Do I need a fuel pump relay!

[1938vw]

I don't understand what they do neither do I understand if I need one?
I'm using a Renault 5 gt turbo fuel pump on my turbo engine.

[UKLuke72]

You ought to have one wired like this


That way if you end up in a ditch on fire the fuel pump cuts out

Nicked the pic from Mr.Whippy http://www.volkszone.com/VZi/showthread ... 633&page=2

[1938vw]

Oops

[1938vw]

Thank you very much. Where would I find the correct "changeover relay"?

[UKLuke72]

http://www.ebay.co.uk/itm/5-Pin-12V-DC- ... 2eb3b02695

[Marc]

The relay needs to be wired so that it will interrupt power to the fuel pump if the engine quits. This could prevent the engine filling up with fuel if a carb float valve leaked while idling unattended, but the most important reason to have one is to stop the flow of fuel should you be in an accident and are unable to shut off the ignition.
There are various ways to "tell" the relay that the engine's quit. Stock L-Jetronic cars monitor airflow into the engine, when it gets below a certain point the pump stops. K-Jet (CIS, as in Rabbits) monitors the "tach" signal from the distributor, and stops the pump when it gets below a threshold RPM. Both of these approaches work well (and incorporate some means to automatically energize the pump when starting the car so no bypass switch is required) but each requires a specialized relay
If you want to control a fuel pump using a "plain" SPST load-reduction relay, it'll need to be wired to react to either oil pressure or charging system output. Since the stock oil pressure warning light sender is a normally closed switch which opens when oil pressure exceeds 4psi or so, it's of no use here if you want a "fail-safe" system (which you should) - using the stock switch, the relay would need to energize in order to stop the pump. Any number of problems could arise which might prevent that from happening.
An aftermarket pressure switch that is normally open and closes when pressure exceeds its setpoint solves the problem, but you'll need one with a low enough setpoint that it won't act when you're idling at a traffic light on a hot day. ~10psi models are common, you'll need to look bit longer for the 1-3psi version we need. Many sold on eBay, search for "Hobbs".
Finally there's the option of sensing charging system output instead. This is the way VW controlled the auxiliary freshair fans on `72-up Buses, ensuring that they'd shut down and not waste power if the engine stalled, and it works well enough. Wire relay Term 86 to the charging system warning light wire (D+ on an alternator, 61 on a generator regulator) and Term 85 to ground, fused input power to Term 30 and relay output Term 87 to the pump +.
The circuit in Whipp's drawing, with the pump wired to Term 87a of a SPDT relay, does absolutely nothing to prevent the pump from running when the relay's deenergized and is therefore not "fail-safe". Power goes from + to the pump via Term 30 and Term 87a when the relay's at rest.

[Dale M.]

Simple and easy....

http://revolutionelectronics.com/Fuel_Pump.html

Dale

[Marc]

Next time you're at the boneyard, look at the fusepanel on an early watercooled (Rabbit, Dasher, Audi Fox, etc.). If you see a relay with an integral fuse holder like the one pictured below, snag it if the price is right. It has the same features as the Revolution unit (runs the pump for a few seconds each time the ignition is turned on, shuts it off when the engine stalls) but it's not a fully solid-state device, so these things were never paragons of reliability...if you choose to use one be sure you know how to bypass it in an emergency.

[Piledriver]

The later model ones without the fuse worked better IME, but some had a rev limiter to cut fuel IIRC, could be a bug or a feature depending on application.
(used an external fuse)

Except for the relay mechanism itself they are solid state, 555 or 556 timer (I have slept since then) , a transistor to drive the relay, and some timing components.

The biggest thing wrong with the OEM units I personally have seen is it seems they used some dodgy solder or technique back in the day.... That and those relays were driving the ~equivalent of an 044 pump. (one step lower, same flow, less max pressure) so the wiring/relay got warm, esp with the marginal fuse connection and water dripping onto it after running down the antenna wire on MK1s.

Still available at most FLAPS for ~$35 new, and $1 at most boneyards, very easy to work with.

[1938vw]

Will this one work ok seems to be the same as the first diagram?

http://washford.scene7.com/s7viewers/dh ... rd/184161a

[Marc]

If it's rated at a high enough amperage for your pump it'll be fine. This is the double-throw style (SPDT) that has a current path from 30 to 87a when deenergized. There's no need for that if the wiring's done properly, but it does no harm.

[1938vw]

This is the double-throw style (SPDT) that has a current path from 30 to 87a when deenergized. There's no need for that if the wiring's done properly, but it does no harm.

Sorry buddy I don't understand

[Chris V]

When the relay is in the natural open state there's continuity between 30 and 87a...when it's energized 87a is no longer 'hot', and instead 87 has continuity to 30.

Wiring the relay 'backwards' from terminal numbering standard (where 30 is constant hot and 87/87a are switched) results in making the 'extra hot' terminal a non-issue if it's a simple relay. 87a never has continuity to 87.

A SPST relay is not un-like a simple SPST toggle switch to control a bedroom lamp...a SPDT relay is like having the same switch turn-off the hallway light as the bedroom light is turned on and vice-versa.

[volksbugly]

another really cool thing to do with the relay, especially if its on a baja is to include a tilt switch, so if your car flips upside down it cuts the fuel.
So you can wire in two SPDT one relay is the main and gives power to the FP, it is turned on by the ignition and the other relay being in the correct state, the other relay
is put into the proper state by the tilt switch and by the alternator warning light. you get the correct "state" by tilting the tilt switch either open or closed.

I can draw a diagram when I get home tonight, I cant upload from work.

I don't understand what they do neither do I understand if I need one?
I'm using a Renault 5 gt turbo fuel pump on my turbo engine.

I'll try to attempt to explain my understanding of why you need one.
Two reasons:
1. If you have to much of a load on a switch/wire/circuit, ie trying to pull to much power through a single wire or switch then you need a relay to augment power
2. If you need to have more control over a circuit ie control the circuit with another switch

[1938vw]

I have the relay. Cheers guys, next question is what size fuse and wiring do I need to use? As its a 30amp relay do I need a 30a fuse and 30a wire?