Inertia Switch/Fuel Pump/Relay Problems

Page Revised: 10 May 2019


Article by
Eric Dess

Sometimes you'll have an instance where the car won't start, and you know that you have good spark. Even with a 1983 carbureted engine, there's still a computer controlling parts of the ignition and fuel system. In these cases it's wise to look to the electrical part of the fuel system for troubleshooting.

When you turn the ignition key forward ('ON') but do not fully start the car, a signal is sent from the ignition switch to fuel pump relay in the trunk, then to the inertia switch, then to the fuel pump in the tank (or on the framerail, depending on your model year). The fuel pump will audibly hum for a second or two in order to prime the fuel system and build up pressure. If you do not hear the pump humming, you must look to either the relay or the inertia switch first before you drop the tank for a pump change.

Fortunately you can see that with only a few breaks in the fuel system's electrical components, it's fairly easy to troubleshoot if there is a problem with starting the car. Here are some basic guidelines for finding a problem in the electrical part of the fuel system.

Inertia Switch

If your car has been in a rear-end collision, or if there is some other type of sudden blow to the rear part of your car, chances are it won't start. This is because the inertia switch, also known as a fuel cutoff switch, has been tripped. This switch exists on modern fuel-injected cars as a safety measure when accidents occur, cutting the fuel pump off thereby eliminating the fuel flow to the engine, where a fire or explosion could occur. The system is pretty basic but ingenious, letting a simple spring lock and a few wires do all the work. A white colored button on the top of the switch is always in the down (locked) position to allow power to the fuel pump. When the area around the switch is suddenly jolted, the switch pops up from inertia. All you need to do is simply press the button down until it locks, then try starting your car.

Intertia Switch In 1983-86 Cougars and Thunderbirds, the inertia switch is tucked away in the trunk on the passenger side, behind the metal rail next to the trunklid hinge. For cars with a trunk liner, there is an access hole in the liner on that side to reach the white spring-loaded button on the inertia switch. Cars without liners still have an access hole in the metal.
Intertia Switch The white arrow indicates the access hole through which the reset button is located. In case the switch ever needs to be replaced, the orange arrow indicates the screw that needs removed.
Intertia Switch A tight shot of the inertia switch from the back side of the metal panel. On the top you can see the white reset button.
Intertia Switch In 1987-88 Cougars and Thunderbirds, the inertia switch is on the taillight panel inside the trunk, on the driver's side. There is a hole in the trunk's taillight cover panel through which you can access the white spring-loaded button on the inertia switch. There is no need to remove the cover.

With age and especially in climates with extreme temperature changes, the inertia switch can fail all by itself. The spring lock mechanism that holds the button down can rust or become stretched, and the button will pop up at will. This can happen particularly on a very cold night. Understanding this problem before it happens can be a blessing! If you're stuck somewhere and this is indeed the case, you can temporarily tape or zip-tie the button down securely to get you home. A new switch usually lists for around $80 US and can be easily changed yourself. This may still be a Ford dealer-only item; nevertheless, feel free to shop around.

Fuel Pump Relay

If your car's fuel pump will not turn on and you know that the inertia switch is good, or if the fuel pump keeps running after the ignition is turned off, then you have a bad fuel pump relay. It's the only part on the whole car (aside from the ignition switch) that can cause either problem. All you should need to do is change the relay to fix the problem.

The fuel pump relay is found in the trunk behind the back seat, on the passenger side. The relay itself is about 1"x1"x2" and is a green color, attached to a black housing. The location of the relay depends on the model year:

In 1983-86 Cougars and Thunderbirds, you'll find the relay attached to the seat's metal support brace, or sometimes hanging behind the rear seat cushion.

1983-86 Fuel Pump Relay

In 1987-88 Cougars and Thunderbirds, the fuel pump relay is attached to the trunk support panel. This is directly behind the trunklid hinge.

1987-88 Fuel Pump Relay

Replacing the fuel pump relay is very simple: simply pull hard until it detaches from the black housing, and replace with the new relay. You can find fuel pump relays at the Ford dealer or at parts stores nationwide.

If for some reason changing the fuel pump relay does not solve your problem, then you will have to replace the ignition switch.

Fuel Pump

If you've ruled out the interia switch, fuel pump relay, and ignition switch as the causes of your starting problem, then you are probably looking at a fuel pump replacement. In general, fuel pumps will either slowly die, or just quit without warning. If you're lucky you'll get some warning signs first, like very long starting times, erratic idle, cutting out under heavy throttle, and hesitation. Or you will hear the hum of the pump get noticeably louder, or have a high-pitched whine. After awhile your car simply won't start because the pump refuses to turn on.

You may be tempted to replace your fuel pump with one from the local parts store. Now we won't berate anyone's business, but many readers have reported that generally you will not have good luck with most of these pumps. It's common to have to replace these parts-store pumps every few years, if not sooner. You surely don't want to drop the tank every few years to change the pump, even if it is under warranty! The only tried-and-true factory-style fuel pumps will come from the Ford dealer. Surprisingly they are reasonably priced so don't let the cost factor keep you from making the trip. As with heater cores, the Ford pieces are second to none in quality and longevity. It's not worth saving a few extra bucks on an inferior pump, especially when it's such a necessary part of the car, and something you depend upon daily.

BBK Fuel Pump

We have had good luck with high-performance aftermarket pumps. If you have a 1986-88 5.0 car, or any other car with just an in-tank, high pressure pump, you can install one of these. Places like Summit Racing and Jeg's are good places to find them. You won't find these listed for a Cougar or Thunderbird, but you can use a Fox Mustang pump. Just so you know, you only need the pump itself, not the pickup assembly as are sometimes sold with Mustang pumps. The Mustang pickup assembly will not work properly in your tank, so don't waste your money on something you can't use. As for the ratings, the stock high pressure pumps (5.0L V8, some 3.8L V6) put out around 88 liters per hour (lph). Depending upon your engine's performance parts, you can upgrade to a pump with a higher volume output. The bare minimum considered for a performance pump is 155lph; 190lph is more desirable. BBK is one of the few brands where you can buy just the pump without a pickup assembly. Also, do not be confused by pumps that are listed in gph, or gallons per hour. These flow much more fuel than you'll probably ever need. Stick with a pump that's rated in lph.

When you install a new pump in the tank, whether a stock-type or a high-performance aftermarket pump, you will reuse your old pickup assembly. The new pump may have to be zip-tied to the assembly, as some aftermarket pumps are a little smaller. Also, some rewiring may be necessary to get an aftermarket pump to work. Don't worry: there are only two wires on the pump, black (ground) and red (power). Make sure your electrical connections are super-safe! Also, you can reuse the sock if it looks pretty clean. Sometimes aftermarket socks don't fit our pickup tube very well. When you've got the new pump in, the fuel lines and wiring connected, and the tank back into position, you have to prime the system before starting the car. Turn the key forward ('on') but do not start the car. Listen for the tank to make a noise; it can be a slight buzz or a muffled gurgle. Turn the key back off, wait a second or two, then turn it forward again. Do this 5 or 6 times to fully pressurize the fuel system, then start the car all the way. It should fire up immediately and the pump should run normally. Any sputtering may be electrical in nature so be sure to double check your wiring before reassembly.

Lastly, whenever you drop the tank and/or change the pump, don't forget that you will need to change the fuel filter. This is because you will stir up any sediment that's been sitting on the bottom of the tank, no matter how gentle you are. You don't have to change the filter immediately after a pump install, but you should within 10 miles or so, which is usually enough of a distance to get you to a gas station to fill up the tank, or take the car for a spin to make sure everything's working properly. Basically you want the pump to kick up the big, nasty sediment into the old filter for at least a little while before putting a fresh filter in. Changing a filter immediately after a pump replacement can be premature in this respect.