I would like to run my electric fuel pump only when the engine is running for safety reasons.

This is a race only car. I would like to wire the fule pump to get power from the altnator when running.

I feel that I can do this cleanly by installing a splice before the battery and a diode in line to the fuel pump.

I can run a seperate switch to prime the carb prior to starting and then turn it off once the car fires.

Where can I get a diode?

I do not want to use the oil pressure to activate the pump since I already have too much stuff running off of that source and don't want to have the potential for further leaks.

 

radio shack should have an assortment of diodes that will work

 

What do I ask for?

I assume there are different ratings

The only diodes that I have experience with are the ones in my electronic science project kit.

 

1N5401 to 1N5406 are rated from 50 to 1000 V and 3 A.
I haven't looked at it in detail, but I guess you might need two diodes to do what you want.
I don't know what the current draw from the pump is, so you should measure this before proceeding. Any higher than 3 A usually requires diodes with heatsinks, but I guess you could use a relay instead to amplify the current.

 

You would need a diode between the alternator and the battery. And that would have to be a big diode that's rated for at least what the alt rating is, i.e. 100+ amps.

Have you considered the oil pressure switch that comes standard on our cars? It turns on the fuel pump only when the engine builds up oil pressure. If the race car you're building up is the 91 Z28, you already have the wiring for it on the engine and it should work.

Lou

 

A 3A diode can't be trusted with a fuel pump. A fuel pump is going to want more current, especially when it first starts. This idea won't really work anyways. To try to do this you could install a battery isolator (cheap, big, heatsinked diodes) and put the fuel pump on the alternator side of the isolator. It still isn't a very good way to solve the problem.

As was stated, the common way to do this is to use a relay that is activated by an oil pressure switch. One thing that was left out was that typically the starter selonoid is used to power the fuel pump as well. That way you have power to the fuel pump when you have oil pressure OR when the starter is engaged.

 

Thanks for all the replys!

After posting I realized the diode would have to go to the battery vs the pump and would have to be rated for the altenator output.

Engine is down now So I can replumb the oil lines. I believe that I will add another the oil pressure switch.

I have one in line for the ediot light now. It just cumbersome adding to the an lines and expensive. I thought the doide may have been a cheaper and easier way.

So what is a battery isolater?

 

I don't understand a diode is like a one way street and only lets power go one way. So same as a relay. and there is a fuel pump relay already on the car all you have to do is rewire the relay turn-on wire to what you like.

 

No, not the same as a relay at all.

 

A battery isolator is essentially two diodes with the alternator side tied together. That allows the alternator to charge two different batteries without directly connecting the batteries together. This prevents a load on one battery from draining the other. A few common applications are boats, RVs, or off road vehicles where there may be a starting battery and a deep cycle auxiliary battery.

Because these are commonly used, most parts stores carry moderate amperage models. The good part is that they not only come in sizes that can handle automotive alternator currents; they also come in a heat sink package that can keep the diode cool under load.

I still would not go about this problem by using a battery isolator.


To address 2fast4u92z; a relay shares no characteristics with a diode. A diode is something of a "one way street." A relay is just a switch that is turned on with electricity passed through an electromagnet. When the relay is activated the contacts act just like a piece of wire. When the relay is deactivated the contacts act like a cut wire. Obviously, a wire does not have a "one way street" behavior; electricity can easily pass in both directions.

 

^ I agree ^

But I guess I miss read what he was trying to do. And I know what you are talking about I have a huge battery Isolater and on it has 3 posts ALT,battery1,battery2 and huge heat sinks.

I thought he wanted the fuel pump to turn on when the alternator stared to kick out power or with a switch.

I perfer relays to diode due to the falure rate of diodes.

 

A properly chosen, protected, and installed diode will rarely fail. Most of the time when a diode fails it is because it is in a poorly designed circuit or simply inadequate for the application.

A good example would be placing a 3A diode on a fuel pump. The current and voltage spikes seen when the motor starts and stops would cause the diode to fail.

Relays are more forgiving when pushed beyond their designed limits, but they will also wear out and can cause problems when the contacts do begin to pit and burn.