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MG MGB Technical - New Electronic SU Pump Install

So I have a 79 B and I am installing a new su electronic fuel pump. I remember the other pump (replacement no name electronic unit) the unit already in the car only has the two banjo fittings and nothing else and with the new su there are what looks like vaccum fittings etc on the pump. Im just wondering where everything hooks up. There is one rubber hose right next to the fuel pump that looks like it should be connected to the new su but im not sure to which one etc. If at all. Also I remember hearing on here to run a fuse on the fuel pump. What fuse is suitable ?
James

Pumps have vents on the end cap covering the electrics and on the body near the diaphragm. The vents are necessary to equalise the pressure inside the pump as it is of the reciprocating type. Where the vents are outside i.e. underneath the car the pipes prevent dirt and moisture being sucked into the pump

On a chrome bumper car there should be two plastic pipes going into the boot terminating in tees. On a rubber bumper only one and that goes on the body vent of the pump (on a late model rubber bumper the other end may go to a nipple on the chassis rail and not a Tee in the boot). The electrics end on a rubber bumper is already in the boot so doesn't need a pipe.

It's good practice to fuse the pump (and the overdrive) as these circuits are not fused from the factory and it is not uncommon for the wiring to get shorted which can severely damage the harness. The easiest way is to get an in-line fuse carrier which takes the same fuse size as in the fusebox and fit bullets to each wire. Find where the white (or white/brown) wire from the rear harness joins the main harness near the fusebox at a double connector, pull the rear harness white wire out of the double connector, and insert the fuse using a new single connector on the rear harness white. Use the same rating fuse as in the main fusebox, as you have (or should have) two spares in the fusebox lid. There is nothing gained by having different ratings or different types of fuse for additional fuses, it only means you have to carry more spares.

For the overdrive, for a car with the manual switch on the dash or column, find where the yellow wire from the gearbox harness joins the yellow wire in the main harness, in the same place as before, and insert another fuse as before. For UK cars with the manual switch on the gear lever it is a white wire in the gearbox harness and the same white (or white/brown) double connector as for the fuel pump. For North American spec cars with the manual switch on the gear lever a single white comes off the inertia switch to a double-connector nearby. That has one white to the gearbox harness for the overdrive, and another to the rear harness for the fuel pump. You can fuse these individually as before, or if you pull the wire coming from the inertia switch out of the double connector a single fuse will protect both.
Paul Hunt 2

James - E-mail me and I'll send you information on the fuel pump vents. Regardles of hte year of the car, the only vent that needs to be routed into the trunk (boot) area is the one at the base of the coil housing. The vent on the end cover (if there is one there) has a check vavle in it so that ai can only be expelled from that area and not drawn back in. The vent at the base of the coil housng is open to air going both ways and needs to be routed into a relatively dry area so as not to get moisture into the pump above the diaphragm.

I don't know what the current draw for the O/D solenoid is so I can't say what size fuse is required in that area, but for the fuel pump a 10 amp fuse will protect the wiring and still stand up to the current being drawn (about 4 to 5 amps on the new pumps). Cheers - Dave
David DuBois

Thanks guys, just one question as to why you need to run the fuse near the main harness? For the reason of it being just a lot easier than tracing wires down, I could just splice in the inline fuse in the trunk, correct ?
James

You want the fuse as close to the power source as possible to provide maximum protection. If you place the fuse in the rear and the wiring from the front of the car shorts out before it gets to the fuse it will fry the wiring from under the hood to the fuse at the rear of the car.
John H

Got ya, thanks
James

The later LH-type solenoid takes about 800mA. The earlier D-type take about 17 amps pull-in and 2 amps hold-in. So whilst the LH-type could work with a 2 amp fuse quite satisfactorily, there seems little point in using a value different from everything else in the car, for which you have spares in the fusebox. As well as not having to carry a selection of spares, the fuse protects the wiring which is perfectly capable of taking the current required to blow the standard fuse. And as John says, the nearer the fuse is to the supply the more wiring is protected (which is why on modern cars the main fusible links are often right on the battery post). With dashboard OD manual switches you can put male and female spades on the in-line fuse and install it on the back of the switch (which is what I've done), but I didn't want to confuse the issue any more than I needed to by giving yet another alternative.
Paul Hunt 2

This thread was discussed between 25/06/2007 and 26/06/2007

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