Fuel pump relay?
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Junior User
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Fuel pump relay?
1994 F150/4.9 engine, automatic. Having a weird issue with my fuel pump relay. Let the truck sit for a few hours and the fuel pump won't run.
-If I swap the relay out with any other identical relay in the box (horn relay for example) it starts right up.
-If I test whichever circuit is now running on the fuel pump relay (horn), it works fine. (!)
-If I shut if off and put the relays back in their original spots, it still starts right up.
I initially thought it was simply a bad fuel pump relay. However...
-If I let it sit for a few hours with the relays swapped, it won't start again and I have to swap them back. What on earth??? Doesn't seem to matter which relay I have in the fuel pump circuit, it won't start unless I replace it with a different one.
Any ideas?
-If I swap the relay out with any other identical relay in the box (horn relay for example) it starts right up.
-If I test whichever circuit is now running on the fuel pump relay (horn), it works fine. (!)
-If I shut if off and put the relays back in their original spots, it still starts right up.
I initially thought it was simply a bad fuel pump relay. However...
-If I let it sit for a few hours with the relays swapped, it won't start again and I have to swap them back. What on earth??? Doesn't seem to matter which relay I have in the fuel pump circuit, it won't start unless I replace it with a different one.
Any ideas?
4wd Low
Joined: Jan 2020
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Is the relay hot to the touch when you pull it out? This sounds like an issue I had with another vehicle of mine. The wiring had gone bad and the draw in the circuit was enough to overheat the relay. It would stop working for a little bit until it cooled down.
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Junior User
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Hot? No. Warm, a little. If the relay was overheating, wouldn't that prevent the truck from starting when warm? It warm starts just fine and never stalls when driving, but let it sit for a few hours and you have to swap relays to make it start again.
Lead Driver
Joined: Oct 2016
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From: Houston/Sugar Land
Do you have dual tanks? The next time you go to start the truck, go ahead and try to start it. When it doesn't start, remove whatever relay is in your fuel pump slot and just put it back in the same slot and see if it will start. Sandy
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Hi Sandy. It originally had dual tanks, but the rear tank has been disconnected (no fuel pump, lines are capped.)
I'm not so sure if the issue is actually relay-related at all. I inserted a brand new relay this afternoon, and it did nothing. It seems that power isn't always getting to the relay. My battery is at a healthy 13 volts.
Do the fuel tank selector switches on these trucks fail? I'm wondering if this is the issue.
I'm not so sure if the issue is actually relay-related at all. I inserted a brand new relay this afternoon, and it did nothing. It seems that power isn't always getting to the relay. My battery is at a healthy 13 volts.
Do the fuel tank selector switches on these trucks fail? I'm wondering if this is the issue.
Thread Starter
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Junior User
Joined: Feb 2017
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Hi Sandy. It originally had dual tanks, but the rear tank has been disconnected (no fuel pump, lines are capped.)
I'm not so sure if the issue is actually relay-related at all. I inserted a brand new relay this afternoon, and it did nothing. It seems that power isn't always getting to the relay. My battery is at a healthy 13 volts.
Do the fuel tank selector switches on these trucks fail? I'm wondering if this is the issue.
I'm not so sure if the issue is actually relay-related at all. I inserted a brand new relay this afternoon, and it did nothing. It seems that power isn't always getting to the relay. My battery is at a healthy 13 volts.
Do the fuel tank selector switches on these trucks fail? I'm wondering if this is the issue.
Lead Driver
Joined: Oct 2016
Posts: 6,185
Likes: 633
From: Houston/Sugar Land
Yes the switch can go bad. You can probably google for a schematic. The power goes through the tank select switch before it gets to the relay I think, but I don't know for sure. Sandy
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Lead Driver
Joined: Jan 2019
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From: N.W. Alabama
could try to toggle the switch front/rear in the truck, when it wont start, it sounds like it may be carbonized up, could try spraying wd40 in switch and toggle front/rear, to try clean off carbon buildup, window switches will do this also, cant get them apart, just put the plastic straw on end to direct spray in the sides of switch
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Fleet Owner
Joined: Oct 2005
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From: Kentucky
I do not see where the OP actually measured the input voltage to the fuel pump relay coil or verified the computer (PCM) is applying the ground to the coil of the relay.
Lead Driver
Joined: Oct 2016
Posts: 6,185
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From: Houston/Sugar Land
Thanks for the clarification Randy. I assume the OP should follow your recommendation for the above testing. Sandy
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