mjr wrote:So I take it that the L jet deals with shutting the fpump down in a shunt by way of a closed AFM flap? (what if the afm flap is tuck open though?), surely thats why the inertia switch is a secondary? my 83 still has an inertia switch wired in, I have never tried activating it, to see if it works though.
So if MS is configured to say 300rpm crank to turn the pump on properly, it doesn'it deal with shutting it off also? hence why a modification is required to deal with this? sorry for all the questions, but I have just ordered my MS11 3, and am a bit confused about the fuel pump situ after reading various threads about it.
Yes, it does deal with turning it on and off. Let's go back to the beginning and clear up any confusion.
1) The MS is responsible for turning the fuel pump on and off as a safety precaution. Perhaps this is a legal thing for the US market since most cars have this safety feature from the factory, including Alfas.
2) Speaking to #1, if it was mandated by law from the manufacturer, that would explain why the intertia switch is there for Spica cars. I can't speak to why it was conitnued on the early L-Jet cars. Alfa didn't ask my opinion.
3) It's possible the AFM could stick open but unlikely. On the Milano(75) the fuel pump is switched purely from the AFM switch. On L-Jet Spiders, it's switched by RPM just like the MS.
4) The MS - being a generic solution - needs to accomodate this and, IMO, the design is sound using RPM. Many OEM's use this method so we probably can't credit B&G with the design, just the good sense to incorporate it.
5) The L-Jetronic cars turn on the fuel pump through pin 88a of the combi relay. The MS turns on the fuel pump by supplying 12v to an external relay. This scheme will not work electrically with the stock L-Jet relay so a modification to the artwork of the MS is needed to supply ground to 88a to turn on the fuel pump.
6) The software functions exactly as designed using RPM to turn the pump on and off and is unaware of the electrical change to the pump switch itself.
7) The pump turns on momentarily for the priming pulse then turns off until it detects at least 300rpm during cranking. When it detects less than 300 RPM, like if the engine were to stall in an accident or rollover, the MS turns the pump off.
![Cool 8)](./images/smilies/icon_cool.gif)
The fuel pressure regulator stock delivers 36psi to the fuel rail and returns the excess fuel to the tank by bleeding the pressure to the return line. This serves 2 purposes. To cool the fuel as it returns to the tank and to ensure there is enough fuel and pressure in the rail to supply the injector pulsewidth at any RPM. Obviously, the lower the RPM, the less fuel is used and the more fuel is returned to the tank.
9) If one of those lines were to rupture in an accident, you'd have high-pressure, low-flow on the high side and low-pressure, high-flow on the low side of the fuel line. Neither of which you want in a crashed car. When the pump turns off, no fuel is flowing through or out of the lines to cause a fire, supply a fire, or poison the occupants (remember the return line travels *through* the interior of the car).
10) Therefore, turning the pump off when the motor isn't running is a really good idea.
That said, if you were not concerned with this kind of thing, like racing where you might have an external shutoff mandated by your sanctioning body, you might hotwire the pump to the ignition. The pump starts when the key is turned on a street car, or the pump is started by an internal switch by the driver.
On my cars, the Milano has the pump hotiwred to the ignition and the Giulia Super race car has an internal switch for each of the 2 pumps I run, and both ignitions which are also controlled by the master shutoff switch.
Hope this long essay clarifys things.
-Peter