Fuel Pump Relay Fried

[RedTwo]

I rewired my fuel pump as per VFAQ (aside from using a SPST over a SPDT), everything was well and good for over a year until the other day I managed to kill the relay. Terminal 87 is pretty fried but the 15A fuse (across terminal 30) and the other terminals are untouched.
I've had a quick check through all the wires people have suggested before (o2, MPI relay, etc) and they all look good. I reconnected the wires from 86 and 87 and the car fired up.

Can anyone explain why 87 (to fuel pump) fried before the terminal and/or fuse coming off the battery did? Would this mean that the car/pump was actually running when the relay blew?

 

[toybreaker]

What was the relay rated at?

It may be as simple as the relay contacts got oxidized and the increased `resistance created enough heat to melt the terminal. /ubbthreads/images/graemlins/dunno.gif

For what it's worth, I've seen issues with the potter brumfield brand relays in that application. They just don't seem to hold up to the continuous duty that a fuel pump application requires.

I'd fire in a 40 amp rated Bosch unit, and call it done...

If the cause of the failure is still bugging you, open up the old relay and shoot a few pics. That'll tell the tale.

You can also measure the fuel pump current draw, as the other potential cause for that type of failure is a pump on it's way out. (Many pumps will draw extra amperage when they are on their last legs.)

edit Do you have an anti-fly back diode installed in this circuit?

 

[FlyingEagle]

Quote:
edit Do you have an anti-fly back diode installed in this circuit?



A wigywho? This sounds like a good idea, but what does it do?

 

[toybreaker]

Quote:
Quote:


edit Do you have an anti-fly back diode installed in this circuit?





A wigywho? This sounds like a good idea, but what does it do?




/silly,

it prevents issues with Musca Domestica Linneaus entering teh fuel system and wreaking havoc with teh fuel trimmage... /ubbthreads/images/graemlins/wink.gif

//silly

Actually, don't feel bad, I didn't know about this little device for years. /ubbthreads/images/graemlins/blush.gif

I always wondered why I was burning up relays and switches, when I thought I had properly rated them, (or in the case of electric motors, over-rated them by a factor of three).

The contacts of the switches would be all burned/pitted, and over time, the resistance of the contact point set would increase to the point that the circuit cease to operate consistently.

I was lucky enough to encounter a saaavy old school guy and he explained a few things in plain and simple language that even I could understand.

It seems that when an electric motor is operating, a magnetic field is created in the windings. (that's what attracts/repels the magnet segments on the rotor and makes it spin)

When the current is switched off, the magnetic field collapses.

This induces a counter electromotive force, or a power surge in the opposite direction of the original current flow.

This sends a power spike back up the wire. (The concept is very similiar to how a secondary circuit in an ignition coil works.)

This spike will arc/jump across the contact points in the relay/switch like a little lightning bolt, and over time, it will erode/damage the surface of the points.

Installing a suitably sized diode (or capacitor) in the circuit will shunt that power spike to ground, saving the points from the constant arcing.

This will prolong their service life considerably. /ubbthreads/images/graemlins/smile.gif

Many of the higher end relays, (and most bosch 4 pin relays) have this feature built in, but many of the cheaper and/or car stereo type relays don't have them, and you will need to an install one yourself.

 

[FlyingEagle]

Oddly enough we just learned about this in our first autmotive electrical classes.
It was called a clamping diode, but that is neither here nor there as it does the same thing. Current and or voltage will run around in a circle in the device until it's gone, or low enought to not cause said spike back into sensitive devices. I wonder if my fuel pump relay kit was supplied with a good relay. EEP!

 

[iceman69510]

Quoting toybreaker:
Quote:
Quote:


edit Do you have an anti-fly back diode installed in this circuit?





A wigywho? This sounds like a good idea, but what does it do?




/silly,

it prevents issues with Musca Domestica Linneaus entering teh fuel system and wreaking havoc with teh fuel trimmage... /ubbthreads/images/graemlins/wink.gif

//silly

Actually, don't feel bad, I didn't know about this little device for years. /ubbthreads/images/graemlins/blush.gif

I always wondered why I was burning up relays and switches, when I thought I had properly rated them, (or in the case of electric motors, over-rated them by a factor of three).

The contacts of the switches would be all burned/pitted, and over time, the resistance of the contact point set would increase to the point that the circuit cease to operate consistently.

I was lucky enough to encounter a saaavy old school guy and he explained a few things in plain and simple language that even I could understand.

It seems that when an electric motor is operating, a magnetic field is created in the windings. (that's what attracts/repels the magnet segments on the rotor and makes it spin)

When the current is switched off, the magnetic field collapses.

This induces a counter electromotive force, or a power surge in the opposite direction of the original current flow.

This sends a power spike back up the wire. (The concept is very similiar to how a secondary circuit in an ignition coil works.)

This spike will arc/jump across the contact points in the relay/switch like a little lightning bolt, and over time, it will erode/damage the surface of the points.

Installing a suitably sized diode (or capacitor) in the circuit will shunt that power spike to ground, saving the points from the constant arcing.

This will prolong their service life considerably. /ubbthreads/images/graemlins/smile.gif

Many of the higher end relays, (and most bosch 4 pin relays) have this feature built in, but many of the cheaper and/or car stereo type relays don't have them, and you will need to an install one yourself.



I love this place.

 

[ktmrider]

LOL, good description IMO...
Here is a great link click that covers exactly what the others are talking about ( altho they call it a "Quenching" diode ).

 

[89coltgt]

They also call it a freewheeling diode.

click