SU fuel pump component question

Inside my fuel pump black cover was a small cylindrical device, held in place with clip. Not all my other cars with SU pumps have this device. I recently replaced it with a Moss supplied dual polarity diode (green wire item) they show as the modern update, Other than the fact that the great state of California says it can cause reproductive harm and cancer, exactly what is the purpose of this device? My LH pump seems to be working fine with it installed and I may fit one to the RH pump as well,

I believe it is a capacitor. Purpose is to attenuate high voltage back EMF, or voltage, generated by the inductance of the pump solenoid. Diodes do the same thing but better. Don’t know why CA doesn’t like. EMF causes arcing of the points. IMHO.

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Not sure what EMF is but I suppose these diodes are intended to reduce sparking at the points thus improve their life?

Electromotive force.

To me - dual polarity diode sounds like a bidirectional Transil or TVS.
This is a hefty zener diode that limits spikes to given voltage. Ubiquitous types are 600 Watt and 1500 Watt size transils. The one pictured looks like a 1500 Watt. So my best guess would be 1.5KE-18CA device, 18 Volt, bidirectional. Their advantage is immensly fast reaction and quite high current spike immunity.

Electro-magnetic force in this case.

Right. Sorry if following nerdy comment is a bit long.

Inductors (any coil, for purposes of discussion) are used as solenoids (as here) or transformers (ignition coil) or electric motors, as well as for other purposes, Inductance is the electronic equivalent of inertia (as in a flywheel). With a steady current applied, the inductor behaves sorta like a spinning flywheel. If you try to change the current, the inductor “fights back” by generating a voltage opposing the change. That voltage was called back-electromotive force (back-EMF) during the prehistoric days of my education. I use it from habit.

Anyway, turning off the current altogether is opposed by a large back-voltage, which can be tens or hundreds of volts. That’s enough to shock you from a 12V device, which is surprising. This large voltage can arc across mechanical points in fuel pumps and of course contact points ignition. The latter uses a capacitor not only to prevent point arcing but to induce a bit of oscillation that prolongs the created by the coil’s secondary. For that reason, capacitors remain in that application even though diodes have replaced them in fuel pumps.

With electronic ignition you still get the back voltage when a transistor interrupts the current, but there are no mechanical points to arc. You still have to protect the transistor from the back-voltage though… That was the purpose of the 350V Zener diode in the early Jaguar ignition amplifier. IMHO.

35v zener…

It clamps the maximum reverse voltage spike to 35v. Allowing a 350v spike is no clamp at all!

kind regards

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The other reason that diodes are not used in ignition systems is that a diode would prevent the flyback action that produces the high voltage for the spark plugs. As Robert said, when the current through a coil is interrupted the coil “tries” to keep the current flowing by generating a sufficient voltage to maintain the current. If the interruption is caused by points opening the voltage needs to arc across the points to maintain the current that was flowing when the points were previously closed. A diode across the points directs that current away from the points and through the diode, which snubs the high voltage that would have been generated to arc across the points. However, the whole purpose of an ignition system is to utilize that high voltage to spark across the plugs. The primary and secondary of the ignition coil are both coils, so when high voltage is generated at the secondary for the spark plugs there is also a somewhat lower high voltage produced at the primary, and that voltage can reach into the hundreds of volts. Prove it to yourself by touching the primary coil wire while your engine is running. When the secondary voltage rises to the value where the plug fires, the voltage stops rising in the secondary and also in the primary. However, if the spark plug is open or disconnected there is nothing to keep the voltages from going even higher than what it takes to jump a .024" gap:.

Dark Spark S

I used a 500 Volt transistor in my electronic ignition, but if a spark plug is disconnected the primary voltage can go much higher than that, so that is why a 350 Volt zener is used to protect the transistor. It is not used in normal operation, but is there in case someone pulls a plug wire. If you were to connect a 35V zener across your points the engine would immediately stop running due to a lack of spark.

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I was referring to the fuel pump and its points cradle. I didn’t realise we’d moved on to ignition, hence the crossed purposes. They’re obviously not the same.

kind regards

Then you must have misread the sentence where Robert mentioned the 350 Volt zener…

Guilty as charged.

EMF … what it is depends on what is being referred to .

Electrical systems:

  1. EMF is electromotive force, which is voltage, and is the driving force that causes current flow through an electrical system.

  2. EMF is electromagnetic field, which is created by current flowing through a conductor, and when that conductor is wrapped around and around into a coil, the magnetic field that is created is what pulls relays in and makes (or breaks) contact within the relay to accomplish something (energize or de-energize something).

EMF as in electromagnetic field is what causes electrical interference by the frequency that the magnetic field is created and allowed to collapse.

With ac (alternating current) systems, that frequency is typically design-set at 50 cycles per second (50 Hz such as Europe and other locations) or 60 cycles per second (60 Hz such as North America and other locations).

With dc (direct current), such as battery systems, there are no cycles (0 Hz) until such is created by something continuously switching on-off-on-off, creating a ‘cycle’ (and the frequency of the cycle can vary).