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MG TD TF 1500 - SU fuelpump questions

I'm about to change the points in my early SU (L-type?) fuelpump in my YA because it only works when asked friendly or persuaded with a mighty blow on the head!

I'm sure I will be able to change and adjust the points, but to prevent the points pitting again I've heard of fitting a (zener?)diode.

Can anybody please give me the right specification/series number for that diode, so that I don't make a complete fool of myself at the electronics-store? I converted my Y to negative earth.

AND can anybody explain how to fit the diode? Preferably with a picture!

Lastly, whilst researching this question I stumbled upon someone recommending a bidirectional TVS-diode (transient voltage surpressor) because it lasts longer when pumping air when you ran out of fuel. Apparently his 24V zenerdiode gave up very quick in this (rare) condition; anyone care to comment?
Willem van der Veer

Willem - I’ve been told by an electrical engineer that a Zener diode is too slow reacting to provide much protection (I've been using the on my modifications). I do know that a standard diode across the coil will slow the action of the pump considerably and with the old L type pumps, the coils need all the help they can get pump enough fuel. The transient voltage suppressor should work fairly well in this situation and is the thing that I am considering using in place of the zener diode. I would go with one that is rated at about 47 volts, but at this time I don't know what the part number would be. If I can find out I'll e-mail it to you. Another choice is to use a resistor/diode combination in series across the coil a Burlen Fuel Systems does in their later fuel pumps. This combination eliminates the sluggish performance of a diode only. I would use a 1N4005 diode and a resistor that is somewhere around 150 to 200 ohms. The resistor and the diode are placed in series and the band of the diode should be pointing the end of the coil that is at the most positive potential (ie. if you have a positive ground, the end of the diode with the band on it would be attached to the upper points. You could also order one of the diode /resistor units that Burlen Fuel Systems uses. If you will send me your postal address and the polarity of your car, I can probably send you one of these units. You will need one of the stepped or domed end covers to use this particular unit. Cheers - Dave
David DuBois

Dave,

Are you saying to wire the diode/resistor combo parallel to the coil?

Like this: (for positive ground)


points

(+)-----------

@ -
@ ^ Diode
@
coil @ >
@ <
@ > resistor
@ <

(-)-----------

Thanks,
Evan
Evan Ford - TD 27621

Ooops!

The diagram above looked good before I hit submit.

Sorry....
Evan Ford - TD 27621

Evan - One end of the resistor to the power terminal One end of coil), other end to plain end of diode, end of diode with bar on it to the upper points along with the other end of the coil. Cheers - Dave
David DuBois

Thanks Dave!
Evan Ford - TD 27621

This thread was discussed between 17/04/2006 and 18/04/2006

MG TD TF 1500 index

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