.

You do not hear the fuel pump "hum" when you turn the key to ON.

You smell a distinct "burning plastic" odor inside the cabin, specifically near the back seat.

The Cause (It's not just the relay!): The fuel pump relay is located in the Rear Fuse Block under the rear passenger seat.

The Design Flaw: GM designed the metal frame of the rear seat bottom too low. When passengers sit in the back, the metal seat frame physically presses down on the fuel pump relay.

The Result: This pressure wiggles the relay while you drive, creating electrical resistance. Resistance creates heat. Eventually, the relay gets so hot it melts the plastic fuse block around it, breaking the connection.

The Inspection:

Lift the bottom cushion of the rear seat (pull up firmly on the front edge to pop the clips).

Locate the black fuse box on the passenger side.

Remove the plastic cover.

Pull the Fuel Pump Relay (Check the diagram on the lid).

Look closely: Is the plastic around the pin slots melted, brown, or black? Is the relay leg scorched?

The Fixes:

Option A: The "Official" Fix (Expensive)

Replace the entire fuse block (GM Part # varies by year, approx $300-$500).

CRITICAL STEP: You MUST use a hacksaw or grinder to trim the metal bracket on the bottom of the seat cushion that sits above the relay. If you don't do this, the seat will melt your new fuse box in 6 months.

Option B: The "Bypass" (Smart & Cheap)

You don't need a whole new fuse box. You just need to move that one circuit.

Buy a "Fuel Pump Relay Relocation Kit" (vendors like Renegade Solutions make a plug-and-play one).

These kits plug into the melted slot but move the actual relay to an external heavy-duty holder that hangs off to the side, safe from the seat frame.

Does anyone have photos of a melted fuse box they can share?

 

[4 barrel]

I just fixed mine without a new $500 fuse box. My relay was over 225 degrees. I used a 10mm wrench --turned the fuse box over and spliced into the rear of the box with two wires-gray to #87 fuel pump--green and white to #86<key> I put a 14 gauge to # 85 ground to the body and a 14 gauge with a 20amp fuse to 12v on the fuse box-hot all time and the relay is now on the outside of the box running 145 degrees. I drew a diagram on the floor of the fix.

I have a better relay but this works. I took the fuse box apart and from the bottom it looks new- You can see the upper corner Where I removed the fuse. It's a pretty simple fix once you have done one. I took me two days being careful and watching videos of different ways to rig but the next one should be less than two hours. You can jumper #30 and # 87 on the fuse box for the pump to run if you break down. I never figured why the relay got so hot and the fuse box didn't.

The fuse box is air temperature now. Anyway the relay is a normal temperature also. I also installed a new Delphi pump <in the trunk>it didn't need but it's a keeper. The net said to cut 1/2'' off the end of the metal seat bracket over the fuse box that GM claimed was rubbing the top cover but no one has sat in the back seat since new. I did that with bolt cutters.

In the third photo about 4'' right there is another 40amp breaker the same as the one I replaced. You can't remove the key if it's removed. I ran 1/4 tank of gas through it and both relays run the same temperature.--145 degrees. I feel this is safe or I would have installed a new fuse box. Where I removed the relay will still work--but the relay is in a different location--no wires cut. These relays are sealed tight and looks like they would run cooler vented. I am planning to cut one apart to make a couple of vents and see if they run cooler. When I get time I will put in a smaller fuse to see if it blows. It looks like the 10 amp fuse beside the 40 amp relay should have blown first.

 

[4 barrel]

VERY EASY TO CHECK--For a short cut before you do anything-to see if the relay will cool down. A new relay is $30- I bought 4 on line for $12. I took a new relay and twisted a piece of wire around #30 terminal and bent the pin to the outside away from the 12v plug in. This plugs back in to factory box for testing. I made a simple jumper wire with a fuse and pugged into a spare 12v circuit in the fuse box. You do not have to drive the car to check the relay. Let idle for 30 minutes <I ran 1/4 tank at idle>and the relay should be about 125 degrees. Best I can find a fuel pump relay runs normally 125-165 degrees after 30-40 minutes.

 

[2007LucerneCXL]

On 2008 and up model years, 2006 and 2007 are different design rear seat fuse boxes.

If the rear seat fuse box is damaged internally a bypass/external relay may not be work or depending on the box damage may shorten the fuel pump life as a original relay mat do.

There is a shorten GM fuel pump relay that should be used which is listed in the GM TSB on this problem. As a note the seat clip into fuse is a questionable problem, poor fuse box design or QC is a more likely suspect.

Yes there are posts and photos in the Lucerne forum use the search function to find.

 

[angie2short]

Does anyone have the wiring diagram of the fuse box? Boyfriend won't let me do any work to the wiring system without it. Ugh

 

[2007LucerneCXL]

Carmin - Apps on Google Play

Automotive Electrical Wiring Diagrams

play.google.com

Here's a app that has the wiring schematics.

 

[angie2short]

Carmin - Apps on Google Play

Automotive Electrical Wiring Diagrams

play.google.com

Here's a app that has the wiring schematics.

Thank you

 

[JLF_1977]

This is a problem I'm fighting again. Had the pump changed out about 2.5 years ago. It's killing relays again. I can park the car and come back and it's dead. I suspect that there is too much draw at key on. Unsure if that's going to be a pump or if the relay relocation think will do the trick. Since the back seat isn't even in the car right now, I suspect I may have to do both. My relay was about 125*F but the fuse was so hot you couldn't touch it. How it never blew, I'll never know. It was burnt.

 

[2007LucerneCXL]

On the rear seat fuse box a bad/blowing relay will eventually kill the fuel pump. The relay relocation modification is a work around but it is somewhat dependent on what or how much damage has been done to the fusebox itself. It's a matter of a good inspection on the box and then deciding on a bypass, used or new box as it's not really a one size fits all solution.