Fuel Pump Fuse Blows

Rickyll7 · Sep 20, 2008

Ok... so the fuel pump fuse keeps blowing every time I try to start the motor. The engine turns over fine, but wont start. The fuse blows before the pump even pressurizes the system. I had the battery tested and it tested "good" but only had 400 cranking amps out of a rated 800 cranking amps. So I charged the battery and the fuse still blew before the pump even turned on to pressurize the system. I've even tried a 30 amp fuse and that blew too (it's only supposed to have a 20 amp).

The first time this happened, I replaced the fuse (with a 20 amp), and it ran fine. A couple days later, I started the car and the fuse blew again after it ran fine for a few minutes.

I'm not sure where else to look...

Any ideas?

HOOCBB · Sep 20, 2008

first and foremost...

Fuses don't just blow, something is causing a higher amperage draw than the circuit is intended for. You will need to figure out exactly what all is on the same circuit, then figure out what is causing the fuse to blow.

It could be a locked up fuel pump (or in the process of locking up) causing a high amperage load. It could also be a shorted power wire to the fuel pump.

You will be better off using a wiring diagram and tracing the circuits on that fuse. If you are uncomfortable with electrical work, take it to a reputable shop.

Rickyll7 · Sep 21, 2008

Does anyone have a wiring diagram for a 1998 gt fuel pump circuit? Even with the wiring diagram, is there a good way to find these wires, pictures, a good book? Where is the relay for the fuel pump system? If something is shorted, it must be after the relay, because the fuse only pops when I turn the key to pressurize the system. Thanks!

Roblu98 · Sep 21, 2008

Don't put a larger fuse than what the circuit calls for. The wire is only rated to handle that current. Any larger fuse can cause a fire. If the fuse blows as soon as you turn the key you could have a short some were.

stangnutlx · Sep 21, 2008

Just disconnect the fuel pump and replace the fuse. Turn on the car and see if it blows. If not replace the fuel pump, if it does start chasing wires.

Rickyll7 · Sep 21, 2008

It's a short. I tested it with a multimeter. The fuse shorts to ground when the key is turned. Does anyone know where the wires route or at least where some of the components are such as the relay? What is the best way to chase these wires and where should I start? Any pictures or diagrams would be much appreciated.

Thanks again!

stangnutlx · Sep 21, 2008

Rickyll7 said:
It's a short. I tested it with a multimeter. The fuse shorts to ground when the key is turned. Does anyone know where the wires route or at least where some of the components are such as the relay? What is the best way to chase these wires and where should I start? Any pictures or diagrams would be much appreciated.

Thanks again!

Usually the easiest way is to start from the fuel pump plug and go from there. Finding shorts are a sucky job no matter how you look at it.

Rickyll7 · Sep 21, 2008

I put in a new fuse and unplugged the fuel pump and the inertia switch, then I turned the key. The fuse popped so it's got to be a short.

stangnutlx · Sep 21, 2008

Yeah definately a short.

Rickyll7 · Sep 21, 2008

Does anyone know where the relay for the fuel pump is? The manual I have calls it a "Constant Control Relay Module", I think. The short must be between this and the inertia switch as the fuse only blows if I turn the key to on, and still blows if I unplug the inertia switch.

Thanks!

BobHyatt · Sep 21, 2008

stangnutlx said:
Yeah definately a short.

It isn't necessarily a short. The motor is copper wire windings, which connect +12v to ground to make power. An ohmmeter may well measure 0.00 ohms because it doesn't have anywhere near the amperage in the built-in battery to turn that motor over. You need to drop the tank, and disconnect the pump there and then see if you get zero ohms to ground when you turn the key on. I'd bet the pump motor has failed, which can produce a dead short internally...

BobHyatt · Sep 21, 2008

Rickyll7 said:
Does anyone know where the relay for the fuel pump is? The manual I have calls it a "Constant Control Relay Module", I think. The short must be between this and the inertia switch as the fuse only blows if I turn the key to on, and still blows if I unplug the inertia switch.

Thanks!

This is in the passenger-side fenderwell. Easiest access is to remove the right front wheel, then the wheelwell liner. You will see it mounted up high near the front. has the fuel pump relay, A/C compressor relay, fan relay, etc. All built into one module. Why it was done that way is unknown as you can't replace an individual relay, you replace the whole thing which is around $150...

stangnutlx · Sep 22, 2008

BobHyatt said:
It isn't necessarily a short. The motor is copper wire windings, which connect +12v to ground to make power. An ohmmeter may well measure 0.00 ohms because it doesn't have anywhere near the amperage in the built-in battery to turn that motor over. You need to drop the tank, and disconnect the pump there and then see if you get zero ohms to ground when you turn the key on. I'd bet the pump motor has failed, which can produce a dead short internally...

Read the the 5th post....

Rickyll7 · Sep 22, 2008

Ok, this may take me a couple weekends to get to it, but this is what I'm going to try...

I'm going to switch the multimeter to continuity mode and connect it to ground and the power wire that powers the pump. I should then hear a steady tone signifying the power wire is grounded somewhere. Then I'm going to start wiggling wires between the inertia switch and the relay and hope that the annoying tone stops soon before I have to remove too many panels. Hopefully it will work and I can correct the short. I'll let you all know what I find.

In the mean time, does anyone know how the power wire is routed from the relay in the passenger fender well to the inertia switch in the trunk?

COramprat · Sep 23, 2008

Have you done any work immediatly prior to this problem?

The CCRM could be the problem because there is a relay inside of the module for the fuel pump. The relay is not generally replaceable so you would have to get the whole module. I have an extra used one if needed.

Which fuse is it exactly? Everything listed in your manual for that circuit or the fuse number.

BobHyatt · Sep 24, 2008

stangnutlx said:
Read the the 5th post....

Read my comment again. If you take an ohmmeter from the fuse to ground, unless the motor is burned up, there _must_ be a zero-resistance path thru there, otherwise the motor won't run when it is energized. My point was that zero ohms from the fuse to ground just shows continuity, which we know is there whether there is a short or not. You will get zero ohms if there is a short. You will get zero ohms since there is a path to the pump, thru the brushes, thru the commutator, thru the armature, and to ground. Either way looks the same. If you unhook the pump, and still get 0 ohms, you do have a short. With the pump hooked up, you can't just measure resistance from fuse to ground, see a 0.00 and conclude it is a short.

Rickyll7 · Sep 25, 2008

COramprat said:
Have you done any work immediatly prior to this problem?

The CCRM could be the problem because there is a relay inside of the module for the fuel pump. The relay is not generally replaceable so you would have to get the whole module. I have an extra used one if needed.

Which fuse is it exactly? Everything listed in your manual for that circuit or the fuse number.

No work none recently, and it's the fuel pump fuse only. I believe only the fuel pump is connected to it and its under the hood. It's right next to the ECU fuse.

nyuk98gt · Sep 25, 2008

RickyII7:

The Owners Manual diagram is WRONG for 1998. The FP fuse is the one nearest the fender, the PCM fuse is the one nearest the engine. Both are yellow 20A fuses.

GL,

Chris

Notchbck93 · Sep 25, 2008

A lot of times, the chiltons/hayes manuals have little diagrams in the very back, check your local library or get a used one or double check the back of the book before you buy it from your local retailer.

Finding a used FSM can be consuming/frustrating when you are anxious to find out what the problem is.

the proper way would be to work from the fuel pump back.

I would take the power wire of the fuel pump and check for it to be grounded---continuity.

Unplug the harness from the relay control module and check again. Then you get to start to tear out your interior looking to see where it could have rubbed through to metal/and or over heated and melted insulation and is touching a ground circuit wire.

If I had to have the car for work, I would wire in a new relay, build my own little harness, fuse the circuit, and rock and roll. I personally would rather fix it right, but a plan B option is just dandy as long as you use the proper size wires, amp fuse, and appropriately rated relay with harness.

BobHyatt · Sep 25, 2008

Here's the main thing. 99 out of 100 such problems are a bad pump motor, not a short. You can not tell whether the wire from the fuse is shorted to the body, or whether the continuity you are reading is through the pump motor. So to find a short, you are going to have to remove the tank anyway to disconnect the pump. Once you do that, you can now telll with 100% accuracy whether the power circuit is shorted or not. Or you can ignore than and simply connect the pump to your battery, being sure to use a 20a fuse for protection in case the motor is frozen/shorted internally.

But the tank is going to have to come down to diagnose this. I can guarantee you that any normal mustang will show continuity from the fuse to ground, because the motor is providing the continuity. It has to be removed to see if there is a short...

Let's not turn this into another "the clutch cable needs to be shortened as the clutch wears" type discussion...

Fuel Pump Fuse Blows

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