Electric fuel pump safety switch using Alt' WL term'

[miked]

Guys,

Anybody done this? Sorry for length. Hope it makes sense.

I just got around to fitting the fuel pump to my S4. All piped up with a filter king reg' etc. Managed to fit my pump in the corner of the spare wheel area on the inner wall. I previously cut out a trap door for the handbrake tree and diff' plug access and fitted an ally' plate. So I put on this with a relay.

I have fed the pump via a fused supply direct from the battery (saves volt drop to and fro' the front) using the local relay in the positive leg. I have found a spare wire in the loom to signal the realy coil from the front of the dash area. Have also installed one of those impact switches on the bulk head. Second hand from Ebay. I looked into fitting of an oil pressure switch to kill the pump in the event of an accident with the engine stopping with ignition still on. Spilling fuel etc. I already intended to fit a prime switch that allowed me to start the pump when the ignition was on. Saw no point in that business using the cranking signal as a first supply as it shows with the pump instructions. The whole point is to get fuel in the carbs before you crank. Is it not???? so you are not pulling your battery down to fill carbs and slowing your cranking speed.

Well, I have seen (at work) old generator and diesel fire pumps panels with auto start systems using the WL of the charge alternator to also ground a relay coil in addition to any excitation supply or ignition warning light. This has been used for the "crank disconnect" or "crank terminate signal". On these systems the engines are given a start signal and something is required to sense when the engine is running so the starter is withdrawn from the ring gear. Otherwise - smoke. Today it is done with magnetic pick up's and also main Alt' a.c. volts on generator sets. Oil pressure has been used.

On the old system, it is like this; As the engine starts and gathers speed the WL terminal (which was a negative) lifts to a postive potential and the relay de-enegsies. Also the ignition light goes out by the same principle. The relay removes the starter (crank signal). I am going to use this principle. I will parallel the coil of a relay with the ignition warning lamp so that it enegises as the ignition it put on. It will switch out the pump supply. I will then use the prime switch to short the relay (now open contacts) to fill the carbs. When primed, I crank and the engine starts and the relay de-energises. The contact used is a NC, so the pump is supplied by the de energised relay when the engine is running. If at any time the engine stalls the pump is stopped as the charge alt' is no longer spinning and the relay is energsied again as the WL terminal is made negative again.

The only down sides I see are very small. If the alt' packs up, the ignition light comes on and my pump is stopped. USE prime switch to get home, no bother.

If the fan belt slips or the brushes start to give out and the light glows dimly, use same the same switch.

If the fan belt snaps, you are not going to drive anyway because of the water pump being stopped. Fit your spare belt .

If the relay coil circuit goes open by fault, the relay will not energsie to isolate the fuel pump. I am think of fitting a small LED via one of the spare relays contacts to see when the relay is energised. Will hear the click anyway.The circuit is tested very time you start the car. You can say it is not truly "fail safe" having to energise to isolate. However, on balance am I on my own not wanting to fit one of the oil switches because of the messing. If so, it does add significant safety if you have not fitted any kind of device.

It may sound long winded as I have tried to describe it but it is only one relays and a few wires and a prime switch which I believe others use with the oil pressure method anyway. I have decided to do this as it seems a pain in the bum trying to break into the oil pressure circuit. I did not fancy a "T" into the gauge pipe nor the use of that outlet under the exhaust manifold. Are there others I have missed


Any thoughts. Will post how I went on but see no poblems.

Mike

[steveww]

Sounds complicated to me

On my car the electric fuel pump is switched on (via a relay) from the ignition circuit i.e. coil positive. When starting the car the ignition is switched on and you wait a short time while the pump fills up the float bowls, then crank as normal.

I have though about fitting on of those impact switches to kill the fuel pump in the event of a bump but it is still on the "to do" list. The car does have a master battery switch which can be used in the event of disaster which of course turns everything off.

[alan71]

I just use an inertia cut-out switch (and relay) wired to ignition on, no need for a prime switch.
The only way I can see this being a safety risk is if you roll the car or break the fuel line without triggering the cut-out, is that possible?

Alan.

[miked]

Alan & Steve,

Thanks for reply. I get a bit paranoid about safety. Ex mining elec' and seen some odd stuff.

I to drew the conclusion that I would have to have a failure of the impact switch coupled with a fuel spill and be stunned into not switching off the ignition. I am glad I wrote this down and got feedback as I think I am going to far. The impact switch is a ball bearing sat on a switch with rubber housing. The chance of it not dislodging in an impact so severe to rupture the fuel circuit is very very slim.

I will not bother with it nor the prime switch. I will just pick up the ignition switch supply as you have done (to my impact switch) and leave it at that.

There is no Emoticon for "paranoid", if there were I would posted it.
Or has somone hid it?


Thanks Mike