XK Tranistorized Fuel Pump

I’ve found older posts (RJ_ Reilly, etc) on how to transistorize the XK fuel pump. It appears the original scheme was to use a TIP 31 NPN transistor (for positive ground) a diode and a 25 ohm resistor. This was later revised to a TIP 31 NPN, a 1 K ohm resistor, a 100 ohm resistor and a 0.1 uf capacitor. I would prefer to use the updated scheme. Does anyone have a diagram on how the parts are interconnected? Pictures would be great as well. The drawings that I did find in the XK forum are very difficult to understand.

Pat H.


Here are a couple of original pages with instructions on making the installation/conversion.

I don’t know if it would fall into the “updated” category or not.

It took some practice to get it all to fit in the somewhat restricted area but I have had excellent success with the conversion.



I believe DuBois requested his page be removed from jag-lovers, but he may not have been the originator of the scheme, it may have come from the vintage MG folks, who passed it on to the Classic Jaguar Association in 1989.

There have been a few updates over the years, and the latest version available from Burlen has a printed circuit board and eliminates the points.

I have done about 12 of these in this manner.

In my pictures the brown blobs are RTV sealant covering solder joints.

Parts you need:
one transistor…for negative ground get a TIP 32 PNP
…for positive ground get a TIP 31 NPN
one diode = 200V 1A microminiature type 1N4003…same for either ground (or an alternative diode 1N4002 can be used)
one resistor = 1 K Ohm
one resistor = 100 Ohm
one capacitor = .1 uF

First disconnect the one coil wire from the blade contact point small fixing screw, and remove the capacitor if yours has one. The pump in my 1950 Mark V didn’t, so it really arced something fierce. The other coil wire is your 12V input, and stays as is.
The transistor has a mounting tab, which I drilled out to fit on one of the two existing large mounting screws for the bakelite bridge. Use either one you like, since you won’t be reinstalling the old capacitor.
Note the transistor has an emitter, a base, and a collector. Regardless of whether you are doing a PNP for negative ground or an NPN for positive ground, the connections are the same.
The collector is also the mounting tab, which is thus grounded to the coil body, as is the wire from the flipover contact points.
The base terminal is soldered to a wire, which connects to the small screw which holds the blade contacts.
The emitter is soldered to the coil wire that was removed from the small screw above.
The 1000 Ohm resistor is soldered to connect the input wire and the base.
The 100 Ohm resistor and .1 mf capacitor are soldered in series connecting the base terminal and ground.
Now for the diode: For both positive and negative ground the diode is soldered between the emitter-to-coil joint and the input terminal, but in opposite directions. The forward direction looks like an arrow running into a wall.
For positive ground (earth): the diode forward direction is from the negative input terminal to the emitter.
For negative ground: the diode forward direction is from the emitter to the positive input terminal.

Technique tips:
Be sure to ream out the hole in the mounting tab on the transistor big enough so the screw head will not damage the transistor before cutting off the excess. I found it easier to put spade connectors on the screws and then soldering the wires to the connectors in place, rather than trying to solder to loose connectors or wrap wires around screws. All the capacitors and resistors come with wire ends way too long. Stick them through the connectors with the excess hanging out, solder, then cut off the excess. For soldering to the transistor terminals, load a bit of solder on the terminal and the wire end separately first, then touch them together and its a quick job to solder them together. The center collector pin terminal is not used so I break it off completely.
For joining the capacitor and resistor, twist the ends fairly close together, solder the joint, then cut off the long excess. One of the wires coming up from the coil is too short, so you have to add an extension wire to it to reach over to the transistor. A lot of this job is figuring out how to do it with only two hands, and the rest is packaging, figuring out how to fit it all inside the cap without any crossed wires. Insulate any potential short circuits with a dab of RTV and let it dry before closing it up. I always make a paper label to stick on the outside of the pump, indicating whether it is positive or negative ground.
The result is a pump that rattles away with not a bit of visible arcing and sparking at the points. An added bonus for those who care is the mod is completely hidden from concours judges by the plastic cap.
Piece of cake, right?

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That is EXACTLY what I was looking for. Thank you so much. Parts on
order. If that doesn’t work I may go for the Burlen circuit board unit.

Pat H.

This is an interesting thread. I am slightly worried by the ad hoc nature of the soldering.

I am working on a little circuit board which will simply be a screw in replacement for the current connections. It will need no soldering and functionally, it’ll deliver the same circuit.

I’ll report back when it’s done.

kind regards


Here is my version.
It’s a ready made pcb with no soldering required to fit it. You just unscrew the relevant terminals and bolt them back in a slightly different combination to make the transistor do the work.

kind regards

su pump pcb Image1


I bought an SU electronic pump. End of all problems such as banging it at traffic lights.

Wow…really nice work!

iirc, some schematics were removed as the author perceived he may be sued if anyones car burned (presumably as a result of poorly executed solder work in the limited space)

Question; is it possible to mount the circuit externally to the fuel pump?

Woah, if that were true none of us would ever offer any repair advice over the internet.
A fire would be no more likely than with the original points arcing away like mad in there. The pump diaphragm shouldn’t be leaking fuel to the coil anyway.

I suppose one could put the electronic stuff in a separate box somewhere else on the car, but you would need to run more wires to the pump.

Without an ignition source, it makes no difference where the circuit is. With no points arcing, it’ll make no difference where it is mounted.

As another example, every car of this vintage has a fuel gauge driven by a rheostat wiper sitting inside the fuel tank. It doesn’t make any sparks because the there isn’t enough current to do so even though the wiper arm wipes against a wire, potentially connecting and disconnecting itself from its 12v source as fuel load changes or you go over a bump in the road.

In either case, you’d need petrol:air vapour ratio to be within certain limits to form a combustible mixture in the first place and experience shows that that isn’t the case.

kind regards

wracking my brains here, but I am thinking if a positive ground vehicle is “dioded”, and then later, an unknowing owner converts to negative earth, the diodes will go up in a puff of smoke, and that was the basis of concern.

like yourself, I would not be unduly concerned.

If a search is made in the old archives, under diode fuel pump conversion, instructions will be seen to be deleted, and the discussion is way back somewhere.

It is also possible the writer deleted it wholly or partly for other reasons, such as copyright

The diode won’t go up in a puff of smoke.

If you look at the circuit diagrams above and apply 12v and earth the other way around on the lower diagram, then the current will simply flow straight through the diode and by blocked at the transistor. Unfortunately, it’ll also go straight through the 1k resistor, through the points and to earth. The wiring will melt somewhere along the white wire path as Jaguar never thought to fuse the circuit (other than with the submersible pump on very early cars).

The circuit board and the fuel pump will be unaffected; the loom will melt first. This will happen long before you have had a chance to start the car or even pump any fuel, so the idea of a big explosion is not likely.

In the event that anyone with a positive earth car wants to buy one of these, all they need to do is be warned to attach a sticker saying they have a positive earth fuel pump which needs two components swapped prominently to the battery terminal post.

In any case, I suspect that positive earth owners will probably “keep their cars original” as I haven’t sold a single positive earth upgrade board yet.

kind regards

I re-posted my old instructions on Sept 3 at the top of this thread.
Searching “Transistorized SU Fuel Pump” I see my report of my first conversion was on June 9 1995. All the pumps I have done, 4 for myself and 8 for other people, have received a label identifying whether they are positive or negative ground.
There was never any mention of copyright. The Classic Jaguar Association published the instructions in 1989, and I believe they got the idea from the Vintage MG folks so it is even older.
Marek and Burlens have both improved on the concept with their printed circuit boards, making it a lot easier.

Marek, I wouldn’t mind being the first positive earth XK owner to use your upgrade board. Could you post a photo of your upgrade board installed in an SU fuel pump?

Dear Mike,

There is a picture in the classified Ads section of this website.
Here are another couple of pics.


These pictures are before you put the end cap back on with its spade terminal, otherwise there’d be nothing to see.

The fitting instructions are in the form of a MS Word document with 13 more annotated pictures.

kind regards

Marek, You are truly versatile!!

Mike, I bought lenses Marek manufactured for e-type S1, and I have to say they are spot on. He does wonderful things!..

For my next trick, I am embedding high brightness LEDs into the Lucas round s1 etype rear reflectors. You won’t see them until you hit the brakes - and then everyone will see them.

kind regards

1 Like

Can ya make some for my Rover?


You’ll have to dream on Paul
Sadly I am tied up doing headlights for the forseeable future.

kind regards