The SU Pressure Pump Type “L” (Part 1)


The Type L pump was introduced by SU in the early nineteen-thirties, probably in 1932 or 33 and together with the instruments is one of the few electrical components of our cars not made by Lucas. Prior to that, SU offered the ‘Petrolift’, which dates from the late 1920s and was SU’s first electrically powered fuel pump. The Petrolift was not universally acclaimed, proving to be less than reliable in use.

From its introduction, the Type L pump quickly became popular with British car manufacturers and was made in both 6 volt and 12 volt versions to suit the vehicles of the day. Its use continued right through production of the T series and beyond, changing from a low pressure AUA25 mounted high in the engine bay to a high pressure AUA54 mounted low down near the petrol tank part way through production of the TF.

The Type L Pump from a
very early datasheet

Fig 1 shows the original and now much sought after single-piece brass base. This was replaced in 1948, no doubt to reduce cost, by a two-part alloy base. Note the knurled metal terminal nut in the above illustration rather than the later and more familiar black plastic terminal nut and the spring ‘J’ which was not fitted on later pumps. It is my assertion that all brass-based pumps were low pressure types. Many will argue with this, but I believe the high pressure pump did not see the light of day until after the change to the two-part alloy base. I’d be interested to hear any irrefutable evidence to challenge this assertion. Certainly there are some high pressure pumps with brass bases around today but I would suggest these may have been assembled from a collection of bits and pieces.

Fig 1 – Photo of a very early brass based pump with its seal and label intact. The label reads “IMPORTANT DO NOT BREAK THIS SEAL BEFORE READING THE INSTRUCTIONS.”

So how do all these versions of the pump differ, and how can you tell them apart?

6 Volts v 12 Volts

To differentiate between a 6 volt pump and a 12 volt pump should be really easy. All 6 volt pumps originally had a brown plastic cap with ‘6V’ clearly embossed into the moulding, whilst 12 volt pumps originally had a black cap with 12V embossed into it. However the caps are easily damaged (more of this later) and easily swapped, no doubt resulting from the ‘mend and make do’ mentality that prevailed in the post-war era. As a result, the cap colour can no longer be relied upon to tell you which voltage pump you have, so it may be necessary to be a bit more inventive to tell the two pumps apart. A second clue is the colour of the leads that emerge from the body of the pump under the cap, 6 volt pumps had green leads and 12 volt coils had red leads. Here again it is easy to be fooled because the leads will by now have discoloured and will all have changed to an identical shade of brown (more about this later). A third way to separate the two involves dismantling the pump by unscrewing the six 2BA screws that hold the body to the base. Inside, on the brass disc on which the volute spring bears, you will find a number stamped. PT 1686 tells you it is a 6 volt pump, whilst PT 1687 confirms you have a 12 volt pump. If there is no number at all, or if the disc is aluminium, then again you almost certainly have a 12 volt pump. If all else fails, the only certain way to know which voltage pump you have is to measure the coil resistance. A 12 volt coil measures around 4 Ohms and a 6 volt coil around half that. A 12 volt pump will not ‘tick’ in a 6 volt car, and whilst a 6 volt pump will tick in a 12 volt car it will soon over heat. Swapping is not to be recommended.

Low Pressure v High Pressure

The low pressure version of the pump is always fitted in the engine bay, at or around carburettor level. It has a powerful electromagnet which when energised via the contacts, pulls on the diaphragm, which in turn draws petrol into the pump body. It can lift petrol from about 40 inches. When the electromagnet releases the diaphragm the charge of petrol is expelled by a fairly weak spring, so the push stoke can only raise the petrol a further few inches above the pump body.

The high pressure pump on the other hand has a more powerful spring to expel the petrol from the body so can lift the petrol to a height of nearly 48 inches above the body. However, the electromagnet of the high pressure pump has to compress this spring as well as lift petrol into the pump body; hence it can only lift petrol from a few inches below, even though it has a more powerful magnet than the low pressure version. For these reasons low pressure and high pressure pumps are not interchangeable.

So again, how do you tell the two versions apart?

Firstly, if you are lucky, you will find a part number tag fixed to the pump body. AUA25 is a low pressure pump, so is the AUA66 which was originally specified for the Morris Minor. It differs from the AUA25 only by the outlet pipe spigot, which can easily be changed to the more familiar T Type right-angled version. If your pump’s label says AUA54 then it is a high pressure pump as fitted to later TFs. On most pumps however these original labels no longer survive so we need to look for another clue. On later alloy based pumps there is an earth (or ground) terminal at the side of the body. If this is a 2BA screw, the same size as the six screws that hold the body to the base, it is a low pressure pump. If it is a smaller 4BA screw, the pump is a high pressure type. Simple? Not quite that simple I’m afraid. There is always an exception to every golden rule. The AUA54 pump, fitted to later TFs has a 2BA earth screw but is a high pressure type. It is however easily identified because it has a uniquely longer body, which at 75mm is around 15mm taller than the standard pump body.

Pre 1948 brass based pumps did not have this side earth terminal screw. Instead they had a 2BA earth stud which replaced one of the six screws that hold the body to the base. Again, if my assertion is correct, if it has a brass base it is always going to be a low pressure pump.

If there is still any doubt the definitive test is to dissemble the pump and measure the diameter of the magnet core (labelled Q in the illustration). The low pressure pump has a 15mm core and the more powerful magnet of the high pressure pump has a 18.5mm core.

Why did MG move the pump from the engine bay in mid production of the TF? I believe it was because vaporisation of petrol within the pump was beginning to become a problem. Why else would they have moved the pump to the wet and dirty and generally inhospitable location above the back axle? The additional benefit with the high pressure pump is that the petrol in the fuel line to the front of the car is at above atmospheric pressure, and hence less likely to vaporise than petrol in the fuel line of a low pressure pump system, which is at below atmospheric pressure.

The Contacts

The Achilles heel of the Type L pump has always been the contacts (labelled U and U1 in the illustration) used to energise the electromagnet. These quickly erode as a result of the arc that is produced as they break each time the pump ‘ticks’. They also tend to corrode until they no longer make contact if left in a damp garage over winter. SU tried numerous modifications to extend the life of these contacts during the evolution of the pump, including doubling up the contacts on later versions. Unfortunately in terms of longevity twice ‘not very long’ is still ‘not very long’. Owners of course, perfected the art of hitting a dead pump with a soft hammer to breathe life into again, albeit for a short period of time. This probably accounts for the damage caused to the pump cap, and why they may have been swapped.

SU’s first attempt at snubbing the arc was to include a burden resistor in parallel with the coil. This took the form of another winding, this time of resistance wire, wound on top of the coil. For reasons best known to SU they threaded the ends of the resistance wire through the green or red sleeving that identified 6 volt pumps from 12 volt pumps and vice versa, which was the cause of the burning that turned both colours to a similar shade of brown. Next they tried a condenser across the points which is intended to serve the same purpose as the condenser in the distributor, i.e. to provide a path for the current to take for long enough for the points to open sufficiently wide so that they can’t arc, but condensers of the time were not as reliable as now, and it was not a great success.

Further attempts at extending the point life included the use of a diode to snub the arc, which works well, but makes the pump polarity conscious. Not a good idea if you lend a pump to a friend with a car wired with the opposite battery polarity, only to see all that expensive smoke, lovingly fitted by dear old Joe Lucas, aka “The Prince of Darkness”, escaping from the loom. The ideal solution to extend contact life is of course to fit a Transil, but more of this in Part 2.

Eric Lembrick

2 thoughts on “The SU Pressure Pump Type “L” (Part 1)

  1. gianfranco donati says:

    good morning I’m looking for:


    Should you tell me where can i find it or who can i contact for shopping?
    Thanks in advance

    Gianfranco Donati

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