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(Negative Ground)


by David DuBois

Several years ago, after my wife stalled thirteen times while driving across town, I decided to do something to improve the reliability or the S.U. fuel pump. A check with a meter showed that when the pump is operating it is drawing nearly three amps of current through the contacts. This explains why the contacts burn so quickly and start sticking, causing the pump and the car to stop running. My solution was to install a transistor in the circuit so as to reduce the amount of current through the contacts to approximately 0.4 amps. Since installing this modification I have put somewhere around 20,000 miles on the car, and at last check the points looked brand new.

Parts required for the modification are: PNP transistor {Ed. - Radio Shack has a TIP42 (part number 276-2027) rated at 3 amps collector current. This should work although I have not tried it.}or similar flat epoxy PNP power transistor (1); 1N4003 diode or similar diode rated at 1 amp, 100V or greater PIV (1); 25 ohm, 1/4 watt resistor (1); 1/8" shrink sleeving or other insulating material (6"); Solder lugs (2).

All parts can be obtained from Radio Shack or similar sources for about $3.00. Once the parts are assembled proceed as follows:

1. Disassemble, clean and adjust the fuel pump in accordance with the shop manual. Before reassembling the posts they should be cleaned and shaped using an ignition file and then polished using a small, fine sharpening stone or polishing paper. If the old points are beyond salvage, install new ones.

2. When attaching the stationary point (fig. 1, #1) to the mounting pedestal, do not put the coil wire (fig 1, #3) under the mounting screw as you normally would. Instead, put one of the solder lugs under the screw (fig. 2, #9).

Fig. 1: This is what the fuel pump pedestal looks like before conversion.

3. Enlarge the hole in the mounting tap of the transistor (fig. 2, #8) ­ I filed it out to a forked shape ­ and mount it under the left­hand pedestal hold­down screw (fig. 2, #2). Clip off and discard the center (collector) lead of the transistor.

4. Solder one end of the resistor (fig. 2, #6) to the left­hand (base) lead of the transistor. Insulate with shrink tubing.
Bend close to the transistor and solder the other end to the solder lug attached in step 2 (fig. 2, #9).

Fig. 2: And this is what it should look like after transistorization of the pump is completed.

5. Solder the wire from the coil which you left disconnected in step 2 (fig. 2, #3) and the end of the diode (fig. 2, #7) oppposite the band to the right hand (emitter) lead of the transistor. Insulate with shrink tubing.

6. Place the second solder lug under the nut which secures the pump's "hot" or input terminal (fig. 2, #5) to the bakelite pedestal. Solder the end of the diode with the band to the lug.

7. Check to be sure all leads are insulated from each other and from screws, etc. which could short circuit them, and that they do not interfere with the operation of the points.

8. Check your work by grounding the body of the pump to any convenient ground on the car and attaching a wire from the input terminal (fig. 2, #5) to the positive (+) battery post. If everything has been done right the pump should start pumping like mad.

Fig. 3: For those of you who understand such things, here's a proper schematic of the transistorized pump's electrical system.

It may be found, when fitting the bakelite cap back onto the pump, that it will be necessary to grind off the projections on the inside of the cap in order for it to fit all the way down onto the pump body. This can be done using a small hand grinder. Once the cap is on, there is no visible sign of the modification.

In the unlikely event that the transistor should fail somewhere down the pike and stop the pump from operating, it is a simple matter to convert back to the original configuration and continue on your merry way. Clip the coil wire (fig. 2, #3) from the right­hand (emitter) lead of the transistor. Remove the resistor lead from the stationary point (fig. 2, #9), and connect the coil wire to the point. Remove the diode lead from the input terminal ( fig. 2, #5). This can all be done by the roadside with a minimum of tools; a knife or fingernail clipper plus a screwdriver will do in a pinch.

The Sacred Octagon

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