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Fuel Pump failure causes electrical fire?

Hi everyone, got a frustrating situtation here I hope to get some insight into. I was driving my 1976 Carrera 3.0 and it suddenly dies at a stop light. I start it back up without an issue and it proceeds to die again about 100 feet later, never to start again. Once I get it back into the garage I take a close look and find the fuel pump relay with a burnt hole through it and fused to the relay socket. Also, about 1.5" of the red feed wire of the fuel pump relay fuse is melted, and fuse looks severely hot but NOT blown. Additionally, I see the backup light/brake light/turn signal fuse blown. I track down a short to ground on the wire coming from the backup light sensor to the 11 pin connector in the engine compartment.

That doesnt solve the fuel pump issue, but does allow me to replace the lighting fuse with no issues. At any rate, I still have some short on the fuel pump line. My hypothesis is that the fuel pump has gone bad and shorted to ground inside. I experimented with "hotwiring" the pump and it did make its standard noise but seemed to struggle a little bit before blowing the fuse. Has anyone ever experienced this issue when the fuel pump goes bad? I dont want to go through the process of replacing the pump if it is not bad, as those arent cheap and not returnable. Anyone have any suggestions to test the pump? Maybe resistance values across the terminals?

I am done working on it this weekend, but hope to have some more ideas for next weekend! Thanks everyone!
Old 12-07-2008, 01:33 PM
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Can't help but I can certainly sympathize. I had the same blown fuel pump and relay problem but only a lot worse to the tune of about $5,000 in electrical repairs. This can happen to you if that short finds its way to the electrical harnesses so I would certainly proceed with caution if you are going the DIY route.
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Old 12-07-2008, 02:00 PM
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Three people in another forum told me to check out the elec. connections at the fuel pump, a task I'm waiting for spring to do as it's only about 18 degrees now.

http://carnuts.us/forum/viewtopic.php?f=10&t=248&start=0
Quote:
Originally Posted by Jagman
I think you're on the right track with the fuel pump, but before you replace it - check the connection right at the pump. We used to have a real problem with those plastic fittings corroding the wire connectors inside of them and causing the wire to break right where it plugs into the pump. A couple of new spade connectors on the wires and some silicone to encase it and all was good again. You may just have a bad pump too, after 31 years...........
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Old 12-07-2008, 02:03 PM
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sounds like the pump is drawing a ton of amperage. it's typical to see burnt wires at the fuse and the relay pins melting the rubber socket. also could have started at the fuse connection due to loose wire connections. a loose connection is a hot connection.
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Old 12-07-2008, 02:27 PM
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Jagman:
Hm........... most fuel pumps I have seen on a 911 have wires connected with eyelets, not spade connectors, to ensure a solid connection.

Whether Bosch or Pierburg, they are in the range of 5-6 bar and should not exceed 10 Amps max.
Check the draw.
You don't want to have the pump fail while you're in the middle of nowhere, especially with a full tank.
If the pump is really old with Mileage unknown, get a new one and replace the short fuel line at the same time.

Consider this when ordering a new one: does it have an internal or external check valve?
Some pumps with internal check valve still need an adapter to connect the fuel line.
Is it compatible for the pressure needed in the system? Is it a Banjo-connection or??

Look at your pump first before ordering.

To install, drain the tank with a long hose into suitable containers with a fire extinguisher close by. Some people just clamp the short fuel line but it can fail and make a mess; be careful.
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Last edited by Gunter; 12-07-2008 at 02:52 PM..
Old 12-07-2008, 02:50 PM
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I had a simliar problem 3,500 miles ago last year.

Driving with the kids, engine quit, cranked over but would not start, then smoke started flowing through the dash.

Someone in the past installed a balde fuse by-passing the original fuse holder for the fuel pump since it was broken and there might have been high electrcial resistance causing the fuse to melt.

Pump is still fine, it was more of a wiring issue and I replaced the 10 unit fuse holder,all fuses, fuel pump relay socket and window relay socket.

I was very lucky.


Bentley has a good description testing the fuel pump for current draw.
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Last edited by GothingNC; 12-08-2008 at 04:54 AM..
Old 12-08-2008, 04:51 AM
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GothingNC:
Why do you think the window relay socket melted as well?
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1995 seafoam-green 993 C2, LSD, Sport seats.
Abstract Darwin Ipso Facto: "Life is evolutionary random and has no meaning as evidenced by 7 Billion paranoid talking monkeys with super-inflated egos and matching vanity worshipping illusionary Gods and Saviors ".
Old 12-08-2008, 10:50 AM
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It was next to the flames from a bypass fuse that melted.
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Old 12-08-2008, 10:56 AM
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Is it just harness positoning issues? Should these wires go "bad"? DAMN....what nightmare..

Scary!\\

Best,

Doyle
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Old 12-08-2008, 04:38 PM
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A good reminder to look over the fuses and relays.
Check visually for discolored wires or melted insulation.
Remove the relays and look for signs of melting.
Put your nose close to the fuse holders and relays; overheating has a distinctive smell.

I usually take the fuses out and check the tension on the holders; they can be squeezed/restored.
The same with the relays: The pins can get tired and can be restored by using a hobby-knife and gently pry the split open just a little.

A small film of dielectric grease on the relay-pins and fuses prevents corrosion.
I use dielectric grease on anything electric like the 14-pin connector and any other electrical connection.
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1995 seafoam-green 993 C2, LSD, Sport seats.
Abstract Darwin Ipso Facto: "Life is evolutionary random and has no meaning as evidenced by 7 Billion paranoid talking monkeys with super-inflated egos and matching vanity worshipping illusionary Gods and Saviors ".
Old 12-09-2008, 09:13 AM
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Old 12-09-2008, 09:13 AM
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