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Fuel pump relay repair (Injection)

Posted: Wed Jul 29, 2020 5:33 pm
by Not_Anumber
The fuel pump relay is a known point of failure on Injection cars that can fail without warning.

This may help you repair a fuel pump relay to good as new/ better than new condition or prevent a problem occurring.

From the outside the thing looks like a standard relay, though with 5 rather than the usual 4 connections. The genuine Ford ones are a pinky purple colour though the 3rd party replacements tend to be black. On the inside a small PCB is host to a simple circuit which checks the engine is running (via a cable to the ignition coil ). A miniature relay is also mounted on the PCB which does the final part of the job of switching the live feed to the fuel pump on and off.

There are 2 sources of heat in this device, The coil of the relay warms slightly whilst current passes through it to keep the contacts closed. Additionally though there can be heat produced by the switching operation itself. The pump can draw a fair amount of current especially when it is a few years old and this can warm up the relay's switch contacts plus the copper tracks on the PCB that connect these to the external terminals. Over time this heat can degrade the solder joints on the tracks which can then cause arcing which can make the solder joints fail - stopping the fuel pump.

The fuel pump relay is mounted with the other relays under the driver's side lower dash just above the footwell kick panel at the side, to the right of the pedals. If you are very lucky you may be able to access it just by removing the thin plastic cover over the pedals- most though will need to remove the lower steering cowling and the right hand part of the under dash panel. The relays are clipped to a rail - they should all be clipped with the connections pointing downwards, if not then unclip them and put them all up the right way. They are susceptible to moisture damage if the wrong way up and this is so easy to prevent.

The way to check whether the fuel pump relay is actually the culprit is to temporarily unplug it and plug in the heater rear window relay in its place. (The HRW relay is dead easy to spot as its the only one with a clear plastic cover on top.) Doing this will make the fuel pump run whenever the ignition switch is on, it does away with the need for the engine to be running, a safety feature in the event of an accident. Yes the car is now driveable working round a duff fuel pump relay but please only use this as a fault finding trick as you dont want to permanently remove a safety feature.

With the fuel pump relay removed from the car use a small flat bladed screwdriver to carefully open the casing from the terminal end. It will slide off as a 5 sided box leaving you with the terminal end and it's PCB. Find the switched contacts of the miniature relay and look at the underside of the PCB in this area for damaged solder joints. Carefully clean and resolder the joint and if any nearby solder joint also look damaged its worth re heating them and adding a tiny bit more solder if necessary. Be careful on soldering any joints to a transistor as they can be heat damaged if the soldering iron is left in contact with them for too long.

I like to improve things if I can so to prevent the same thing happening I added an additional jumper wire, above the pcb, direct from the inside of terminal 87 to the body of the mini relay itself. This duplicates the connection on the PCB so prevents the likelihood of the solder joint failing again over time. You could do this as a useful bit of preventative maintenance.

The fuel pump relay terminals and its connections are

1.. Green wire to coil (picks up the pulse of the engine running)
15. Black wire. This is a switched Positive from the ignition switch
30 Thick red wire. This is an unswitched Positive from the battery
31. Brown wire Negative / earth. (Grounds to the body of the car)
87. Pair of black/red stripe wires. This is the +supply outwards to the fuel pump (and also feeds the warm up part of the fuel injection system)

Re: Fuel pump relay repair (Injection)

Posted: Tue Feb 09, 2021 3:48 pm
by ollyw

This is a great article, yes you are quite correct, the fuel pump relay often just has dry soldering joints on it. Most people just swap them and chuck the old one away but as you say they can be easily repaired with a soldering iron.

Olly W