Extend the life of your SU fuel pump – Fit a transil!

Following Part 2 of Eric Lembrick’s article on the SU Pressure Pump Type “L”, published in the August 2011 issue of TTT2, I have received a number of enquiries about fitting a transil. It seems that what has given rise to these enquiries is the following advice given in para 4.12 of the article:

“Finally refit the contact blade and a Transil to protect the points”.

Why fit a transil? The points on your SU fuel pump are at the mercy of the high voltage (up to several hundred volts) that is generated each time they open, causing them to arc. The basic explanation for such a high voltage (bearing in mind that your battery is only 12 volts!) is that it is an effect that happens each time a current through a coil is interrupted.

To negate this high voltage (i.e to limit voltage transients) the transil pictured below comes with a rated voltage. Below the rated voltage there is no connection between the two terminals, but above the rated voltage the terminals are connected together (dead short). Consequently, when the points break the high voltage which is generated across them is shorted out by the transil, so saving their burning and pitting.

Fitting the transil Fitting is simplicity itself. The transil is supplied with ready made solder tag connections. All that is required to fit it is a screwdriver.

Photo 1 – shows a transil with ready made solder tag connections along with transils awaiting fitment of the connections.

Note: Some T-Types will have been fitted in the past with a diode across the coil. Whilst this has proved to be an effective solution to reducing the voltage across the points it needs to be borne in mind that once a diode is fitted the pump becomes polarity sensitive. If a diode is fitted incorrectly, or, if fitted to a car of the wrong polarity, a pump fitted with a diode will fail instantly, causing the associated wiring to overheat or even catch fire. (The night after typing this I learnt from a TTT 2 ‘hard’ copy subscriber at our local ‘noggin and natter’ that he actually experienced this with his J2.

Photo 2 – shows a transil fitted to an SU pump.

The advantage of a transil is that it is completely non-sensitive to battery polarity. Hence a transil can be fitted either way around and a transil pump can be fitted to any car. It can also be left undisturbed if the polarity of the car’s battery is changed at a later date.

Transil kits, complete with fitting instructions are obtainable on a non-profit making basis from John James, 85 Bath Road, Keynsham BRISTOL BS31 1SR, UK. Cost, including postage is £3.50 (UK), and £4.00 outside of the UK. Payment by PayPal is acceptable.

If paying by PayPal please e-mail me at jj(at)octagon.fsbusiness.co.uk (substitute @ for at) and I will send you a PayPal invoice.

Your transil kit will be sent inside a normal correspondence envelope and when you open the envelope you will find the transil kit and fitting instructions inside.

John James

6 thoughts on “Extend the life of your SU fuel pump – Fit a transil!

  1. admin says:

    Barrie Jones has commented on the Transil article as follows:

    I have just read the article on converting SU pumps with a Transil.

    I would suggest a warning to anyone trying to source their own.

    Standard Transils are normally polarity sensitive (i.e. they only work one way round).

    The Transil described in TTT2 Issue 9 is a bi-directional Transil.

    By the way, the name is just a shortened version of the function – Transient Limiter.


  2. Tom Pollak says:

    Just installed a bi-directional Transient Voltage Suppressor (TVS) in place of the condensor in the SU pump.
    part number 625-1.5KE24CA-E3
    MFG PN: 1.5KE24CE-E3/54
    Described as:
    1500W 24V Bidirect Vishay TVS Diodes – Transient Voltage Suppressors

    Very reasonable price, extremely fast service!

    • Duncan McKellar says:

      The MFG PN is actually 1.5KE24CA-E3/54. CA is an industry indicator of bi-directionality in diodes. Fairchild Semiconductor and Littelfuse also make these TVS suppressors, along with Vishay. A Littelfuse almost identical to the one you mention would be MFG PN 1.5KE24CA with a Mouser # 576-1.5KE24CA. But these two are 5.3mm dia. x 9.5mm L. The pictures in this article show a bidirectional TVS suppressor that is near 3.56mm dia. x 7.62mm L, and is likely for a 6 volt pump; along the lines of a Fairchild bi-directional TVS with a Mouser # 512-P6KE13CA and a MFG # P6KE13CA. Mouser is great, and I also like Allied Electronics out of Ft Worth. http://www.alliedelec.com

      • JOHN JAMES says:

        A Transil is a transient voltage suppressor. The brand name was originated by ST Microelectonics but now many other semiconductor companies produce transient voltage suppressors and they are widely available from a myriad of electronic component distributors around the world. The type mentioned in the article is rated at 600 watts peak and can be used on both 6 volt and 12 volt pumps. The body size varies from manufacturer to manufacturer, but is typically 6mm long and 3mm in diameter.

    • Tom Pollak says:

      Installed the TVS 625-1.5KE24CA-E3 in the fuel pump of my 1960 MGA last fall and have been running it since. So far all is OK. I am looking forward to checking the points for wear, pitting & etc.
      At this point, I have no reservations in recommending the use of the TVS in the SU (British) fuel pump. The bi-directional TVS gives the added benefit of being able to be used on the cars with negative or positive ground systems.

      • Duncan MacKellar says:

        I added a Littlefuse (brand) from alliedelec.com on an AUA 25 low pressure TC pump with new double contact points earlier this year. Working wonderfully. Before I finalized the install I ran the pump off a 10A 13.7VDC power supply and observed the point spark. When I swung the TVS suppressor into place the sparking lessened by a good 80%, with no loss of pressure or performance in any way. I am pretty good at soldering, and it was tricky to get soldered, as the “wire” coming off either end of the TVS melts at a low temp. I also tried the same TVS suppressor in an autopulse model 500 I rebuilt, and similar results. Though Tom Pollack and myself are using somewhat experimental transils, most people probably will find it easier to go with the pre-built transil and fittings that is mentioned in the article; from John James in Bristol.

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