// Jag-Lovers // The Jaguar XJ6 //

 

Transistorised SU Fuel Pump

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By Rob Reilly.

Hey, just in time for Alastair and Nick, I was putting together a report on how I rebuilt my Mark V fuel pump with a modification.

First, for Dave, your injected car has a different kind of pump from the carburetted cars. SU pumps on carburetted cars have an electromagnet, a diaphragm and a set of points. The steel disc on the diaphragm is what clicks.

Yours is an electric motor which I think should whir only when the engine is either cranking or running. If the engine is shut off or if after cranking it fails to start, and the pump keeps whirring, I believe that would indicate a problem in the pump relay or the air flow sensor.

I just finished rebuilding the original pump for my 1950 Jaguar Mark V last week, and this time I added a transistor and a diode according to a scheme published by the Classic Jaguar Association in July 1989 and Jan 1990. They got it from the Vintage MG folks. Most of your SU pumps will have a capacitor which sort of protects the points for awhile but eventually they burn out. My MkV didn't even have a capacitor, so the points really arced something fierce.

Apparently the MG mod lets the transistor do the coil switching work, so the points last indefinitely, no more tapping the pump with a hammer or slamming the car door to get it going again.

 

Parts List

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From Radio Shack or another electronics supplier you need:

• one transistor: for negative ground get a TIP 32 PNP, for positive ground get a TIP 31 NPN
• 0ne diode: 200V 1A micro miniature type 1N4003 (or a 1N4002 can be used instead) same for either ground

Get these parts from one of the after market Jag parts suppliers:

• one contact points set and one diaphragm for a single pump, or two for a double pumper.

You might need:

• one-way valve discs (some are plastic, and they do break)

Notes:

1. Get the double contact type points, not the single.
2. There are two lengths for the diaphragm stem, Mark V is the longer one, I think XJ6 should be the shorter one.

 

Procedure

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Remove six screws around diaphragm and gently peel it off the aluminum body. This contact face is a sealing surface, so don't gouge it with a screwdriver like I did my first time. Clean the crud out of the body and check your one way valves (a gozzinta and a gozzoutta). You can push up on the diaphragm to see the toggling action of the points. Note how much the stem is screwed into the center pivot of the points, and try to duplicate with the new diaphragm, but you don't have to be super accurate about that.

The diaphragm has either a plastic centering spider or 11 centering brass discs to center it in the cast iron coil body. Unscrew diaphragm from old points, replace points and screw in new diaphragm. Push on the diaphragm to check toggling action of the points and bend the tab to adjust the travel of the moving points (I found a couple of new sets of points didn't flip over right, had to bend the coil spring a bit). Bend and/or adjust the stationary points to get a good contact with the moving points. Good contact and good separation is what you're after. Put in centering spider or discs, put fuel sealant on contact faces, and tighten down six screws diagonally and uniformly. Note the small drain hole in the coil body is supposed to point downwards. Bench test it with jumper leads from your battery, it should rattle away at about 3 or 4 cycles per second. Installed, it should fill up the carb bowls and then slow down to like one click every two to ten seconds. At this point your pump is just as good as it ever was...

 

Modification

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From this moment it becomes either a positive- or negative-ground only pump, so decide which you want. First disconnect the one coil wire from the blade contact point small fixing screw, and remove the capacitor. 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 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 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.

Now for the diode: For both positive and negative ground the diode is soldered between the emitter-to-coil joint and ground, but in opposite directions. The forward direction was shown on the package I bought, looks like an arrow running into a wall.

For negative ground (earth): the diode forward direction is from ground to the emitter.

For positive ground: the diode forward direction is from the emitter to ground.

The result is a pump that rattles away with not a bit of visible arcing and sparking at the points.

Second attempt at an ASCII circuit diagram, this time paying more attention to the <space bar>.

This is the negative ground version:

 

o______/\/\/\/\/\/\____________o +ve 12 V input
|         coil
|
|
| TIP 32 PNP transistor
o-------emitter--->----base---------------o = blade contact
|     |
|     |
| collector
|     |
|     |
o-----|<----------------o----------------o = flip-over contact
           1N4003 diode |
                        |
                        |_________o -ve ground

 

Following is the positive ground version:

 

o______/\/\/\/\/\/\____________o -ve 12 V input
|         coil
|
|
| TIP 31 NPN transistor
o-------emitter---<----base---------------o = blade contact
|      |
|      |
|  collector
|      |
|      |
o----->|----------------o----------------o = flip-over contact
          1N4003 diode  |
                        |
                        |_____________o +ve ground

 

 

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