I have a custom 1966 AC Cobra. I just got finished rebuilding it in the last couple of weeks. I am having an issue with the fuel pump relay getting hot and either melting the relay or blowing the fuel pump 30 amp fuse. I have a new pump (second one), new filter (pre/post fuel filters), I checked the amps which are 9.2 amps on the pump when running. The ground is good and I am running at a constant 43psi of fuel pressure. I am running 12 guage wires for the ground and hot to the fuel pump and 16 guage wires to the engine management system. All seems good, any help or suggestions would be appreciated. -MM
I assume you have replaced the relay also. Check the voltage at the pump connection while the pump is on and make sure you have at least 12 volts there. If that is ok then only thing I can think of is there must be another path that the current is taking. Did you measure the current at a point near the relay that included the total current paths from the relay? Make sure the relay is rated for at least 25 amps of continuous current.
Tell you what I’ll do. Just leave that car with me for a few weeks and I’ll repair it and only charge you for the parts. It may take some road testing to get to the bottom of that problem, though…
Just a few questions first.
What type of pump are you running? The amps seem a bit high.
Fuel return line in place from the eng. compartment to the fuel tank and it’s not obstructed?
There will be a current surge every time the fuel pump starts running so you might consider checking that initial current surge and see how much amperage the pump draws on startup.
If it’s pulling 25 amps (still high IMHO) that could be the cause of the problem.
What a dilemma to be in!
The fuel pump is a MagnaFuel in-line pump. It has 12+ volts to the pump and is pulling 9.2 Amps when I force the pump to run constantly via the on-baord engine management system (computer). The relay and wires get slightly warm when I do this for about 15-20 minutes with the car is in my garage. The strange part is it seems to get hot when I am driving the car on the road. All grounds and hot wires have been tested for resistance while the car is not running. Additionally, the 9.2 amps and the 12+ volts were verified when I had the car on a dyno three weeks ago. Anything else I need to check? The power is coming directly from the battery which is about 3 ft away from the fuel pump. The ground is going to the frame 2 inches away from the pump. Any other suggestions?
After looking at the specs for some of the pumps like you have it appears that the current draw you measured is about normal. If the relay is getting hot after some run time then I would suggest you get a relay with a higher current rating. The one you are using now doesn’t appear to be able to handle the load. I would get one that is rated for at least 25 amps of continous current. I also suggest you replace the 12ga. wire with 10ga… The rated current load is at the max for 12ga. wire. Not a good way to run things if you want to have a reliable system.
Check that the connections to the relay are tight. It sounds like a loose connection at the relay socket, or a loose socket.
If you have a carburetor, there is way too much pressure. Some electric fuel pumps for carbs only put out 2.5 PSI.
The Cobra is fuel injected. I have changed the power and ground wires to 10 ga. wires and I changed the relay to a weather pack heavy duty 70 amp. I will let you know if I have any further issues. Thanks!