Fuel pump stays on when ignition is off

mercury

#1

The fuel pump on my 1989 Mercury Grand Marquis is still running after turning the ignition off, causing the battery to drain. Replacing the fuel pump relay stops this from happening for a short time but it soon reoccurs.



Is it possible there is a short in the ignition system, or is this a sign that the fuel pump is going out?


#2

I believe you have a bad ignition switch. Power should be removed from the relay when the ignition is off. If you keep a relay powered up long enough it will fuse internally and cause the problem you are experiencing. You need a good wiring schematic for your vehicle and check that power is removed from the relay when the ignition switch is “off”. A Haynes or other repair manual for your vehicle will have the needed info.


#3

Take a look at the failed relay by opening it. If the contacts are welded together then your pump is drawing way too much current. Check the pump leads for a hot short to the chassis. Some how more current is being pulled through the relay than the contacts can handle.


#4

euryale1 makes a good point. Here is a simple schematic of your wiring to make things easier:


#5

It could be that replacing the relay which caused the problem to go away was conincidental.
The trigger for the relay is grounded through the ECM. The ground is provided electronically for a few seconds when the key is turned on and is maintained after that due to a pulse from the ignition module.

It could be due to an erratic fault in the ECM. If you have a VOM or test light you might check the diagnostic connector where the codes are pulled for a tan wire with a green stripe.
This wire is an offshoot of the pump relay wire that grounds through the ECM.

If the car is currently acting up probe that particular wire and note if it shows a ground or not. There should be none. (key off of course)


#6

True but a hot wire poorly grounded(ie not dead short) can weld the relay, possible that the pump is pulling too much itself as well. The ground via ecm is the relay ground not the pump ground. Something held the pump relay closed and the ecm is not the best candidate but a good second choice.


#7

There is hardly any current at all involved with the relay trigger though. That’s measured in milliamps.

I mention an ECM fault because I’ve seen this a couple of times.


#8

Try rapping on the fuel pump relay with the handle of a screwdriver or similar. If this shuts off the pump, the relay is sticking. I had exactly your problem, and it happened to be on a 1987 LTD Crown Vic I had, a very similar car.

When you say “Replacing the fuel pump relay stops this from happening for a short time”, do you mean you took the same relay out and put it back, or that you replaced it with a new relay and the problem still occurs?


#9

I have replaced the fuel pump relay with a new one. I have done this 2 times and the fix lasts for about 2-3 days of driving.


#10

Remove the mounting pad were the relay is located and turn it over. Look for melted connections. Since you now have 3 relays, open one of them up and look for fused / burned / melted parts…If so, there is a short in the fuel pump circuit or the pump itself is drawing way to much power, overheating the relay. But this should blow the fuel pump fuse before meltdown occurs…


#11

What you should do, while the car is acting up, is probe that wire lead I told you about to determine if a ground exists when the key is in the OFF position.
This would take all of a whopping 10 seconds and not only would it weed out the possibility of an ECM fault it would require far less time than replacing the relay; again and again and again.


#12

If it IS the ECM, you can simply put a switch in that relay ground wire, mount the switch on the bottom edge of the dash, and control the pump that way. A replacement ECM would total the car…