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Fuel Pump Relay


Antony W. Serio's Avatar
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07-26-02, 03:40 AM   #1  
Antony W. Serio
Fuel Pump Relay

I am trying to diagnose a cold starting problem on my truck. I've heard tell that if the fuel pump relay is bad, then the fuel pump will not kick in until after the engine builds up enough oil pressure to trip the oil pressure relay. I do
not hear the pump kick in when I first turn the key before cranking, so I suspect this is the problem. So, I thought about swaping the A/C relay with the fuel pump relay (assuming they are the same type), to see if my symptoms change. The problem is, that I have not been able to find the thing.

I have checked the factory service manual, and it says that there should be three relays in a row along the left side of the engine bay. In the manual (page 4-30), they are labeled (front to back) oil pressure, air conditioner, and fuel pump relay.

Here is the problem: my truck only has two relays in this location, and I do have air conditioning. There is a bracket for a third relay, although there is no relay or pigtail. On page 6D-46 of the manual, there is an engine wiring diagram, which only shows two relays along the side (F-R labeled A/C and Fuel Pump relay), with two more along
the firewall labeled (L-R) Oil Pressure Relay (Gauges only) and Fuel pump relay (Gauges only). I have not been able to locate the firewall relays anywhere else in the book (although my truck does have them), and I am not in the mood to trace wiring diagrams right now.

All of the relays are the same type. Assuming the second diagram is right, which fuel pump relay should I swap out; the firewall relay, or the one above the wheel well, and why the * do I have two of them? The Chiltons and Haynes manuals all have 'possible locations' for the relay, so they are not much help.

 
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Joe_F's Avatar
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07-26-02, 03:43 AM   #2  
Joe_F
They must be not only the same type, but the same part number to be swapped for the test. You might pick up a replacement instead, they are only a couple of bucks at any parts store.

 
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07-26-02, 04:56 AM   #3  
Antony W. Serio
The four relays all have the same part number. I was just wondering which one I should swap out, as I can not identify which one is the fuel pump relay. If they are cheap enough, I may just swap out all four.

 
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07-26-02, 05:26 AM   #4  
Joe_F
Identify which accessories are controlled by the relays and remove them one by one. Then, try those accessories. Whatever doesn't work (which did a minute before) is what that relay controls.

Take a permanant marker and mark it on the relay for future reference.

 
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07-26-02, 06:02 AM   #5  
Antony W. Serio
The only relay that is for an 'accesory' is the A/C relay. The others are labeled in the service manual as 'fuel pump relay' (X2), and 'oil pressure relay', assuming the manual is right. Of course, the manual could be wrong. What would the oil pressure relay do if I unpluged it?

 
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07-26-02, 06:41 AM   #6  
Joe_F
The oil pressure relay is likely for the fuel pump because the oil pressure switch is wired into the fuel pump circuit. The fuel pump turns on once it senses the engine has sufficient pressure. I'll bet the one marked oil pressure also works the fuel pump. I'll also bet your oil sending unit is a multiterminal, multifunction switch.

 
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07-26-02, 07:26 AM   #7  
Antony W. Serio
I don't think so. I definitely have seperate oil pressure and fuel pump relays. The service manual explains how if the fuel pump relay does not activate the fuel pump, then the oil pressure relay will turn it on. It could be that the oil pressure relay is a backup for the fuel pump relay or something like that. I am not sure about the sending unit.

 
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07-26-02, 08:18 AM   #8  
Joe_F
Swap 'em around and see what happens. Just mark their original locations so you know where they go. Draw a diagram if you have to so they all wind up back in the original spot.

 
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