MK II Fuel Pump Question

Hi Rick,

The capacitor was connected across the points. The diode should be connected across the coil. The two coil wires emerge from a hole in the back of the solenoid. Observe that one of them connects to the power terminal and the other connects to the points. You should connect the diode between those two places. The banded (cathode) side of the diode attaches to whichever side of the coil is connected to positive voltage. If you have negative ground the cathode connects to the positive power terminal, and if you have positive ground the cathode connects to the points terminal.

With electricity there is no "partial credit" for getting it almost right. If you do anything wrong it simply won't work, and there can be disastrous results. Write back if you need any further clarification on this circuit.

Thanks for the reply, Michael. Yes, I had done a straight swap, having assumed your directions equated with the capacitor's location. A bit embarrassed to ask, but could you post a diagram indicating the correct location of the diode? (I can send a photo, which perhaps you could mark up.)

Thanks for the reply, Michael. Yes, I had done a straight swap, having assumed your directions equated with the capacitor's location. A bit embarrassed to ask, but could you post a diagram indicating the correct location of the diode? (I can send a photo, which perhaps you could mark up.)

Hi Rick,

Did you literally replace the capacitor with a diode? I mean, did you take the capacitor out and put a diode in it's place? If so, reread my original post about where to install the diode. If you put it in the correct place in the circuit you should not be able to see any arcing at all in a dark room.

Well, I replaced the capacitor with the diode, and the fuel pump is now pumping away. My concern, though, is with the original statement that mentioned that the capacitor was needed to suppress arcing. There is some arcing now, Should I be worried? I could create a short video to demonstrate exactly what I see if that would help any.

Thank you, gentlemen, for your replies. I'm a bit torn, as I'm anxious to get the car running for the first time since I picked it up. Is it actually possible that the capacitor could be the sole cause for the dead fuel pump? The odds will affect whether I pursue the polarity reversal and possible entire fuel pump swap, or just hunt for a new capacitor.

Thank you, gentlemen, for your replies. I'm a bit torn, as I'm anxious to get the car running for the first time since I picked it up. Is it actually possible that the capacitor could be the sole cause for the dead fuel pump? The odds will affect whether I pursue the polarity reversal and possible entire fuel pump swap, or just hunt for a new capacitor.

Mike that is a funny post! After 40 years of British cars I still never understand why these simple devices that ladt almost forever give folks such problems.

I hope they are not Electrical Engineers! The solenoid has no permanent magnets associated with it and works equally well with either polarity. The slug of steel is attracted equally well to a North pole as to a South pole.

Yes, on a Positive ground car the cathode would connect to the points side of the coil. The original capacitors did not have polarity sensitivity but after 50 years no one knows whats been replaced in there. Some clocks are polarity sensitive. Those made after 11/63 with the markings "CE1111/01" will be destroyed if you reverse the polarity, even if they are not running. Best disconnect your clock before reversing the cars polarity. That way it will be able to be repaired when you decide to do that. Radio too.

Two of us engineers here believe you should not reverse the diode on a fuel pump. Rather, the pump is designed with the solenoid working in one direction depending on current flow. If you reverse car polarity you need to get a different fuel pump (SNG-B for example lists 2 different part numbers based on polarity) or mount your old pump so it is electrically isolated from the body. The pos. ground car expected positive polarity at the body of the pump so do that. Neg. went ot the terminal so ground that for a neg. grnd car.

The caution now is that the body of the pump is "hot" relative to the car so you must be certain it cannot contact anything that would short it to the (negative) car body. An in-line fuse at the pump would be a good idea. Also, if you put the fuse somewhere accessible you could open it to theft-proof the car.

Check into a updated pump with solid-state contacts so you don't have to worry about relaibility. Or, for example, use one of Auto Zone's rotary in-line pumps for about $45. Get a low pressure model for use with carbs.

Yes, on a Positive ground car the cathode would connect to the points side of the coil. The original capacitors did not have polarity sensitivity but after 50 years no one knows whats been replaced in there. Some clocks are polarity sensitive. Those made after 11/63 with the markings "CE1111/01" will be destroyed if you reverse the polarity, even if they are not running. Best disconnect your clock before reversing the cars polarity. That way it will be able to be repaired when you decide to do that. Radio too.

Thanks for the excellent advice, Michael. So if my car is (still) positive ground, the banded end goes toward the ground side?

Another stupid question: I understood that there were only a limited number of things to do to switch to negative ground. I don't recall the fuel pump being on the list of items to be modified. If it is on that list, would it be as simple as flipping the capacitor 'direction'?

Hi Rick,

There is no resistor associated with the fuel pump. The original pump had a capacitor across the points to suppress arcing. Later replacement pumps used a diode. If I were you I would go with the more modern component and connect a 1N5400 diode across the coil, with the banded (cathode) end towards the positive coil terminal. Which terminal that is depends on whether your car is positive or negative ground.